20A 240V plug/receptacle choices, and running 5HP <20A motors on 12AWG circiuits?

[Dave MacArthur]

Hi all. I've recently purchased some 240V machines with 3-5HP motors. All came wired with locking plugs on them, 3 poles and ground (4 prongs), 20A plugs of type L1420P. Don't know why 3 poles, motors all require only 2 plus ground on these, I'd have expected L6-20P...http://www.electri-cord.com/nema/images/L14-20c.gif

My shop and machines are currently wired with non-locking 6-20P.
http://www.electri-cord.com/nema/images/6-20.gif

Both are fine according to NEMA charts (here's a nice reference from previous thread at SMC asking about 240 v plugs: http://www.powercabling.com/nema.htm )

Question 1: Thoughts on using straight bladed plugs instead of locking ones currently on the machines, which would let me use my currently installed receptacles? Just personal preference it seems, but anyone have any supporting stories?


Question 2: Can I replace these locking plugs on the 5HP machines and run them on 20A circuits, 12 gauge wiring with 20A CB to a subpanel I installed myself, without issues? Previously I just assumed that a 5HP motor would need a 30A circuit on 10AWG wiring, but now looking at the motor plates and seeing them all rated less than 20A, I'm thinking my 20A circuits are OK? Garage shop, hobby use. As I read the NEC, 12AWG can do 25A, over-current protection to be 20A. So I'm thinking if the machines don't trip the 20A, should be good?

Here's the machines, motor ratings and motor faceplate or instruction manual amperage:

SawStop ICS 5HP 19.7A from both motor plate and manual. Manual says saw should be wired with a 3 wire (hot/hot/ground) and NOT a 4 wire plug/cable, tells you to cap the neutral if you have 4 wires run.

Oneida 5HP cyclone DC, 5HP Baldor, 19.5A, good at 20A if my CB can handle the start up (slow trip one). Also shows only 2 wires and ground on their motor wiring diagram.

Anyone out there running these or other "5HP" motors rated just less than 20A on 20A circuits and have any issues or thoughts? I'm seeing my way to NOT running new 30A circuits, doing sanity check ;)

Thanks!

[Peter Quinn]

I'm no electrician but I'd think running 19.5A on 12/2 20A circuit would be a poor long term strategy. I think the figure for motorized circuits is not to exceed 80% the circuits rating, so 20A gets you 3HP, not 5. Might it start and run the machines? Probably, but you'll be heating the wire beyond what is advisable over the long haul.

As far as receptacles , I like twist locks and strain reliefs on all my machines as my boxes are mostly ceiling hung, but in reality in a home shop it's of little importance as long as they match the current rating. I have some of both.

[Curt Harms]

I'd bet the saw wouldn't be an issue unless you're a commercial operation. How often do you use the full depth of the blade, and for how long at a time? If you were ripping 12/4 white oak nonstop you could have a problem. The cyclone could be more of an issue but if I had existing wiring I'd be tempted to try it and put a clamp-on type ammeter on it and see how many amps it actually pulls. It'd depend on ducting, filters if any, how many gates open etc. Of course if this is a permit situation, it's whatever the local authorities want. I'm not sure where and how the 80% rule comes into play. As long as the circuit breaker is working as advertised I don't think you'd be creating a danger, just causing nuisance trips.

[Jason White]

I use L6-20 plugs for 20 amp circuits and L6-30 plugs for my 30 amp circuits. Probably overkill to have the locking plugs in my home shop, but I figure overkill is usually better when it comes to all things electrical.


Hi all. I've recently purchased some 240V machines with 3-5HP motors. All came wired with locking plugs on them, 3 poles and ground (4 prongs), 20A plugs of type L1420P. Don't know why 3 poles, motors all require only 2 plus ground on these, I'd have expected L6-20P...http://www.electri-cord.com/nema/images/L14-20c.gif My shop and machines are currently wired with non-locking 6-20P. http://www.electri-cord.com/nema/images/6-20.gif Both are fine according to NEMA charts (here's a nice reference from previous thread at SMC asking about 240 v plugs: http://www.powercabling.com/nema.htm ) Question 1: Thoughts on using straight bladed plugs instead of locking ones currently on the machines, which would let me use my currently installed receptacles? Just personal preference it seems, but anyone have any supporting stories? Question 2: Can I replace these locking plugs on the 5HP machines and run them on 20A circuits, 12 gauge wiring with 20A CB to a subpanel I installed myself, without issues? Previously I just assumed that a 5HP motor would need a 30A circuit on 10AWG wiring, but now looking at the motor plates and seeing them all rated less than 20A, I'm thinking my 20A circuits are OK? Garage shop, hobby use. As I read the NEC, 12AWG can do 25A, over-current protection to be 20A. So I'm thinking if the machines don't trip the 20A, should be good? Here's the machines, motor ratings and motor faceplate or instruction manual amperage: SawStop ICS 5HP 19.7A from both motor plate and manual. Manual says saw should be wired with a 3 wire (hot/hot/ground) and NOT a 4 wire plug/cable, tells you to cap the neutral if you have 4 wires run. Oneida 5HP cyclone DC, 5HP Baldor, 19.5A, good at 20A if my CB can handle the start up (slow trip one). Also shows only 2 wires and ground on their motor wiring diagram. Anyone out there running these or other &quot;5HP&quot; motors rated just less than 20A on 20A circuits and have any issues or thoughts? I'm seeing my way to NOT running new 30A circuits, doing sanity check ;) Thanks!

[Howard Acheson]

Straight or locking is your preference. I personally prefer straight so that the plug easily comes out of the receptacle if I trip over the wire. However, some seem to feel it's safer to not have the plug come out of the receptacle. Your choice.

Second, as to the amperage of the plugs, what does the equipment manufacturer recommend for plugs and wiring? All my 240 volt equipment is on 30 amp wiring and connectors and breakers.

[Steve Bracken]

Straight or locking is your preference. I personally prefer straight so that the plug easily comes out of the receptacle if I trip over the wire. However, some seem to feel it's safer to not have the plug come out of the receptacle. Your choice.

Second, as to the amperage of the plugs, what does the equipment manufacturer recommend for plugs and wiring? All my 240 volt equipment is on 30 amp wiring and connectors and breakers.

That set up allows you to draw 9.6 hp before you have to worry about a tripped breaker.

Most equipment for home and very light commercial runs less than 5 hp, so a 20 Amp circuit is more than enough unless more than one machine is run on the same wiring, at the same time, or someone has some very large machines :)

[Andrew Pitonyak]

If i were running wire, I would run 4 wire rather than 3 for the machines that support the 4 wire connection. May not really be needed, but, if I have a choice, I prefer the extra safety margin that it usually provides.

[Tom Godley]

If the manufacturer indicates that the tools will run on 20 amps then that is all you should need. People put all kinds of plugs on tools -- correct or not.

If this is new wiring -- you can always run 10g and still use 20amp breakers/ outlets.

The first time I wired a shop I used 10g for a select number of runs -- Just in case I needed to switch up to 30 amps. I have done that since -- and I have never needed more than 20 amp!. So its been a waste -- but I am a victim of habit :)

I never use 30amp feed for a 20amp tool. I like the spec grade twist lock plugs - only for the way they clamp down on the feed wire. I don't like using the screw.

[George Clark]

If i were running wire, I would run 4 wire rather than 3 for the machines that support the 4 wire connection. May not really be needed, but, if I have a choice, I prefer the extra safety margin that it usually provides. Today 12:34 PM

Andrew,

Could you explain the "extra safety margin." I was under the impression that the four wires are necessary if there are both 220v and 110v circuits being supplied. Supplying a 220v circuit alone requires only three wires.

George

[Steve Bracken]

Andrew,

Could you explain the "extra safety margin." I was under the impression that the four wires are necessary if there are both 220v and 110v circuits being supplied. Supplying a 220v circuit alone requires only three wires.

George

At a guess ..... Four wires does allow two discrete paths to earth in the event of a fault.

In a three wire system, the return and ground use the same wire.

[Andrew Pitonyak]

Disclaimer: I am not an electrician.... That said, I have expectations for a single phase system when running 220 to something:

Short answer, the fourth wire is ground (kind of like you rarely really need three wires for your 120V electrical outlet).



You have two 120V lines coming into your house.
You run the two 120V lines to the device. Lets arbitrarily say that one is red and one is black.
You run a Neutral (probably white). The neutral takes the electricity back to the box.
You run a ground (probably uncovered copper, but, ground is usually green on your equipment).
Ground is tied to neutral at the panel.

The ground is typically connected directly to the chassis and will only matter if there is a direct short. It is argued that you usually do not need the ground, but, it is safer to have it. Some equipment may have trouble if the ground is not present.

When I look at a big table saw and all of that nice metal, I really like to know that ground is connected to it in case there is a short.. You may usually need it rarely, but, when you do, your life may depend on it.

So, that is my rational. That said, I am generally risk adverse.

[Steve Bracken]

Disclaimer: I am not an electrician.... That said, I have expectations for a single phase system when running 220 to something:

Short answer, the fourth wire is ground (kind of like you rarely really need three wires for your 120V electrical outlet).



You have two 120V lines coming into your house.
You run the two 120V lines to the device. Lets arbitrarily say that one is red and one is black.
You run a Neutral (probably white). The neutral takes the electricity back to the box.
You run a ground (probably uncovered copper, but, ground is usually green on your equipment).
Ground is tied to neutral at the panel.

The ground is typically connected directly to the chassis and will only matter if there is a direct short. It is argued that you usually do not need the ground, but, it is safer to have it. Some equipment may have trouble if the ground is not present.

When I look at a big table saw and all of that nice metal, I really like to know that ground is connected to it in case there is a short.. You may usually need it rarely, but, when you do, your life may depend on it.

So, that is my rational. That said, I am generally risk adverse.

Risk averse, when dealing with 240V (often closer to 250 in practise), is a very good attitude to have.

120V rarely does more than offer a nasty experience. 240V can, and regularly does, kill people.

I have often felt that Americans are a bit too free and easy about electricity, probably because most grow up with relatively non-fatal power supplies.

Europeans do not make that mistake.

All equipment, unless full double-insulated requires a separate ground (three prong). Sure it will run if you ignore the ground, but it could kill you if it develops a fault.

Whole house GFCIs are a good idea too :)

[Peter Quinn]

I'd bet the saw wouldn't be an issue unless you're a commercial operation. How often do you use the full depth of the blade, and for how long at a time? If you were ripping 12/4 white oak nonstop you could have a problem. The cyclone could be more of an issue but if I had existing wiring I'd be tempted to try it and put a clamp-on type ammeter on it and see how many amps it actually pulls. It'd depend on ducting, filters if any, how many gates open etc. Of course if this is a permit situation, it's whatever the local authorities want. I'm not sure where and how the 80% rule comes into play. As long as the circuit breaker is working as advertised I don't think you'd be creating a danger, just causing nuisance trips.

That would be those pesky local authorities, I think they are called the NEC? I was speaking with the electrician we use at work during a RAS upgrade that I was responsible for installing. Motor name plate was 17.9A 220 and he wouldn't run it on 20A. He said there was a specific percent of available current that NEC allowed, after which the next size circuit had to be used. This is a commercial operation. I think he said 80% was allowed. You don't drive your car at red line all the time do you?

[Curt Harms]

That would be those pesky local authorities, I think they are called the NEC? I was speaking with the electrician we use at work during a RAS upgrade that I was responsible for installing. Motor name plate was 17.9A 220 and he wouldn't run it on 20A. He said there was a specific percent of available current that NEC allowed, after which the next size circuit had to be used. This is a commercial operation. I think he said 80% was allowed. You don't drive your car at red line all the time do you?
That would depend on how the red line was established. If the red line was established at 5000 R.P.M. and testing established that damage would occur at 6000 R.P.M. then no. If testing established that damage may occur above 10,000 R.P.M. but the manufacturer set 5000 R.P.M. as the redline to ensure longevity and reliability, then I would not regard operating near redline as all that risky.

More to the topic at hand, I don't know what the NEC would specify in this instance. Local jurisdictions can impose requirements more restrictive than the NEC. I'm pretty sure there'd be more risk to a 12 ga. wire carrying 30 amps for 8 hours than a 12 ga. wire carrying 30 amps for 8 minutes. The code likely accounts for continuous usage. Unless a circuit breaker fails, it's going to trip after several seconds of an over amperage condition.

[Mark Beall]

Disclaimer: I am not an electrician.... That said, I have expectations for a single phase system when running 220 to something:

Short answer, the fourth wire is ground (kind of like you rarely really need three wires for your 120V electrical outlet).



You have two 120V lines coming into your house.
You run the two 120V lines to the device. Lets arbitrarily say that one is red and one is black.
You run a Neutral (probably white). The neutral takes the electricity back to the box.
You run a ground (probably uncovered copper, but, ground is usually green on your equipment).
Ground is tied to neutral at the panel.

The ground is typically connected directly to the chassis and will only matter if there is a direct short. It is argued that you usually do not need the ground, but, it is safer to have it. Some equipment may have trouble if the ground is not present.

When I look at a big table saw and all of that nice metal, I really like to know that ground is connected to it in case there is a short.. You may usually need it rarely, but, when you do, your life may depend on it.

So, that is my rational. That said, I am generally risk adverse.

Umm, no. If you have 3 wires running to a 240V device, you have 2 hots and a ground. There is only a neutral if you also need 120V at that device, the neutral is the 4th wire in a 4 wire connection.

For those that don't understand how this works, look up "split phase electricity".

mark

[Jim O'Dell]

I'm no electrician, but I always heard that a good rule of thumb on motors, not counting for longer than normal runs of wire, was: <1.5 hp-14 guage 15 amp breaker;1.5-3 hp-12 guage 20 amp breaker; 4-5 hp- 10 guage 30 amp breaker. Again there are always exceptions and you should always follow local rules and manufacturer's recommendations, but the above would normally cover you with some measure for safety. Don't know if this is a good source or not, but the back of the box on the Square-D panel I used in my shop had a chart with motor hp and distance of run, and guage to be used. I thought that was pretty slick. Jim.

[George Clark]

Andrew,

In a 220v 3 wire circuit there are two hots and a ground. How does adding another ground make it safer?

George

[Andrew Pitonyak]

Andrew,

In a 220v 3 wire circuit there are two hots and a ground. How does adding another ground make it safer?

OK, back to the disclaimer that I am not an electrician and I have limited experience with 220v, so, assume that everything that I say is wrong and will cause not only death, but debauchery as well.

In your 120V run, you have a hot, a neutral, and a ground. The neutral is tied to ground at the panel. They used to only run two wires. I mean, why do you need that extra pesky ground wire? I think of it as a fail safe. One more thing to fail before I fry. I know that I am being simplistic and there are other reasons for it, but...

What I expect to see in your 3 wire 220V circuit is that the neutral is also the system ground. I know that the last 3 wire that I checked, this was wired to the Neutral panel. Now, at my panel, Neutral is tied to ground. I think, however, that in the mid 1990's, it became standard to always run the fourth ground wire.

So, by running an extra ground wire, I expect that it will provide a slightly safer environment. I was under the impression that this was code these days.

[Dave MacArthur]

We should probably just stick with correct terminology when discussing electrics so as not to confuse folks. Neutral is not ground. Neutral is a current path in 120V intended to carry off the return current, back to "ground"--it actually does carry a current. In a 240V circuit, there IS NO neutral, the two hot legs are 180 out of phase and annihilate each other leaving no excess current to be carried away.

The ground is something completely different, intended to provide a BETTER, EASIER, LESS RESISTANCE PATH for the current to flow than through your hand/heart/legs/feet if for some reason the current finds it's way to a metal surface you touch. It doesn't usually carry a current unless your machine is jacked up.

The fact that at a MAIN PANEL the ground and neutral are tied together is irrelevant, they are not the same. At that point the current/electrons flow indeed into the ground rather than "in the neutral and back out the ground wire" (from info below, it flows from the bonded ground/neutral bus in the main panel back to the supplying transformer via the service neutral). And in fact in a sub-panel the ground and neutral are not bonded together.

The third wire in a 12-3 or 10-3 cable (3 wires plus a ground), when used for 240V, is the neutral, and is only used if you want to run a 120v circuit off that run. You always have a green or bare ground no matter what. Or you use thnn pulled through conduit and use the conduit as the ground.

I'll agree that from a "prior planning" perspective, running 10-3 everywhere would, while overkill, allow you to run any 30A machines on 240 or 120 V anywhere. Wire and plugs/receptacles are more expensive, and you can only run so much of it in conduit, so there are limfacs.

So anyways, does anyone with a 5HP saw stop or 5HP Baldor/Oneida DC, both rated at < 20A, have stories about running them on 20A CB, popping CB on start up etc? I've been reading the NEC and various internet sites for 4 hours, there are many examples and various de-ratings required. But one thing I do note is that continous load is a motor on for >= 3 hours, or one of the specific applications they say must be calculated continuous load. Which a DC and woodworking tools don't need.

[Andrew Pitonyak]

Interesting.... so for 240V, they are 180 degrees out of phase with each other (I infer this from your statements, I did not look it up).

I simply assumed that the neutral was required for the 240V devices. My older 240V connections used to run only three wires, and one of them was NOT ground. This changed when I dropped a new panel into the house, now it is all four wire... So, the Neutral is only required if your device also has components that desire a 120V feed.

It had never occurred to me that you may choose to NOT run a neutral, because I had never seen that done....

[Roger Frazee]

Hi Dave
There have been so many misstatements about grounds and neutrals in this thread I don't want to go there to try and straighten the crooked line.

Lets look at the question you asked. Which I believe is can you run a 5 hp 230 volt 19.5 amp single phase sawstop ics on 12 awg and 5 hp 230 volt 19.7 amp Onieda DC on 12 awg. And also why the locking 4 wire plugs.

As for the plugs my opinion is that the equipment came from the factory without power cords. The power cords and plugs were added by a second party. So simply put the person installed the wrong plugs and possibly 4 wire power cords and did not use the neutral though they may have used the neutral terminal in the plugs to connect the equipment ground which would be wrong but I see this quite often. Ground and neutral often are assumed to be the same by misinformed people butyou obviously know is not true.
And of course as you and others mentioned there is no neutral required for 230/240 volt single phase equipment/tools.

To answer your question 5 hp motor rated 230 volts 19.5 amps or 19.7 amps. Yes you certainly can operate these on 12 awg copper wiring. Typically small conductors like 12 awg and specifically 14 awg, 12 awg and 10 awg copper are restricted to overcurrent protection of 15 amps, 20 amps and 30 amps respectively.

However the ampacity of 12 awg is 25 amps regardless of overcurrent protection rules for small conductors. Those rules are generally meant for general purpose branch circuits where more than one cord and plug or other equipment can be utilized.

There are exceptions to these small conductor overcurrent rules and circuits supplying motors/motor appliances are one of these.

First if your thinking in terms of branch circuits then the circuit should be dedicated (individual branch circuit) to one piece of equipment ...5 hp saw.. etc.. nothing else connected to the circuit. Technically if cord and plug this means a receptacle that is a single (not duplex)outlet.

You also need to make sure that overload protection (not overcurrent) is in place for the motor. It will either be in a motor starter or a reset button on the motor itself.

So your fla on these motors is below 20 amps .. this is well below the ampacity of 12 awg copper wire which is 25 amps and even if we apply branch circuit conductor sizing rules for motor circuits as specified in article 430 of the NEC (125% of the motor fla as specified by the manufacturer or NEC table 430.248 or both hp and fla appear on the nameplate and the motor is a listed (UL) motor. Anyway 125% of 19.5 or 19.7 amps is still less than 25 amps.

Whenever I supply a motor that runs a piece of equipment I always apply this 125% rule because I want the circuit to be able to sustain the service factor amps. You will see this rating on the motor nameplate usually like SF 1.15 or close to that. You are well within this range with 12 awg.

Now as for overcurrent (circuit breaker) you are allowed to upsize as much as 250% above the minimum breaker size. So assuming the manufacturer says a 20 amp branch circuit is ok then 20 amps is your starting point. If the motor trips the 20 amp breaker then start stepping up your breaker size till the motor will start and run. In your case if a 25 or 30 amp breaker will not start your equipment then I would suspect other problems exist.

So hope this answers your questions.

I'm going to throw in a few diagrams (below) to help with the neutral ground and 120 volt/ 230 volt single phase thing and I'll let you explain if anyone asks questions.....:)

A few things to remember as you know .. neutral is a system current carrying wire equal in amps to the hot wire it can and will electrocute you given the correct circumstances.

Ground (equipment ground) is for human safety and only carries current in the event of a fault ... for example a hot wire touches the metal case of the equipment. This fault current returns to the transformer (not earth) as does neutral current...ie...current always seeks its source (transformer). It will use the earth but only significantly if earth is the only way it can get back to the transformer. Or more simply said current will take 'all' paths available to it to get back to its source but it will take the lowest impedance path in much greater values in amps than higher impedance paths. If we construct a very low impedance path back to the transformer ( as we should) then very little current not harmful to human beings will flow on other paths like connections to earth which are at much higher resistances/impedances.

In most cases both fault current and neutral current prefer using the 'service neutral' to get back to the source transformer.... It's much easier that way... :) and it's the only path that has a low enough impedance to get enough current to flow thru the branch circuit breaker to trip it in the event of a fault to ground. Without the service neutral breakers will not trip when a ground fault occurs.

Anyway a few diagrams to help spur the conversation.... in the 120 volt diagram I'm calling the neutral a 'grounded leg' which I will explain if asked. The diagrams are mine so maybe not the best but I think they get the point across.

http://media7.dropshots.com/photos/440526/20091123/b_174841.jpg
http://media6.dropshots.com/photos/440526/20090904/185115.jpg
http://media8.dropshots.com/photos/440526/20100101/070451.jpg

[Rod Sheridan]

Risk averse, when dealing with 240V (often closer to 250 in practise), is a very good attitude to have.

120V rarely does more than offer a nasty experience. 240V can, and regularly does, kill people.

I have often felt that Americans are a bit too free and easy about electricity, probably because most grow up with relatively non-fatal power supplies.

Europeans do not make that mistake.

All equipment, unless full double-insulated requires a separate ground (three prong). Sure it will run if you ignore the ground, but it could kill you if it develops a fault.

Whole house GFCIs are a good idea too :)


Steve, in houses in Canada and the USA, both 120 volt and 240 volt wiring is 120 volts to ground.

This is well above the fatal contact voltage for human beings, and under the correct conditions you will be killed. I doubt whether it matters by how much you were killed, you're still dead.

In Europe, the 220 volt system is one line to neutral of the 380 volt 3 phase system, so it's 220 volts to ground which is substantially more dangerous from a line to ground shock perspective (what most people die from), than our 120 volt to ground system.

Regards, Rod.

[Dave MacArthur]

Roger,
FANTASTIC reply, thanks so much for taking the time to reply and provide those drawings. This topic of electrical basics comes up alllll the time, and I think your drawings and reply will be very helpful in the future.

Your answer on the ampacity of 12 AWG and allowances for motors/overcurrent protection/small conductors is spot on what I was looking for. As you know, the NEC is copyrighted and very difficult to find online, enough of it you are sure you've read all the pertinent parts and exceptions. Tables 310.16 and 310.10, and parts 240.sub-bullets are referenced all over the place, but I could not find them to read and don't have a copy of the NEC (which my city adopted in 2007). The rules for small conductor automatic de-rating to 15/20/30A overcurrent protection devices on 14/12/10AWG wires, sizing for 125% of FLA from faceplate, and applying the exceptions for dedicated motor circuits were crushing me.

Throw in the de-rating requirements of the conductors when used at high ambient temps (in Phoenix I'm assuming the next step up from 104F for the summer), and whether to use the 90deg THHN table for temp derating, but I'm using NM for the runs until it enters conduit after attic where insulation is stripped, then which temp table for 4 current carrying in that conduit de-rating... Well, there's a reason we pay for electricians! I still want to go back and look at my circuits runs and make sure I'm good for the multi-conductor de-rating in conduit, can't figure out if the 24" exception or the <10' and 10% exception I saw apply at all.

Thanks again for your reply, much appreciated and helps make sense of all those tables!

[Roger Frazee]

Dave

Your welcome.



Throw in the de-rating requirements of the conductors when used at high ambient temps (in Phoenix I'm assuming the next step up from 104F for the summer), and whether to use the 90deg THHN table for temp derating, but I'm using NM for the runs until it enters conduit after attic where insulation is stripped, then which temp table for 4 current carrying in that conduit de-rating... Well, there's a reason we pay for electricians!
There is always the possibility where you may need to derate for ambient heat increases and number of wires in conduit.
But don't make this too hard.

BTW you did one thing you should not have done. Technically you are not allowed to strip the outer sheath off the NM cable to expose the individual wires then run those in conduit. Reasoning is that some manufacturers do not put the nylon jacket over the individual conductors making them 90C rated thhn. Personally I have never seen NM-B that did not have thhn conductors. Nm-b is 90 C rated per 334.80 which to me implies the wires inside are thhn. That rating has nothing to do with the outer jacket... so go figure.

First decide what temperature you may have above ambient in the tables .. specifically 310.16. The correction factors correspond to the insulation rating of the wire your using. There are 3 columns ( 60C, 75C and 90C) so find your insulation rating then locate the column for that type insulation).

So if I'm using 12 awg copper thhn I'd be looking at the 90 C column correction factor ...for example if my wires are in conduit in an attic where temperatures are 114F to 122F my correction factor is 82% and if I have 8 current carrying conductors in the conduit my correction factor for that is 70%. Remember that thhn has 90 C insulation and therefore for derating ampacity purposes I am allowed to use the 90C column to start my deration process. You cannot however use the 90C column for the ampacity of the wire only the 75C and 60C columns. The ampacity of 12 awg copper thhn in conduit is taken from the 75C column which is 25 amps.

So 114F for ambient correction and 8 current carrying wires in the conduit and all 12 awg thhn copper.

Putting this together 30 amps X .82 X .70 = 17.2 amps usable ampacity for the 12 awg.

In your case the load requirement is between 19 and 20 amps and the derated ampacity in this case will not be sufficient. I would need to install 10 awg thhn rated 40 amps in the 90C column.

Now it becomes 40 amps X .82 X .70 = 23 amps approx. This is sufficient for the load.

Now most thhn is multi-rated often being thhn/thwn or thhn/thwn-2. If I'm installing the wire below ground in conduit I'm in a wet location and I must use the THWN rating for the application. Notice that THWN is rated 75 C but THWN-2 is rated 90C. So if I'm using thwn as my insulation rating then I must use the 75C column to start my deration process.

So you must start with a wire ampacity that will supply the calculated load before any correction factors are applied then after correction factors the wire must maintain an ampacity sufficient for the calculated load.

Just apply the above to your situation. I get 30 X .87 X .80 = approx. 21 amps. So this is good for your application. It's a little under if you push the service factor amps of the motor but I wouldn't be too concerned.

If your a long way, say over 100 feet of wire, from your power panel I'd be tempted to run 10 awg but I'd wait to see how things go before changing out the existing 12 awg.

Hope this helps.

[Roger Frazee]

I still want to go back and look at my circuits runs and make sure I'm good for the multi-conductor de-rating in conduit, can't figure out if the 24" exception or the <10' and 10% exception I saw apply at all.

No they don't. This simply means that if your wires are in a short conduit or 'nipple' 24 inches or less no deration is required for number of wires in conduit..

As for the 10 foot rule this applies to direct buried conductors that emerge from the earth into a raceway not greater than 10 feet in length and that raceway is used for physical protection purposes.

Speaking generally if your installation is a full conduit run greater than 24" then deration for number of conductors in conduit and any ambient considerations will apply.

[Chip Lindley]

Roger makes excellent, concise sense! I find it upsetting and annoying that these electrical subjects are usually taken all around the world and stray so far away from the questions posed, that the OP's concerns are forgotten.

HUGE POINT FOR FUTURE REFERENCE: wiring of electric motors does not need a 4th neutral wire, unless 120v needs be provided to that machine!

Contact the manufacturer of your new machines and complain about the 4-prong "generator" plugs supplied with them! Ask the vendor to send you correct 6-20 plugs to allow future continuity of plugs and receptacles in your shop. (hint: those L14-20s are a lot more expensive than 6-20s! Sell them off on eBay at a profit)

Personally, I use 10/3 (left over from a remodel) and 30A breaker for my 5hp compressor. I have two 30A 10/3 wall outlets with 25' 10/3 extensions to power all machines 5hp or less. Yes, a bit of "overkill", but not by much! The price was right on L6-30s and 10/3 cord....so why not?