Question About Amperage

Setting up the freezers and coolers in the new store and have a question about my amp usage.

My panel has a 200 amp service.

The total of my coolers and freezer amps is 158, then the air conditioner that is built in about 20 amps, plus im accounting for an additional 10 amps for small things (registers, printer, computer, etc.)

Bringing my total amp usage to about 190.

However, do my appliances draw 158 amps all the time? Or is that what they draw when working at full capacity only?

If that is the case, i would think it would be almost impossible that all appliances would be running full capacity at any one time.

So in the opinion of the SDMB, do you think i can get away with a 200 amp service or should i contact the electric company about adding additional service.


You want to leave some overhead in your breaker box, so you don’t want to load it to more than 80 percent of its rated max. In other words, even though it is rated for 200 amps, you want the max to work out to 160 amps.

While your load will be typically a lot less than the max you’ve calculated, if you have a power outage, then when the power is restored, all of your freezers and coolers will probably all come on at once. So it’s not that impossible.

Are those 240 volt freezers or 120 volt freezers? If they are 120 and you’ve divided the load between the two phases, you’ll draw half of the current that you’ve calculated on each phase. If they are all 240 volt then you’re about 30 amps shy of what you need from your breaker box.

And as a quick question, does lighting run through this same panel?

Everything is 120 volt, except possibly the compressors for the walk-in cooler, which is 208/230 1 phase

Yes, all of the lights are low voltage fluorescent lights. Amperage i do not know specifically, but possibly would consume that other 10 amps bringing total to 200

While your load will be typically a lot less than the max you’ve calculated, if you have a power outage, then when the power is restored, all of your freezers and coolers will probably all come on at once. So it’s not that impossible.

The motors are rated at full load amps, but when a motor first starts, it pulls “inrush current” for a short time which is much higher, and with all your motors trying to start at the same time…

You need to talk to a professional.

I might be confusing you a bit with the terminology here, since what we’re really talking about is a split center tapped transformer coming off of a single phase. But people commonly refer to the two lines coming in off of the transformer as two phases (you might also be getting two phases out of a three-phase system). But basically for 200 amp service, what that usually means is you’ve got 200 amps on line 1 and 200 amps on line 2.

So your 208/240 cooler gets its amps added to both lines/phases. The other freezers, fridges, registers, computers, etc. get added to whichever line/phase they are attached to.

I think you’re probably a bit low on the amperage for the lighting (either that or it’s a pretty small store) but I figure you’re probably closer to 120 or 130 amps or so instead of the 200 you’ve calculated by adding everything together as if they were all loading both lines/phases.

That 200 amp panel should be 200 amps at 240, or 200 amps per each 120-volt leg of your service. Your main breaker should actually be two 200 amp breakers with their handles bracketed together.

That’s 400 amps worth of 120, so your combined load of 158 amps @ 120 will be fine.

That explains why i was able to run two of my 12 amp coolers on the same circuit no problem. Thought i getting lucky running 24 amps on a 20 amp circuit, but i was only using 12 amp :smack:

Uh, no, that sounds like you’ve overloaded your circuit.

Let’s say you’ve got 5 freezers, A, B, C, D, and E. Freezer A is 240 volts at 20 amps. Freezers B through E are all 120 volts at 15 amps (just some made up numbers). L1 and L2 are the two lines coming into the breaker box. The voltage from L1 to neutral is 120 volts and the voltage from L2 to neutral is 120 volts, and L1 to L2 is 240 volts (your basic split phase service).

A is on its own breaker.
B and C are on the same circuit, which is connected to L1.
D is on a separate circuit on L2.
E is on a separate circuit on L1.

All circuits are 20 amps.

The total load on L1 is A+B+C+E or 20+15+15+15 = 65 amps.
The total load on L2 is A+D = 20+15 = 35 amps.

(Side note - This isn’t a very well balanced load. Moving either B or C to L2 would balance it much better, but I’m doing it this way for a reason)

So on your 240 volt breaker box, the load is 65 amps (the higher of the two individual lines).

But - B and C are overloading their circuit, because they have 30 amps on a 20 amp circuit. You don’t divide their amperage by 2 in this case because you’re talking the amperage of that specific circuit.

The way you were doing it was adding A through E together, which would give you 20+15+15+15+15+15 or 95 amps. Since the 120 volt loads are split between the two lines, it ends up being a lot less than that.

These are imaginary numbers of course, designed to illustrate the point. Do you see what you did wrong and what you are still doing wrong?

Short answer…

Basically if your main circuit breaker never trips, then don’t worry about it!

(Only what is on at the same time counts so far as main breakers are concerned.)

Bad answer.
He’s trying to avoid finding out the hard way if his main breaker will possibly trip.

Except when something does go wrong … and the insurance company says you overloaded the entry service … [giggle] … and they’ll not pay a dime on your claim.

It is very rare that a main breaker trips. If it works, don’t fix it!

You still don’t get it.
He’s trying to avoid losing thousands of dollars worth of frozen goods.
You don’t just say “well, let’s try it and see.”

We had an old house, and just to have normal stuff running, we had the box on max. One day we noticed the main breaker was arcing. We immediately shut off everything, and replaced the main breaker (I was a power generator mech/maintenance in the USARNG, and DH is the son of a general contractor who spent summers from age 15 to 21 helping his dad). It was really old, and not big enough for the breaker box. We put in a new, bigger one, and then upgraded the breaker box. No more problems. But it’s a good thing we saw that before it set the house on fire.

Ok! Thanks for the lengthy example, it’s clear to me now!

Silly question perhaps but how do you know if your main breaker was undersized?

Well if he must, I would first recommend having an electrician use a clamp amp meter to measure the amperage load. That should not cost too much.

And I would not recommend doing this yourself as there is always live electricity present in a breaker panel - even with the main breaker off.