Determining "Capacity" of an existing electrical panel

[Marty Tippin]

I have a 100 amp sub-panel in my workshop and am trying to determine how much "capacity" remains for adding more circuits to the mix.

Is this a mathematical calculation based on the data plates of the various machines and appliances installed and/or the amp ratings of the existing circuits, or can I empirically measure the power used today in a "worst-case" scenario using my clamp-on ammeter?

If I use my ammeter, how do I accurately determine the total current draw? I can measure the amps on each leg of the incoming feed, but I'm not sure what to do with those numbers.

Other than a single 20A 220V circuit that feeds all of my 220V machines (I only run one at a time, ever), I have a 30A 220V circuit for the heat pump, 20A 120V for the 5HP compressor and a 15A 120V circuit that for the lights (9 x 4-light 4' T8 flourescent). I also have 2 20A 120V circuits for general use.

I'm looking at adding a 40A 220V circuit for a Grizzly 3HP cyclone (circuit size is spec'd by Grizzly, full-load amps is 22A).

[Lee Schierer]

Basically if you have space add breakers you have not exceeded the capacity. The fact that your breaker values add up to more than 100 amps isn't relevant. The capacity of the box is 100 amps. The breakers are to protect individual circuits, not to permit you to run all the circuits at or very near to their breaker capacity. Your main breaker or fuse will trip/fail if you exceed 100 amps. If any combination of what you plan to run at any one time exceeds 100 amps you need a larger capacity panel.

[Marty Tippin]

Your main breaker or fuse will trip/fail if you exceed 100 amps. If any combination of what you plan to run at any one time exceeds 100 amps you need a larger capacity panel.

That's pretty much what I understand, but I guess my confusion is in knowing "how many" amps I'm currently using to know whether I have the capacity to put more load on the circuit. (It does me no good to add the dust collector and find out after the fact that I don't have enough capacity)...

If I turn all the machines that I'm likely to use simultanously on and measure (for example 20 amps on one leg and 27 amps on the other leg of the incoming feed, what does that tell me? Am I at 27 of 100 amps capacity or 47 amps of 100 amps capacity? Or is it not that straightforward?

[Mike Heidrick]

You are using 27amps. A 100amp single phase panel is 100amps each hot leg.

[Dan Friedrichs]

If I turn all the machines that I'm likely to use simultanously on and measure (for example 20 amps on one leg and 27 amps on the other leg of the incoming feed, what does that tell me? Am I at 27 of 100 amps capacity or 47 amps of 100 amps capacity? Or is it not that straightforward?

It is that straightforward - you can, in fact, use a clamp on meter and measure the current flowing on each leg (hot1, hot2, and neutral). If either hot leg exceeds 100A, it will trip the main breaker (so, it's the larger of the two numbers, not the sum, that you're concerned with - just like with a "standard" 220V breaker, exceeding the rating on either leg will trip the breaker).

That said, no, you CAN'T just turn on all the machines and do this test, because the power drawn by some equipment will vary (considerably) under load. A table saw, for instance, will draw a lot of current on start-up, but if it's just left running, it only "needs" enough power to overcome things like friction losses (etc), so it will draw very little power.

Realize, though, that the breaker rating is likely MUCH higher than the average draw of these circuits. Based on this list of loads, you have plenty of capacity to add the dust collector.

[Julie Moriarty]

Other than a single 20A 220V circuit that feeds all of my 220V machines (I only run one at a time, ever), I have a 30A 220V circuit for the heat pump, 20A 120V for the 5HP compressor and a 15A 120V circuit that for the lights (9 x 4-light 4' T8 flourescent). I also have 2 20A 120V circuits for general use.

I'm looking at adding a 40A 220V circuit for a Grizzly 3HP cyclone (circuit size is spec'd by Grizzly, full-load amps is 22A).

Marty, you have a lot of stuff on a 100A panel after adding the cyclone.

While I am rarely concerned with the total load on a panel (existing and planned branch loads don't usually cause that concern), I would be curious what the startup draw of that cyclone would be. You have loads that turn on automatically (heat pump & compressor) and adding that cyclone to the mix would make me proceed with caution.

Lights are on. TS is on. DC is on. Heat pump kicks in. Compressor kicks in. That can be quite a load on that 100A panel. I would need to know more before proceeding.

[Charles Lent]

+1 for what Julie posted.

Charley

[Ole Anderson]

Start up loads are for such a short period of time, less than one or two seconds, they are inconsequential in panel sizing IMO. My 2 hp Oneida cyclone draws 10 amps on each leg under load, (2 pole 20 amp breaker) but sucks 70 amps momentarily on startup. Breakers are designed to deal with those start up loads. That includes the main breaker.

[Marty Tippin]

Marty, you have a lot of stuff on a 100A panel after adding the cyclone.

While I am rarely concerned with the total load on a panel (existing and planned branch loads don't usually cause that concern), I would be curious what the startup draw of that cyclone would be. You have loads that turn on automatically (heat pump & compressor) and adding that cyclone to the mix would make me proceed with caution.

Lights are on. TS is on. DC is on. Heat pump kicks in. Compressor kicks in. That can be quite a load on that 100A panel. I would need to know more before proceeding.

Maybe I should clarify that the heat pump is dual-fuel, so there are no resistive elements for supplemental heat; it switches to gas when necessary.

With just the lights and heat pump running last night, I measured about 26 amps. Of course, I have no idea what the start-up current draw is for any of the machines or whether it's a problem. I also don't know if the current draw from the heat pump changes in relation to the outside temperature (or, when it's running as the A/C, whether it draws a different current - I suspect not, but no idea).

My other worry is what's happening back at the main panel in the house - 200A service, but it's a big house with all-electric appliances and a heat pump. I haven't tried to measure the current draw at the main panel yet with everything running in the house, so no idea where I'm at there either.

So what are the consequences of "pushing the envelope" - my guess is I'd see lights dimming as big current drawing machines turn on and in the worst case tripping the main breaker.

I could consider having new service run to the workshop, though it wouldn't be cheap - it's about 200 ft to the transformer box and I'd have to bore under a 40 ft wide driveway and carefully miss the buried propane lines...

[Dan Friedrichs]

So what are the consequences of "pushing the envelope" - my guess is I'd see lights dimming as big current drawing machines turn on and in the worst case tripping the main breaker.

Lights would only dim if the wire gauge was thinner than ideal and the runs were long. If you push the envelope, the only thing you're "risking" is nuisance tripping the main breaker.

I still contend that you're being worried about a non-issue. What are your options, here? You could: (1) Not use dust collection - a very bad idea, (2) Buy a much smaller (~1HP?) dust collector - which would be mostly ineffectual, or (3) Buy the DC you want and deal with the breaker issue, later, should it arise at all.

Even if you have issues with tripping the main (which, again, I don't think you will), there are ways to avoid this (short of running new service) - you could make sure the air compressor is off before turning the DC on, etc.

Again, though, you don't need to worry about this. You said you're concerned about the 200A main service - a quick back-of-the-napkin "sanity test" would be to figure how much you're paying for electricity each month and back-calculate how much current you draw, on average. If you were drawing an average of 50A, you'd have a $1,000/mo electric bill. I assume you're nowhere near that figure. Obviously this doesn't take into account peak vs. average, but my point is that I think you're not even in the right ballpark of concern. Buy the DC and enjoy it.

[John K Jordan]

>though it wouldn't be cheap - it's about 200 ft to the transformer box

I did that for my shop. I didn't install a new service from the utility company with a second meter (you get charged more for the electricity around here) but added a new meter panel box next to the transformer with two big breakers there, one 200 amp to feed the house and a 100 amp to feed the shop. (My transformer is in the woods near the house, 7200 volts 600 ft underground from the pole)

I sized my conductors for acceptable voltage drop at worse case and installed larger conductors than many would - #1awg copper for the power in 250' of conduit. I have a lot of things that suck power including 5hp motors and a heat pump with resistive heating elements and have not seen an issue. One thing I would recommend if you dig a new trench: put in an extra conduit! I laid an extra 2.5" conduit in the trench with nothing but a rope inside for future use. I also ran cat-5 cable in conduit for shop WiFi and femtocell tower for reliable cell service under the metal roof. Life is good.

JKJ

[Curt Harms]

Do you require a permit & inspection? If so, talking to the inspector seems like a good early step.

[Julie Moriarty]

Maybe I should clarify that the heat pump is dual-fuel, so there are no resistive elements for supplemental heat; it switches to gas when necessary.

With just the lights and heat pump running last night, I measured about 26 amps. Of course, I have no idea what the start-up current draw is for any of the machines or whether it's a problem. I also don't know if the current draw from the heat pump changes in relation to the outside temperature (or, when it's running as the A/C, whether it draws a different current - I suspect not, but no idea).

My other worry is what's happening back at the main panel in the house - 200A service, but it's a big house with all-electric appliances and a heat pump. I haven't tried to measure the current draw at the main panel yet with everything running in the house, so no idea where I'm at there either.

So what are the consequences of "pushing the envelope" - my guess is I'd see lights dimming as big current drawing machines turn on and in the worst case tripping the main breaker.

I could consider having new service run to the workshop, though it wouldn't be cheap - it's about 200 ft to the transformer box and I'd have to bore under a 40 ft wide driveway and carefully miss the buried propane lines...

With just the lights and heat pump on, you measured 26 amps. That's a lot for a "no-load" workshop with a 100A panel. (By "no-load" I mean none of the tools are running.) If your heat pump was putting out heat, there has to be resistive loads to generate that size draw, unless you have some other large draw in it and/or the the heat pump is beginning to fail. Lights in a home workshop could only account for a few amps.

Your DC will be running any time a dust-creating machine is running. Grizzly recommends it be placed on a 40A, 2 pole breaker. That's the size of an electric oven, one of the highest drawing loads in the average home. Most homes today have 200A services. You already have a big draw of 26 amps on a 100A sub panel and you haven't started working yet. If you will be hooking the DC up to an auto-start tied to other equipment, that could be a substantial start-up draw. Yes, it could be pushing the envelope.

What could happen? Lights could dim in both the shop and your house. You could trip breakers. You could hear an electric buzz when a large load kicks in. Maybe you can live with that. But you are taxing your electrical system and everything tied to it. Every time you do so, something could be deteriorating. Things can begin to break down and you wouldn't know it. A big surge out of your shop could tax another load calling for power and cause it to work harder. I've seen large draws bypass smaller branch breakers and trip larger ones.

People who don't understand what overloading an electrical system can do might consider these, at worst, minor inconveniences. Experienced electricians consider them problems that need to be corrected. If a breaker trips more than once, there's a problem. I've seen a lot of strange things happen in improperly designed electrical systems. That's why I want to know everything I can.

Again, I don't have enough information to tell you you need to install a larger panel or a separate service. I don't know the actual loads of the equipment. I don't know the start-up draw of your equipment and I don't know how you work or what you know about what to look for when your system is showing signs of being overloaded. So I can't say you are good to go or you are going to have problems. But what you have provided is sufficient for me to need investigate the situation further before throwing in a 40A, 240V machine that will probably be running almost as often as the lights.

Marty, I am not trying to be an alarmist. I am only advising you to get more information. At the very least, amp the start-up loads and the operational loads of everything in your shop and write them down. Amp the line loads on your house panel during those times you typically work in the shop and add them to your list. Run through different scenarios that mimic how you work - lights on, heat pump running, you turn on the jointer and the DC kicks on. What are the loads as machines start up? What are the operational loads? What happens if the compressor kicks in? Is your wife in the kitchen with an electric cooktop and oven running? What if the house A/C kicks on? Imagine different real-life scenarios. If you see yourself passing a continuous 80% of the rated loads of the breakers, you are pushing the envelope.

Sure, you can throw in that DC and see what happens. But you may not know all that happens if some of it comes in the form of shortening the life of electrical machines, appliances and other devices. I have seen enough in my career that I need to know as much as possible about what's happening in a system. Never just to keep adding to it until something fails, unless you like rolling the dice.

[Brian Hood]

After 20 years in the electrical trades and now a consultant, I would go with Dan. You are most likely fine.
About inrush current - when starting a motor this is not normally seen by the circuit breakers bi-metallic thermal strip, which bends as it heats up and trips the breaker, kinda like a mouse trap. In fact there is an added element, the QO device, which trips the breaker based on the magnetic field generated by a spike in the current flow and that is the element that will trip the breaker and it is designed to allow considerable inrush (starting) current. This trip curve is published data. Circuit breakers without the QO are legal and easy to get because of loads with high inrush that will trip a QO equiped breaker 100% of the time. Streetlights for example have capaciters that, when you load up a long string of them, require huge amounts of current (a capacitor is a sort of bucket for holding electrons and filling up a lot of those buckets can be a problem).
With respect to determining if you have an issue, yes, one way would be to look at an incandescent light bulb to see if it dims, better to put a volt meter on the circuit that drives your table saw and test the voltage. Make sure you have lots of other things turned on and that you have load on the saw blade. Typically and I think by NEC you want 5% drop or less.
I think folks should be more aware of the branch circuit wiring. I'm running 12 amps through #10 wire to my table saw because it has hard to replace expensive Danish motors (3 of them) and want them to have all the electrons they can take.
Brian(J)

[Marty Tippin]

With just the lights and heat pump on, you measured 26 amps. That's a lot for a "no-load" workshop with a 100A panel. (By "no-load" I mean none of the tools are running.) If your heat pump was putting out heat, there has to be resistive loads to generate that size draw, unless you have some other large draw in it and/or the the heat pump is beginning to fail. Lights in a home workshop could only account for a few amps.

Lights alone pull around 10 amps - 9 fixtures, 4 x 4' T8 bulbs per fixture. The heat pump compressor is 3 ton (a little big for a 40x40 shop but it's what I could find...) There are no resistive heating elements, just the compressor and the blower motor.


Your DC will be running any time a dust-creating machine is running. Grizzly recommends it be placed on a 40A, 2 pole breaker. That's the size of an electric oven, one of the highest drawing loads in the average home.

Resistive loads like an oven are a bit different than a motor, aren't they? Amps is amps, I know, but the motor on average won't draw near it's "full load" rating while the elements in an oven are at full-bore all the time.

And as has been discussed, the full-load rating of this DC is "only" 22 amps; Grizzly apparently specified a 30A breaker some time ago for the same DC but then upped their spec to 40A - the speculation is that they got too many complaints of tripping on startup.


Marty, I am not trying to be an alarmist. I am only advising you to get more information. At the very least, amp the start-up loads and the operational loads of everything in your shop and write them down. Amp the line loads on your house panel during those times you typically work in the shop and add them to your list. Run through different scenarios that mimic how you work - lights on, heat pump running, you turn on the jointer and the DC kicks on. What are the loads as machines start up? What are the operational loads? What happens if the compressor kicks in? Is your wife in the kitchen with an electric cooktop and oven running? What if the house A/C kicks on? Imagine different real-life scenarios. If you see yourself passing a continuous 80% of the rated loads of the breakers, you are pushing the envelope.

Good advice. I'll certainly try to gather some more information. Not sure how to measure the startup loads on the various machines though unless my cheap clamp-on ammeter will pick up the spike during the first second or two after power on.

[Julie Moriarty]

Lights alone pull around 10 amps - 9 fixtures, 4 x 4' T8 bulbs per fixture. The heat pump compressor is 3 ton (a little big for a 40x40 shop but it's what I could find...) There are no resistive heating elements, just the compressor and the blower motor.

Resistive loads like an oven are a bit different than a motor, aren't they? Amps is amps, I know, but the motor on average won't draw near it's "full load" rating while the elements in an oven are at full-bore all the time.

And as has been discussed, the full-load rating of this DC is "only" 22 amps; Grizzly apparently specified a 30A breaker some time ago for the same DC but then upped their spec to 40A - the speculation is that they got too many complaints of tripping on startup.

Good advice. I'll certainly try to gather some more information. Not sure how to measure the startup loads on the various machines though unless my cheap clamp-on ammeter will pick up the spike during the first second or two after power on.

Thanks for the additional info. I always err on the safe side when giving advice online. There are too many unknowns when you can't be there to test, measure and assess in person.

There are a lot of little signs I picked up over the years that I could never teach through words. But what I do know is a well designed electrical system rarely fails and rarely causes other electrical loads to break down. I've seen a perfectly good appliance fail because it was "deprived" by another load. The homeowner was not happy with their former electrician.

As for your ammeter being able to pick up the start-up spike, you might be able to see it if you have someone watching it while someone else turns the machine on. It may not be perfectly accurate but maybe you can use whatever it does as a ballpark. The objective is to at least have a reasonable idea what is happening and when and why. That way, if you are ripping some 12/4 hard maple and something goes wrong, you won't just reset the breaker or shrug your shoulders when the neighbors storm your house with torches for causing the power grid to go down. ;)

[Mike Heidrick]

Julie or other members in the know, can you recommend a decent priced ( not crazy fluke) clamp meter for me/us?

[Julie Moriarty]

Lights alone pull around 10 amps - 9 fixtures, 4 x 4' T8 bulbs per fixture. The heat pump compressor is 3 ton (a little big for a 40x40 shop but it's what I could find...) There are no resistive heating elements, just the compressor and the blower motor.

Resistive loads like an oven are a bit different than a motor, aren't they? Amps is amps, I know, but the motor on average won't draw near it's "full load" rating while the elements in an oven are at full-bore all the time.

And as has been discussed, the full-load rating of this DC is "only" 22 amps; Grizzly apparently specified a 30A breaker some time ago for the same DC but then upped their spec to 40A - the speculation is that they got too many complaints of tripping on startup.

Good advice. I'll certainly try to gather some more information. Not sure how to measure the startup loads on the various machines though unless my cheap clamp-on ammeter will pick up the spike during the first second or two after power on.

Sorry, Marty, I didn't answer all your questions.

Any load has resistance. Even wire has resistance. That is why very long wire runs require you to factor in what is called voltage drop.

Often, "resistive" loads are considered loads from heating elements. But in reality, every load has resistance. Look at it like this, a heating element that is under load draws a certain amount of power. Power (Watts) = Amps x Voltage. A motor running under a continuously steady load will draw a fairly consistent amount of power. But apply an opposing force to that motor, like when feeding a board into your TS blade, and the load increases. The only way a heating element load increases is if the element is failing or there is a call for more heat and there is some controller that increases the power to the element.

Did I explain that well?

[Julie Moriarty]

Julie or other members in the know, can you recommend a decent priced ( not crazy fluke) clamp meter for me/us?

All my electrical testing tools are Fluke. I went the cheap route early in my career and it was a waste of money. I have Fluke instruments over 25 years old and they work perfectly fine. Sorry I can't suggest something less costly. You can look at it this way, it only hurts once, then you use it for life.

Go to Mike Holt's website. Maybe someone there can help.

[Dan Friedrichs]

As for your ammeter being able to pick up the start-up spike, you might be able to see it if you have someone watching it while someone else turns the machine on.

You don't need to measure the start-up transient, though, because (as mentioned upthread) breakers are often designed to tolerate short transients without nuisance tripping. If you wanted to measure the peak (operational) draw, you could enlist a helper to monitor the meter while you loaded a machine to the hardest you usually do (like ripping 12/4 hard maple on a table saw).

Measuring 60Hz AC is fairly trivial. Even a Harbor Freight clamp-on meter should be reasonably accurate.

[Ole Anderson]

A good, middle of the road meter, and widely used in the industry is the Amprobe line, I have the compact ACD-14 Plus version and it serves me well. My only disappointment is that it does not read DC amps, which would be useful in automotive work.

http://www.amazon.com/Amprobe-ACD-14-PLUS-Multimeter-Temperature/dp/B000EM7CDW

https://www.instrumart.com/assets/acd-14plus-500.jpg
And here is a quick video of my 2 hp Oneida at startup using a very cheap direct-from-China digital gauge (which reads FLA within 3% of the Amprobe):
https://www.youtube.com/watch?v=XGIeq_kDGaI

[Brian Hood]

I have a 100 amp sub-panel in my workshop and am trying to determine how much "capacity" remains for adding more circuits to the mix.

(1) Is this a mathematical calculation based on the data plates of the various machines and appliances installed and/or the amp ratings of the existing circuits, or can I empirically measure the power used today in a "worst-case" scenario using my clamp-on ammeter?

(2) If I use my ammeter, how do I accurately determine the total current draw? I can measure the amps on each leg of the incoming feed, but I'm not sure what to do with those numbers.

Other than a single 20A 220V circuit that feeds all of my 220V machines (I only run one at a time, ever), I have a 30A 220V circuit for the heat pump, 20A 120V for the 5HP compressor and a 15A 120V circuit that for the lights (9 x 4-light 4' T8 flourescent). I also have 2 20A 120V circuits for general use.

I'm looking at adding a 40A 220V circuit for a Grizzly 3HP cyclone (circuit size is spec'd by Grizzly, full-load amps is 22A).

Hi Marty,
I'm requoting your OP above, I realized that you asked the right questions and never recieved exact answers.
(1) Yes, an electrical engineer or professional electrician would begin by calculating the required feeder size using NEC rules. The process is, make a list of nameplate amps and volts. Convert to KW. Add 180 VA (watts) per receptacle. Determine the sq. ft. of the shop, there is an adder for lighting I could look up. Then apply the calculation rules.

(2) To start with an ampmeter may confuse the issue without providing relavent information. These are generally used for troubeshooting and not for feeder or load calculation.

An important question, what size is the existing feeder? Like most people, you appear to be looking at the amp number of the circuit breaker, but feeder size is determined by conductor size not fuse or CB size. Do you have #4 copper? Or #2 aluminum? Because that is the most important factor in your situation.

Side note, I like and own really good tools, Festool, Fein, and Fluke among them. But my $14 clip-on amp meter reads the exact same as my $190 Fluke.

[Julie Moriarty]

A good, middle of the road meter, and widely used in the industry is the Amprobe line, I have the compact ACD-14 Plus version and it serves me well. My only disappointment is that it does not read DC amps, which would be useful in automotive work.

Ole, when anyone on the jobsite is looking for an amp reading meter they always ask for an amp probe. Yet I don't ever recall seeing that brand on the jobsite. Doesn't mean they aren't though. Back in the 70's, I started hearing some electricians taking a stand against products made in China and telling others on the job how to identify them and not to buy them. Maybe Amprobe went the China route and fell out of favor. But Fluke isn't seen all that much on the jobsites either.

This is like the one I have but mine is probably an older version. I can't tell you because it's locked in the vault.
http://i.ebayimg.com/images/g/TCAAAOSwrklVUmBL/s-l400.jpg
I bought mine for $200 about 20 years ago, and I got a deal (or so I was told). This one is selling new on eBay today for $126. Oh, to have the Internet back then!

[Art Mann]

For someone who is only going to use a meter a few times and precision isn't critical, I recommend the more expensive meter sold at Harbor Freight. Before anyone starts sneering, I will say that I have been an electrical engineer for 30 years and have had access to the most expensive high precision instrumentation available and yet the Harbor Freight unit is the only one I own for home use. If I need real accuracy, a clamp on meter that costs $300 or less isn't as good as a nice (and less expensive) shunt based digital ammeter.

[Wade Lippman]

My meter was $10 20 years ago, but it catches startup currents quite well.

You might be pushing the 200a more than the 100a sub. You have added quite a load to an all electric house.
Presumably you won't be running the 5hp compressor at the same time as your shop tools.
I would try turning everything on; heat pumps, stove, water heater, compressor, and seeing what the loads are at the panel and subpanel. If you are significantly short of the limits, than you should be fine. As someone said, breakers are designed to handle startup, unless they are near capacity already.

[David L Morse]

Ole, when anyone on the jobsite is looking for an amp reading meter they always ask for an amp probe. Yet I don't ever recall seeing that brand on the jobsite.

This was used by a contractor in the 60s and 70s. On the back it says "Made in the USA".
326975

[Julie Moriarty]

This was used by a contractor in the 60s and 70s. On the back it says "Made in the USA".
326975

Oh yeah, that looks familiar! Takes me back to when I was an apprentice. :) Thanks, David.

I was on a job in the mid-80s where we were having power quality problems and equipment wasn't running properly. It was reeking havoc and burned a few pieces of equipment out. I used the standard testing tools but they only tell what's going on now, and unless I sat there continually taking readings, the problem was a needle in a haystack. The contractor was an EE and rented a Drantez. It constantly monitored power quality and we finally identified the problem, but it took about a week.

[John C Bush]

Hi Marty,
I have a 125 amp feed to my shop sub panel~200ft from the house 250 amp. main panel. DC is 2hp, comp. is 5hp, TS, BS, jointer, planer, sanders are all 220, I frequently run several machines at a time and have never had any difficulties with power deficiency based on --everything still works--. I expressed the same concerns as you when planning with my electrian and he assured me there was plenty of capacity for my needs. I was most concerned about my welders and haven't had any issues with them either. Hope that helps.

[Bill Space]

Hi,

I think there's a lot of "over think" going on in this thread. All Good information though.

Let's look at the question from a different perspective…

100 A at 240 V is 24 kW. 24 kW is 32 hp. Naturally this is into a resistive load, which an induction motor is not. Induction motors are the motors we normally see running our equipment in the home workshop.

With induction motors there is a phase shift between applied voltage and current, so the actual HP produced by an ampere of current is less than the theoretical calculation for a resistive load. A conservative estimate of power available from 100 amp 240 V subpanel (feeding induction motors) may be about 80% of the theoretical maximum. So 32 hp reduces 25 hp.

So looking at connected horsepower, which may be in use at the same time, can give you a good idea of how close you are to maxing out your subpanel power availability.

I have a lot of tools, and a 100 amp subpanel in my shop, and don't fear the lack of power availability at all at all. I only have two hands and can only run one machine at a time, plus my dust collector, and the furnace to heat the shop, and the lights. Nowhere near 25 hp in total. If you have 220 V rather than 240 V, decrease the worst case power availability by 10% or so. But even 22 hp is a lot…

I think you will be fine unless you have several other people in the shop running multiple machines at the same time. That is not something that happens in my shop…

Bill

[Marty Tippin]

(Just in case there hasn't been too much over-thinking already, I'll add the following.... :D)

I took some time this weekend to measure current draw at the 200A main service in the house (from which the 100A sub-panel in my shop is fed).

Inside the house:
* Heat Pump compressor - 12A
* Range (worst case - burners + oven) - 35A
* Dryer - 25A
* Water Heater - 18A

That adds up to 90A and doesn't include current for the typical light and miscellaneous circuits. Call it 110A on the worst day (i.e., the kids are home and have left every light in the house turned on...) And no, it doesn't account for start-up current on any of the devices.

In the workshop:
* Lights - 9A (on 120V - so that's 4.5A 240V?)
* Air Compressor - 15A
* Heat Pump Compressor - about 15A
* Tools (I can only run one at a time, as I only have a single 20A 240V circuit for all of them) - worst case, 15A based on machine labels -- or "worst worst" case, 20A until the breaker pops.
* Dust Collector - estimated 22A (that's the "full load" amperage given on the label)

That all adds up to about 70A.

So it looks like, in the very worst case (everything in the house running and everything in the shop running), I'm at something around 175A, which is about 88% of rated capacity for the 200A circuit. But, that's only in the very worst case which would only last, at most, a few minutes -- the air compressor will only run for about 60 seconds; the range drops off to about 20A once the oven is fully heated, etc. etc.

Bottom line: I'm wiring up the 3HP dust collector and going on about my business. If I end up tripping a main breaker at some point, I'll revisit and consider a dedicated service for the shop.

Certainly, if I ever plan to add any bigger machines to the shop, I'll probably need to put in a bigger panel. And I've been eyeing a 5HP dual-drum sander and a 3HP SawStop quite a bit lately...

Thanks to everyone who contributed - lots of good input all around and I definitely learned a few things.