Amp load on 120/240 circuits

[CedarCreekWoody]

I wanted to start this as a separate thread in order not to create confusion on a thread where someone has an electrical question about their own personal RV.

Several times I've seen the comment that when the two legs on a 50 amp 240 volt circuit are used as two separate 120 volt circuits that the neutral wire is only carrying the "imbalance" between the two loads. I've seen it stated that if the load on both circuits is the same then the load on the neutral wire is actually zero. I just have a difficult time wrapping my head around this concept. Wmtire has posted several excellent articles on this subject, but it just doesn't register in my head.

I just ran a test in my shop using a clamp on amp meter, reading on the cables in the sub panel box. I had a 10 amp load on L1 and a 12 amp load on L2, nothing running on 240 volts. Using the above logic of the neutral carrying only the imbalance I would expect a 2 amp reading on the neutral line. Or, using the logic that the amps add together I would expect a reading of 22 amps on the neutral. But, I found neither. Instead I got a reading around 11 amps, apparently an average of the two loads (which is what I would expect to find.)

I'm hoping that some of the real sparkys or electrical engineers can fill us in with some facts rather than our usual conjecture, as I am neither. Too often I see this discussion pop up on a thread by a new member and I think we really confuse them when it may have no connection to their issue.

[Bluepill]

Google is your friend.

https://www.bluesea.com/resources/86

Unless all of your circuit loads are purely resistive, the neutral current will be other than the calculated difference between the 2 legs. This is due to phase shift created by inductive and capacitive elements in the loads.

Google "Power Factor" for more info. It is a complex, but interesting, study.

[clr]

The two legs are on different phases. AC power on the power line is 3 phase for many reasons but manly for industrial (very heavy ) power use and for ease of transmission. Each phase is 120 degrees out of phase, so you have 3 phases separated by 120 degrees. To get the math to work correctly you need to consider the phases of the currents. You are using DC current formulas that do not consider phase.

[Arctic Wolf]

The load on the neutral should be equal to the hot leg on a 120 volt circuit.
You can think of it this way...
The 240 volt circuit is like two 12 volt batteries in series... To get 12 volts you would draw from the positive terminal of one battery (L1) and the jumper between -&+(N) 24 volts would be +(L1) on one battery to -(L2) on the other battery

[Arctic Wolf]

Originally Posted by clr

The two legs are on different phases. AC power on the power line is 3 phase for many reasons but manly for industrial (very heavy ) power use and for ease of transmission. Each phase is 120 degrees out of phase, so you have 3 phases separated by 120 degrees. To get the math to work correctly you need to consider the phases of the currents. You are using DC current formulas that do not consider phase.

You're going to really confuse this with the three phase.
I'm pretty sure for the sake of this discussion were only dealing with split phase.
The three phase comes about because the generator is basically a 3 "Cylinder" electron pump (that's a simple way to understand it anyway)

[andymil]

Try this link for an explanation:

http://www.samlexamerica.com/support...chCircuits.pdf

[CedarCreekWoody]

Originally Posted by Bluepill

Google is your friend..

He isn't very friendly if you can't figure out how to tell him what you want.

Interesting discussions, but confusing.

[Arctic Wolf]

Originally Posted by andymil

Try this link for an explanation:

http://www.samlexamerica.com/support...chCircuits.pdf

Excellent explanation

[andymil]

Woody, try the experiment with two 100 watt light bulbs.

My cousin had a problem with his garage. When we turned one light on, no light. When we turned a second light on, both lights lit (different circuits). Turning on a third light changed the balance and the brightness changed. I found the neutral wire feeding the garage broke at the weatherhead. With two equal resistive loads, there was no current in the neutral weather it was connected or not.

[Arctic Wolf]

Only because they were acting as a series wired pair on 240 volts.

[Cowracer]

Originally Posted by Bluepill

Google is your friend.

https://www.bluesea.com/resources/86

Unless all of your circuit loads are purely resistive, the neutral current will be other than the calculated difference between the 2 legs. This is due to phase shift created by inductive and capacitive elements in the loads.

Google "Power Factor" for more info. It is a complex, but interesting, study.

I am an electrical engineer, and this link is exactly correct.

AC is a different beast than DC and harder to understand, but if you break it up into instantaneous slices of time, you can look at it somewhat like DC and than makes it easier to understand some things.

In a 220 system one 'hot' will be a positive voltage with respect to neutral and the other will be exactly the same voltage but negative with respect to neutral. Any current carried in the neutral from one 'hot' will be offset by any current carried in the neutral from the other 'hot' because they are 180 degrees out of phase . One hot's current will be flowing one way, the other hot's current will be flowing the other way. If they exactly the same, they cancel out. If one is 10 amps and the other is 8, then the 8 amp current will cancel out 8amps of current in the 10 amp side, netting only 2 amps current in the neutral.

Of course, that is over-simpified, but I hope that helps.

Tim

[wmtire]

Originally Posted by CedarCreekWoody

I just ran a test in my shop using a clamp on amp meter, reading on the cables in the sub panel box. I had a 10 amp load on L1 and a 12 amp load on L2, nothing running on 240 volts. Using the above logic of the neutral carrying only the imbalance I would expect a 2 amp reading on the neutral line. Or, using the logic that the amps add together I would expect a reading of 22 amps on the neutral. But, I found neither. Instead I got a reading around 11 amps, apparently an average of the two loads (which is what I would expect to find.)

Anyway you could post pics of your clamp meter reading at each line with it connected?

[Arctic Wolf]

I just ran a test in my shop using a clamp on amp meter, reading on the cables in the sub panel box. I had a 10 amp load on L1 and a 12 amp load on L2, nothing running on 240 volts. Using the above logic of the neutral carrying only the imbalance I would expect a 2 amp reading on the neutral line. Or, using the logic that the amps add together I would expect a reading of 22 amps on the neutral. But, I found neither. Instead I got a reading around 11 amps, apparently an average of the two loads (which is what I would expect to find.)[/QUOTE]

This makes sense, since the two phases are opposite of each other.

[CedarCreekWoody]

Originally Posted by wmtire

Anyway you could post pics of your clamp meter reading at each line with it connected?

I didn't take photos of the meter readings. The actual numbers were fractional amps of course, but round out to the numbers I posted. I'm not trying to start an argument as this really does not matter in our RVs as long as the proper size cabling is used. I just kept seeing a member asking a question about his particular sitation and this discussion would pop up in the responses when it really has no impact on his issue. Rather than continuing to confuse new members I thought perhaps a good airing in a separate thread would let people express their thoughts.

Have you ever checked this with your own meter?

[CedarCreekWoody]

Originally Posted by Arctic Wolf

This makes sense, since the two phases are opposite of each other.

That's the way I saw it too, but does not seem to be what others are saying. Hence my curiously.

[Arctic Wolf]

Alternating current does some wired things

[BandJCarm]

Originally Posted by Arctic Wolf

Alternating current does some wired things

If that was intended, you get nineteen extra points!!! LOL

[Bluepill]

Try this analogy:

Two identical water pumps connected to a common pool (C). One is pumping INTO another pool (A) and the other is pumping FROM a third pool (B). (i.e. the pumps are exactly"Out of Phase").

In this example Pool C is the neutral line, A is Line 1, and B is Line 2

When both pumps are moving the exact same amount of water, the level in the common pool does not change because amount IN equals amount OUT. If one of the pumps moves less or more water, the level in the common pool will change at a rate equal to the difference between the flows, not the average of the two.

Note: this analogy cannot be used when considering only 240 Volt loads, as the phase relationships no longer are valid because the mid point connection at the transformer (the Neutral leg) is not referenced.

FWIW, this subject took me the better part of 2 semesters of EE to fully understand. If you get it the first time you read about it, please apply to MENSA.

[Arctic Wolf]

Lol

[CedarCreekWoody]

Originally Posted by wmtire

Anyway you could post pics of your clamp meter reading at each line with it connected?

Bobby I re-ran the tests, this time taking photos. Same loads, drill press on L1 and shop vac and lights on L2. And, of course, the results were different. So I ran them again. Again different. Ran a third time with even more difference. I'm not sure what is causing the variance but it was obvious something was going on that I wasn't understanding. In any case this was never a matter of need, just one of curiosity, so I'm just going to accept that I don't understand some things about A/C current and leave it at that. No point in posting the photos.

Thanks to everyone for your input. I sorry that I'm not a better student.