Determining "Capacity" of an existing electrical panel

[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.../dp/B000EM7CDW


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.

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".
Amprobe.jpg

[Julie Moriarty]

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

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.... )

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.