# measuring current in a 220 ac circuit

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• posted on August 14, 2007, 2:21 am
Hi all. A simple electrical question (I hope).
I have a clamp on amp meter and I'm interested in seeing how much current is used at various loads by the feeder line running to my remote garage.
The feeder is has 2 hots and 1 neutral going to a sub panel that divides it into several 110 circuits.
I know that the current on the neutral will be the difference between the current on the two hots (less any stray current lost to the grounding rod), but I'm interested in knowing the 'total' amps of the 220 circuit.
I'm thinking I could measure the current on one hot, then on the other hot, and add the two values together. What I would like to do is install a permanent meter but now I'm thinking that I need two, one for each hot. If I try to measure the current by passing both hots through the 'clamp', since they are 180 degrees out of phase, won't they cancel each other out (and I would end up with the difference of the two like on the netural)?
So my long winded question is this - how can I use a single 'clamp-on' style meter to measure current on a 220vac circuit?
Thanks.

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• posted on August 14, 2007, 2:26 am
kpg wrote:

Run one through in the opposite direction.
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• posted on August 14, 2007, 2:27 am
CJT wrote:

Oh, and divide by two.
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• posted on August 14, 2007, 2:35 am

If you created a loop so that you simultaneously measured one hot backwards and one forwards it ought to work. (actually I don't know, but it seems like it should)
Why do you want to do this anyhow? The information is pretty useless.

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<%-name%>
• posted on August 14, 2007, 3:02 am

Well, I can't argue with you there. For one, I like gadgets. For another, I have a 5hp compressor, some computer equipment, a full time window unit, and a part time window unit, a bunch of fluorescent lights, occasionally a hand drill or table saw.
It all runs on a 30 amp branch circuit and at times the breaker gets quite warm to the touch. I thought it would be nice to be able to monitor the load on the system, i.e., the amps flowing through the feeder line.
And did I mention I like gadgets?
I'll do some experiments with the one backwards divide by two method.

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<%-name%>
• posted on August 14, 2007, 3:19 am

Each item should have a nameplate. The nameplate will give you a rough estimate of how much everything is using.
Everything you have listed could be single phase, which means that each item will cause the reading to go up in only one phase of the circuit. If you do have 240V stuff it will cause both phases to go up at the same time.
Measure each leg with nothing on and then turn each item on and read again.
Unless you are planing to upgrading , try not to use the compressor and the AC unit at the same time.
The breaker is constantly monitoring the use.

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<%-name%>
• posted on August 14, 2007, 3:33 am

Yes, all of the equipment is single phase, some on one leg, some on the other. This is the source of my dilemma. So what I really am asking is this: I know its meaningful to measure the current on one leg or the other, but is it meaningful to measure the current on both at the same time?
Say I was drawing 8 amps on one leg, and 9 on the other. Am I using 17 amps? That seems right, but does it have any meaning?
The 30 amp breaker is 30 amps per leg - So who cares if they both add up to, say, 40?
I'm starting to think measuring each leg independently is the most correct solution.

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<%-name%>
• posted on August 14, 2007, 4:48 am
You need a watt meter to gauge the total used wattage. The 30 amp breaker getting warm and not tripping may mean a loose connection. Turn of the 30 amp breaker, remove from the panel. Check to be sure the pinch plate isn't turning color [getting to hot] and the matching breaker connections are good as well. Check the lugs on the breaker as well.
You might fare better by calling an experience electrician to check it out.
Zyp

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<%-name%>
• posted on August 14, 2007, 10:36 am
kpg wrote:

Nobody cares if they add up to 40 amps in your hypothetical example... simplifying a little bit here, but if you have 29 amps on one leg and 11 amps on another, the current on the neutral will actually be 18 amps (it won't actually in real life, but close enough) if you have 20 and 20, it will be zero.
nate
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<%-name%>
• posted on August 14, 2007, 12:15 pm

That's how it is done.
but is it meaningful to measure the current on both at the

You probably won't get a reading.

Yes.
That seems right, but does it have any meaning?
It means you are drawing 17 amps and the load is almost perfectly balanced. You can also measure amps on the neutral. I usually check the grounding electrode conductor also. Once in a while I find current on it and then I explain to the customer that there may be a problem somewhere.

You are correct.

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<%-name%>
• posted on August 14, 2007, 2:59 am
kpg wrote:

Actually, no it won't. If you had a pure 240 load the current on the neutral would be *zero.* Since you have 120VAC branch circuits, some will have more load than others, and some will be connected to one phase and some will be connected to the opposite phase. So you will have *some* current on the neutral, but it should be significantly less than the current on either of the "hot" wires.
I realize that sounds very, very strange, but google for "Edison circuit" for an explanation - a feeder to a remote subpanel is essentially a really big Edison circuit.

It's probably slightly unbalanced. Measure one leg, then the other.

Current is current. You measure it directly.
nate
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<%-name%>
• posted on August 14, 2007, 5:21 am
wrote:

Isn't this what he said?
Not counting the part about stray current lost through the grounding rod, which I don't really understand. It's rare, isn't it, for any current to go back through the ground or a grounding rod?

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<%-name%>
• posted on August 14, 2007, 1:32 pm

I thought so until I did some research. There are several situations where there can be dangerous current in the ground wire at the ground rod.
Obviously, a fault in the neutral will cause current to use the ground wire as a return path, but also if your neighbor has a fault in his neutral his current could be returning to the transformer via your ground rod.
Moral: before disconnecting the ground wire to service the ground rod always check for current. Even if the main is off.
Thanks for all the replies.

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<%-name%>
• posted on August 15, 2007, 12:36 am
I don't know if any electrician reading this post he must be laughing his head of what is total amps on 220 whatever you are reading on your amprob dummy do you have any knowledge in Ohms law. and neutral at properly design and grounded system IS carrying current it is not at zero ever unless on your screwed up system ground is carrying current which is not suppose to ever. and if you take 220 then split the phase of the same source/cable for 110 the neutral will carry double the current of single hot wire! Tony www.cas-environ.com

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<%-name%>
• posted on August 15, 2007, 2:57 am
You really should learn some punctuation before you call someone dummy.

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<%-name%>
• posted on August 15, 2007, 5:11 pm

Well, I'm not an electrician, this is why I asked the question. Dummy.

I have an Ohm meter. Does that count?

Um, the neutral will carry the difference between the two hot wires. The total current must be the same coming and going - do you have any knowledge in Kirchhoff's Law?

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<%-name%>
• posted on August 15, 2007, 5:52 pm

Sorry I did not mean it as it sound

NO I have few of them and I do not see myself an engineer or expert by any standard and please don't go there because I am anticipating that would be your next step

It is been so long since I stody the basics of Kirechhoffs Law, That I would not atempt to go there Tony

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<%-name%>
• posted on August 15, 2007, 5:33 pm
Tony wrote:

Not true. The two hots in a 240V circuit are 180 degrees out of phase. Only the difference between the loads on the two sides flows through the neutral. Thus, if you have equal 120V loads on each side, the neutral will have zero current flowing in it.

If the neutral carried double the current it would have to be oversized compared to the hots. In reality, it is often *undersized*. The only way this can make sense is if your statement is false.
This is why split-wire (aka "edison") circuits need to have the two hots on opposite legs. If they were on the same leg the neutral *would* carry the sum of the two legs, which could cause overheating.
Chris

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<%-name%>
• posted on August 15, 2007, 6:05 pm

I would like wet one of hand and put it on the ground ant other to the neutral to see if is carrying current in close loop circuit and use something small to close loop like 100 watt bulb that should do it yes why don't try that and let us know because what you saying 0 potential 0 current that should be perfectly safe

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<%-name%>
• posted on August 15, 2007, 6:21 pm

I would like wet one of "YOU" hand put it on the ground ant other to the neutral to see if is carrying current in close loop circuit and use something small to close loop like 100 watt bulb that should do it yes why don't try that and let us know because what you saying 0 potential 0 current that should be perfectly safe