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Author Topic: That seems like a lot of current on the neutral, doesn't it?  (Read 14713 times)

Jeffery Foster

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That seems like a lot of current on the neutral, doesn't it?
« on: March 30, 2015, 09:46:38 PM »

Greetings!

At an event the other night, our small distro was served by a 250/125v 50a circuit (by way of NEMA 14-50 receptacle) that was professionally installed 15' from the 200a breaker panel.

Our amp racks are split exactly even between the two phases.  Things are low-key, and pulling ~15 amps on Phase A and ~16 amps on Phase B.  With everything evenly balanced, doesn't the opposite phases' sine waves cancel out the return current on the neutral?  Instead, at this venue I was showing approximately 22 amps on it consistantly.  I closely monitored everything for heat and got through the night with no other issues.  But still, this bothered me. 

Any thoughts on the cause, or am I incorrect in believing the current should have been close to 0?

Thanks in advance!
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TJ (Tom) Cornish

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #1 on: March 30, 2015, 09:58:24 PM »

Greetings!

At an event the other night, our small distro was served by a 250/125v 50a circuit (by way of NEMA 14-50 receptacle) that was professionally installed 15' from the 200a breaker panel.

Our amp racks are split exactly even between the two phases.  Things are low-key, and pulling ~15 amps on Phase A and ~16 amps on Phase B.  With everything evenly balanced, doesn't the opposite phases' sine waves cancel out the return current on the neutral?  Instead, at this venue I was showing approximately 22 amps on it consistantly.  I closely monitored everything for heat and got through the night with no other issues.  But still, this bothered me. 

Any thoughts on the cause, or am I incorrect in believing the current should have been close to 0?

Thanks in advance!
It depends on the type of the load.  For truly resistive loads, yes - the neutral current will be the difference between the currents flowing in the phase legs.  For reactive loads - power amps, lighting dimmers, LED fixtures, etc., the load creates harmonic currents on the neutral that can add instead of subtract.  In some cases, the NEC actually requires doubling the size or quantity of neutral conductors to accommodate this.

If you have the ability to do so, running as much of your gear at 208v/240v will help significantly, as the neutral wire is not used at all.  Quite a lot of equipment can do this - many things just need a line cord change and can run on anything from 100v - 240v. 
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Tim McCulloch

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #2 on: March 30, 2015, 11:56:37 PM »

Greetings!

At an event the other night, our small distro was served by a 250/125v 50a circuit (by way of NEMA 14-50 receptacle) that was professionally installed 15' from the 200a breaker panel.

Our amp racks are split exactly even between the two phases.  Things are low-key, and pulling ~15 amps on Phase A and ~16 amps on Phase B.  With everything evenly balanced, doesn't the opposite phases' sine waves cancel out the return current on the neutral?  Instead, at this venue I was showing approximately 22 amps on it consistantly.  I closely monitored everything for heat and got through the night with no other issues.  But still, this bothered me. 

Any thoughts on the cause, or am I incorrect in believing the current should have been close to 0?

Thanks in advance!

Are you 100% certain you were on 120/240v split (single) phase *service*, or could you have been on 2 legs of 120/208v 3 phase service?
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frank kayser

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #3 on: March 30, 2015, 11:59:20 PM »

<snip>
If you have the ability to do so, running as much of your gear at 208v/240v will help significantly, as the neutral wire is not used at all.  Quite a lot of equipment can do this - many things just need a line cord change and can run on anything from 100v - 240v.
Slap me across the face again!  Another d'Oh! moment in my life!
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Jeffery Foster

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #4 on: March 31, 2015, 12:08:00 AM »

Are you 100% certain you were on 120/240v split (single) phase *service*, or could you have been on 2 legs of 120/208v 3 phase service?
Good question, Tim. I Didn't consider that it could have been fed by two legs of a 3phase system. I can guarantee these facts; I saw the dual-pole 50a breaker in the panel. I metered the receptacle and showed ~123v L1-N, ~123v L2-N, ~240v L1-L2, and negligible between neutral and ground.
I hope that helps. 
I was just stunned that leading or trailing voltages can create that kind of current.
During the previous night, we were cam'd directly to a venue single phase switch.  We drew a few more amps per phase, but the neutral returned about 2 all night.
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TJ (Tom) Cornish

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #5 on: March 31, 2015, 06:59:07 AM »

Good question, Tim. I Didn't consider that it could have been fed by two legs of a 3phase system. I can guarantee these facts; I saw the dual-pole 50a breaker in the panel. I metered the receptacle and showed ~123v L1-N, ~123v L2-N, ~240v L1-L2, and negligible between neutral and ground.
I hope that helps. 
I was just stunned that leading or trailing voltages can create that kind of current.
During the previous night, we were cam'd directly to a venue single phase switch.  We drew a few more amps per phase, but the neutral returned about 2 all night.
Tim's thought is probably correct. In a three-phase system, the legs are 120 degrees apart, not 180 as they are in a true single phase system. Once you get out of the tiniest dives, nearly every commercial building has 3-phase power. Confirming this is easy - if you have 240v leg to leg, it is single phase. If you have 208v, it is 3-phase.
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Jonathan Johnson

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #6 on: March 31, 2015, 03:55:54 PM »

Tim's thought is probably correct. In a three-phase system, the legs are 120 degrees apart, not 180 as they are in a true single phase system. Once you get out of the tiniest dives, nearly every commercial building has 3-phase power. Confirming this is easy - if you have 240v leg to leg, it is single phase. If you have 208v, it is 3-phase.

Except in *some* older systems that are 3-phase delta, in which case you have 240V any leg to any leg; one secondary is center tapped providing 120V between two main legs and neutral, and you have 208V between high-leg and neutral.
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Stop confusing the issue with facts and logic!

Guy Holt

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #7 on: March 31, 2015, 09:48:30 PM »

I can guarantee these facts; I saw the dual-pole 50a breaker in the panel. I metered the receptacle and showed ~123v L1-N, ~123v L2-N, ~240v L1-L2, and negligible between neutral and ground.

If the OP read 240V line-to-line, the service was clearly single phase, which means that harmonic currents would not be additive on the neutral. There must be something else going on here other then triplen harmonics adding on the neutral.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
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Jeffery Foster

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #8 on: March 31, 2015, 10:15:13 PM »

I'm really sure it was single phase. I sincerely appreciate everyone's input on this.

Q: Could it have been an undersized neutral wire from the receptacle to the panel?
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TJ (Tom) Cornish

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Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #9 on: April 01, 2015, 01:40:06 AM »

I'm really sure it was single phase. I sincerely appreciate everyone's input on this.

Q: Could it have been an undersized neutral wire from the receptacle to the panel?
The size of the wire doesn't affect the current (much) which is why we have to be careful and use over current protection devices.

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ProSoundWeb Community

Re: That seems like a lot of current on the neutral, doesn't it?
« Reply #9 on: April 01, 2015, 01:40:06 AM »


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