- Thread starter Eddy Current
- Start date

Not very if it only has three, is yellow and owned by Del Boy TrotterIMO, the question in analogous to asking, "Can you tell how fast a car can go by how many wheels it has?"

(Google "Only Fools and Horses")

Over here, it is common to buy the unit for residential applications complete with the main breaker fitted (often 100A) and a number of free ways to fit a number of downstream breakers up to the capacity of the board. It may have slots for say, 12 ways but not all have to be used.The rating of the main breaker is determined by various factors including panelboard nameplate rating and/or tap rules.

The load rating in amps of the panel is determined by things like bus size, not breaker count or breaker size.

The actual load can be determined by amp clamp meter on the feeders. If you wanted to get theoretical and had time on your hands, you could validate Kirchoff's current law by measuring the load on each branch circuit and adding the (same leg) currents together.

Yes, it is called the Nostradamus formula. It can be found by utilizing the fourth quatrain form the 1762 letters divided by the years passed since then and muliplying it with your daughters birthday. If you have no daughters, you will be unable to preform this calculation.:lol:Measure with a meter, but is there a formula to find the overall amperage?

- Location
- Austin, TX, USA

Use an Ammeter

If you have an "Ideal", "Fluke", "Commercial Electric", or "Klein" brand Multimeter with an Amp Clamp on it you can clamp each phase (feeder) on the panel under a standard load and flip the dial to the Ammeter setting and it will give you a pretty accurate reading of how many amps are being pulled through that phase. Do this on each phase of the panel and add it up to get the total Amps being pulled. This is the easy way.

There are calculations in the NEC that tell you how much to account for each circuit and motor load when building a service and preparing to wire a house. These calculations tell you what percentage to calculate for the entire house and this is how we electricians figure out what size service needs to feed the home. This is the hard way.

What do you need to figure this out for?

- Location
- NE Nebraska

With the procedure you described it is possible to measure a fully loaded 100 amp three phase panel and come to the conclusion it is drawing 300 amps. Does this sound correct to you?If you have an "Ideal", "Fluke", "Commercial Electric", or "Klein" brand Multimeter with an Amp Clamp on it you can clamp each phase (feeder) on the panel under a standard load and flip the dial to the Ammeter setting and it will give you a pretty accurate reading of how many amps are being pulled through that phase. Do this on each phase of the panel and add it up to get the total Amps being pulled. This is the easy way.

There are calculations in the NEC that tell you how much to account for each circuit and motor load when building a service and preparing to wire a house. These calculations tell you what percentage to calculate for the entire house and this is how we electricians figure out what size service needs to feed the home. This is the hard way.

What do you need to figure this out for?

- Location
- Austin, TX, USA

Not necessarily. You have to amp clamp each leg and see what amperage is being used. Just because you have a tankless water heater on a 20amp breaker, doesn't mean its drawing 20amps of electricity. Its more accurately pulling around 2amps. So when you amp clamp each feeder, depending on the loads, you'll most likely get something random like 21amps on black, 17 amps on red, and 27 amps on blue. Most panels should have pretty evenly loaded phases, and if they don't then you should think about moving some things around to even out the loads.With the procedure you described it is possible to measure a fully loaded 100 amp three phase panel and come to the conclusion it is drawing 300 amps. Does this sound correct to you?

Also, if you have a 100amp 3 phase panel...that means that the panel is rated for 100amps TOTAL, over all 3 phases. If you were drawing 300amps on a 100amp panel, you'd have huge and dangerous problems lol. The wire for a 100amp panel would be way too small to maintain 300amps for continuous use. You can't just add up the breaker sizes and say that is what is being used, because that's never going to be the case.

Does this all make sense?

- Location
- Fort Collins, Colorado

- Occupation
- Electrician, Contractor, Inspector

kwired,Not necessarily. You have to amp clamp each leg and see what amperage is being used. Just because you have a tankless water heater on a 20amp breaker, doesn't mean its drawing 20amps of electricity. Its more accurately pulling around 2amps. So when you amp clamp each feeder, depending on the loads, you'll most likely get something random like 21amps on black, 17 amps on red, and 27 amps on blue. Most panels should have pretty evenly loaded phases, and if they don't then you should think about moving some things around to even out the loads.

Also, if you have a 100amp 3 phase panel...that means that the panel is rated for 100amps TOTAL, over all 3 phases. If you were drawing 300amps on a 100amp panel, you'd have huge and dangerous problems lol. The wire for a 100amp panel would be way too small to maintain 300amps for continuous use. You can't just add up the breaker sizes and say that is what is being used, because that's never going to be the case.

Does this all make sense?

you better go back and rethink all you thought you knew.

- Location
- NE Nebraska

I guess so.kwired,

you better go back and rethink all you thought you knew.

- Location
- NE Nebraska

If you have an "Ideal", "Fluke", "Commercial Electric", or "Klein" brand Multimeter with an Amp Clamp on it you can clamp each phase (feeder) on the panel under a standard load and flip the dial to the Ammeter setting and it will give you a pretty accurate reading of how many amps are being pulled through that phase. Do this on each phase of the panel and add it up to get the total Amps being pulled. This is the easy way.

There are calculations in the NEC that tell you how much to account for each circuit and motor load when building a service and preparing to wire a house. These calculations tell you what percentage to calculate for the entire house and this is how we electricians figure out what size service needs to feed the home. This is the hard way.

What do you need to figure this out for?

Your second post makes sense, (except maybe the 2 amp load on a tankless WH unless it is only 480 watts single phase or 830 3 phase - pretty small WH) but it contradicts what you said in the first one. What was said in the first one is what I commented on as being inaccurate. Maybe I misunderstood or you did not word what you meant very well. I took it as saying clamp all three phases and add the amperages to come up with a total. That means a fully loaded 100 amp panel can have 100 amps per phase and if you add all three together you get 300. This panel is not drawing 300 amps, it is drawing 100 amps per phase. The rating is per pole and not add all poles together. Even if it is all line to neutral load it is still 100 amps per phase and the neutral is balanced and carrying zero (assuming linear loads of course).Not necessarily. You have to amp clamp each leg and see what amperage is being used. Just because you have a tankless water heater on a 20amp breaker, doesn't mean its drawing 20amps of electricity. Its more accurately pulling around 2amps. So when you amp clamp each feeder, depending on the loads, you'll most likely get something random like 21amps on black, 17 amps on red, and 27 amps on blue. Most panels should have pretty evenly loaded phases, and if they don't then you should think about moving some things around to even out the loads.

Also, if you have a 100amp 3 phase panel...that means that the panel is rated for 100amps TOTAL, over all 3 phases. If you were drawing 300amps on a 100amp panel, you'd have huge and dangerous problems lol. The wire for a 100amp panel would be way too small to maintain 300amps for continuous use. You can't just add up the breaker sizes and say that is what is being used, because that's never going to be the case.

Does this all make sense?

- Location
- Portage, Indiana NEC: 2008

The above in red is what is in error.If you have an "Ideal", "Fluke", "Commercial Electric", or "Klein" brand Multimeter with an Amp Clamp on it you can clamp each phase (feeder) on the panel under a standard load and flip the dial to the Ammeter setting and it will give you a pretty accurate reading of how many amps are being pulled through that phase. Do this on each phase of the panel and add it up to get the total Amps being pulled. This is the easy way.

There are calculations in the NEC that tell you how much to account for each circuit and motor load when building a service and preparing to wire a house. These calculations tell you what percentage to calculate for the entire house and this is how we electricians figure out what size service needs to feed the home. This is the hard way.

What do you need to figure this out for?

You can not add together loads on one phase to loads on another phase to determine how much load is on a panel, Kwire is correct.

Last edited:

- Location
- Austin, TX, USA

I did not know this. I was always under the understanding that the panel was rated for the entire load, but now that I think of it it makes sense that the phase is what needs to hold the rating. Sorry for giving out misinformation, I still learn new things every day lol.

- Location
- NE Nebraska

The panel rating is per bus - which I think you have mostly figured out. If you have a 100 amp panel and phase A is carrying 100 amps then that phase is fully loaded (as far as panel rating goes) even if the others are drawing nothing.

I did not know this. I was always under the understanding that the panel was rated for the entire load, but now that I think of it it makes sense that the phase is what needs to hold the rating. Sorry for giving out misinformation, I still learn new things every day lol.

Don't be too sorry - if you hadn't mentioned it no one would have challenged what you said or corrected you and you would still have the wrong idea.

I was misunderstanding it this way also. Glad I read this thread!

I did not know this. I was always under the understanding that the panel was rated for the entire load, but now that I think of it it makes sense that the phase is what needs to hold the rating. Sorry for giving out misinformation, I still learn new things every day lol.