sub-panel question

[Jerome Hanby]

Looks like the distance between the main breaker box in the house and a good spot for a sub panel in the garage is about 10 feet. I don't have any mega sized industrial equipment. My worse case load would be half a dozen fluorescent light fixtures, a 240 volt 2 HP dust collector, a 2 HP 240 volt Unisaw, a Shop fan, and a small air compressor (with my luck it would cycle on when I had everything else going). My initial thought was to run 100 amps to the sub panel, but I've seen several posts talking about 50 or 60 amps being plenty. That would be about 30 amps total for the saw and DC, that would leave 20 or 30 amps leftover for the lights, compressor, and fan. Think all of that would live on 60 amps?

[Don Jarvie]

Add up the amps for all of the tools you figure you may have on at once, TS, DC, etc and get the total. If your borderline with 40 or 50 increase it.

Depending on the cost for the supplies and electrician, it may be a small difference in cost for 50 to 100 amps. If it's not that much more go with 100.

I ran a full panel instread of a sub panel to may garage so I can have 20 slots compared to 6 in the sub. The TS, DC, heater and each wall can run on there own circuit. Makes it easier to expand.

[Rod Sheridan]

Hi Jerome, I have the following on a 35 ampere feeder

- 4 HP jointer/planer and a 4HP saw/shaper

- 1.5 HP Oneida cyclone

For your applications a 2 HP motor is about 10 amperes so

- dust collector (2HP) 10 amperes

- 3 HP machine 13 amperes

- air compressor 10 to 15 amperes unless you go crazy

So you're at 40 amperes plus a few amperes for lighting.

A 60 ampere feeder would be more than you need.

Remember, it's a home shop so the saw or other machines such as planer/jointer/shaper will be intermittent in load, short time use, and lightly loaded.

The air compressor will run at rated load, however it's intermittent.

The dust collector will run at full load, once again for a while, then shut down.

Regards, Rod.

[Darius Ferlas]

60 or even 50 amps for your scenario (similar to mine) is plenty and the margin of safety is sufficiently large. I can't see myself needing more in a hobby shop.

I have a 60 amp feeder breaker, a 2 awg feeder wire (between the main and sub), and 12 awg wires from sub to receptacles. The only reason I have a 2 awg feeder wire (100 amps) is that I got it for free. The side effect is that I can eventually plug in a 100 amp feeder breaker if I need it, but again, I'm not even close to using the 60 amps I have. I would have gone with a 50amp sub-panel but I couldn't find one that I liked and was comfortable with. I bought an 8 slot (or 16 space saver slots) Siemens sub + breakers.

Simultaneous uses in my garage are:

2HP DC
1 machine (1 to 2 HP)
5 double tube fluorescent lights
1500 Watt quartz heater (in winter)

[Jerome Hanby]

Thanks for the info guys. I thought I would be pretty safe at 50 amps and going 60 would leave a little wiggle room.

Being in Birmingham, running a heater is never an issue and open garage doors and a big fan is the cooling system.

I hear you on using the free wire. I've put out a few feelers to see if anyone I know that works with AL Power might have a scrap of feeder cable 10 or 15 feet long...

[Rollie Meyers]

Just a little note: Oversizing the wiring will kick in a rule that may make things sticky, 250.122 (B) will require a increase of the size of the grounding conductor when larger conductors are used. This can make the use of cable type wiring methods not code compliant because of the reduced grounding conductor size.


(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.

NEC 2008 edition this is unchanged from the 2005 Ed.

This also supercedes the sizing rules in other sections.

[E. Paul Wheeler]

Wouldn't the grounding conductor in cable be sized to match the ungrounded conductors? I would think it would be more of an issue if pulling in individual wires in conduit. I could see someone using larger ungrounded conductors but not thinking to increase the grounding conductor. Or am I missing something?

[John Coloccia]

Just a little note: Oversizing the wiring will kick in a rule that may make things sticky, 250.122 (B) will require a increase of the size of the grounding conductor when larger conductors are used. This can make the use of cable type wiring methods not code compliant because of the reduced grounding conductor size.


(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.

NEC 2008 edition this is unchanged from the 2005 Ed.

This also supercedes the sizing rules in other sections.


I believe all this is saying is that if you make your hots bigger, you have to make your ground bigger as well. I'd assume this is so a situation like this doesn't develop:

A contractor has a LOT of 10 gauge wire laying around, so he wires up all the 110 outlets with 10 gauge. He happens to have green 14AWG so he runs that for the ground. 10 years later, a new owner wants to install some 220 outlets and notices that everything is run with 10AWG, so he simply swaps out the breakers. Now he has a 30 amp load, for example, that can potentially go through a 10AWG ground, and that just won't work.

re: sub panel sizing
The only reason NOT to use a 100A panel is cost, and the cost difference shouldn't be major for 10' of wiring. I'll never understand all the hand wringing trying to nail it just right when 100A is readily available for not much additional cost and will serve any need into the future. Heck, you may bring in a welder some day. Don't laugh. I wasn't into welding for many years....until I was. Things change.

[Sam Layton]

Jerome,

What John said... I have a 100 amp panel in my shop. I don't have to worry when everything kicks in at the same time, table saw, dust collector, air comp, AC, etc.

Like John said, you never know what is in the future. A couple of extra dollars spent now will ge a good investment.

Sam

[Jerome Hanby]

I don't have philosophical problem with running 100 amp, in fact I would prefer to. Just trying to find out minimum I need since I'm trying to scrounge up the feed wire. I want copper and only by the foot I've found for sale around here is aluminum...

[Dave MacArthur]

Also agree with John. You can buy a 100A panel at Lowes/HD for about the same as you can the smaller ones as they give their "deal" on this standard.

I would buy a MAIN PANEL with a 100A master CB (just be sure you un-bond the ground/neutral, that is make sure they're not connected). This gives you a "master cut off" for the shop much cheaper than any 100A sw you'd buy.

You can FEED the thing with a 60A breaker if desired from the main--when I wired my shop, I found that a 60A breaker was significantly cheaper than a 100A. And also, the inspector made me do a "whole house" analysis of the main panel feed vs. house use, to ensure I didn't exceed my main panel power supply. 100A feed to the garage shop exceeded the analysis, 70A was fine. So I used a 70A breaker.

[Justin Freund]

I was just looking at this yesterday for my own house. The cost is about double for 100A vs 60A here. I can use SE cable though so that cuts down on the cost of running individual conductors and conduit, but its still a healthy jump in price. I don't know if you've ever tried to run 2AWG versus 6AWG but there is quite a difference there too. At only 10 feet though, its really a matter of wanting to go to to 100 over 60 as the cost difference is not that much, especially when compared to my 75 or so feet.

Like was said before, add up anything that could possibly be on at any given time (table saw, compressor, DC, AC or heat, lights, chargers, and then any conveniences such as fridge or TV) then give yourself a say 30% margin of error to be really safe. You don't want to be right at your rating with your loads and you should have some room for extras.

[Don Jarvie]

I found it cheaper to buy the wire in 1 unit compared to buying it a 4 separate wires. The 6-3 at Lowes was 2.50 per ft. I can't remember the cost but buying 4 lines of THHN was more expensive.

It was a pain to pull it through the conduit.

If you have a friend who is an electrician you maybe able to run all the lines etc and then the elec can hook up the connections.

[Rick Christopherson]

Just a little note: Oversizing the wiring will kick in a rule that may make things sticky, 250.122 (B) will require a increase of the size of the grounding conductor when larger conductors are used. This can make the use of cable type wiring methods not code compliant because of the reduced grounding conductor size.


(B) Increased in Size.

Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according

to the circular mil area of the ungrounded conductors.

NEC 2008 edition this is unchanged from the 2005 Ed.

This also supercedes the sizing rules in other sections.
Knowing code isn't just a matter of memorizing passages, but the reasons behind them. It is the difference between a good inspector and one that has a bounty on his head.

The intent of this passage is that when conductors are up-sized for reason then the ground is increased accordingly. A reason would be voltage drop or temperature rise, not because there is a sale at walmart. If conductors are increased due to voltage drop, the ground is increased to ensure the circuit breaker will trip on a fault condition.

Furthermore, this applies only to the specific wire run, not the subsequent branch circuits.

For the original poster, the only reason to be concerned about this passage is to appease an anal retentive inspector.

[Rollie Meyers]

Knowing code isn't just a matter of memorizing passages, but the reasons behind them. It is the difference between a good inspector and one that has a bounty on his head.

The intent of this passage is that when conductors are up-sized for reason then the ground is increased accordingly. A reason would be voltage drop or temperature rise, not because there is a sale at walmart. If conductors are increased due to voltage drop, the ground is increased to ensure the circuit breaker will trip on a fault condition.

Furthermore, this applies only to the specific wire run, not the subsequent branch circuits.

For the original poster, the only reason to be concerned about this passage is to appease an anal retentive inspector.

The fact is that code section exists & it does not matter the reason why the conductors were increased in size, it supersedes other code sections on equipment grounding conductors if someone chooses to increase the ungrounded conductors.

In the 1999 NEC it was for adjustment for voltage drop, in the 2002 the section was changed to it's current wording ( 2005 & 2008).

If they are upsized for ANY reason the grounding conductor must also increased. BTW, if a metallic conduit is used for grounding there is no need to worry about 250.122 (B ) at all.