Neutral Calculation?

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I have taken the supervisors exam in Oregon and the state is failing me because of my neutral calculations. I know they are wrong and I need as many opinions as possible to build my case. 220.61 states "The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor". I was given a 240/120v single phase service supplying primarily 120v loads and a 240v heating load. I totaled my neutral va and divided it by 240. This is a pretty standard method. If you want to get more precise you would balance your 120v loads on A and B phases and which ever one had the slightly larger va would be used for the calculation. This is the method the state used but when the they came up with the va calculated on one phase they still divided it by 240v instead of 120v. I discussed this with the Asst Chief for an hour and a half. He will not pull his head out of the sand and look at the math. The difference in va for sizing the neutral was off by double. Please agree or disagree please and help me pass this test and force the state to reexamine the tests that they are giving. Thanks
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Can you show the actual math with some numbers? For a 120/240 system you can just remove the 240 volt loads and find the current difference between the all the 120 volt loads on opposite "phases". For example:

16 amp load @240 volts
12 amp load ("A phase") @ 120 volts
2 amp load ("B phase") @ 120 volts

Neutral load: 12-2=10 amps.
 
A. B. N-A phase
Gen lighting 93750. 93750. 7500
Outlets 3150. 3150. 3150
copiers. 12024. 10020. 12024
computers. 4800. 4800. 3840
point of sale 2880. 2880. 2880
sign. 1500 0
heat 240v. 7500. 7500. 0

Phase A Has the maximum neutral imbalance potential at 29394 va. On paper the loads look balanced but in reality they would rarely be. Nobody is forced to use equipment and devices in a balanced manor. They turn on lights in one area and not another. They uses certain outlets and not others. Certain copiers and not others. The state is agreeing with this premise. Where we are differing is wether or not the unbalanced current should be divided by 120 or 240v. The maximum unbalanced current from the neutral to phase a is at 120 volts. If the majority of power being consumed comes off of A or B doesn't matter. The neutral needs to be able to carry the entire unbalanced 120v load on that phase.
 

Smart $

Esteemed Member
Location
Ohio
Can you show the actual math with some numbers? For a 120/240 system you can just remove the 240 volt loads and find the current difference between the all the 120 volt loads on opposite "phases". For example:

16 amp load @240 volts
12 amp load ("A phase") @ 120 volts
2 amp load ("B phase") @ 120 volts

Neutral load: 12-2=10 amps.
Ummm... that is incorrect.

The maximum calculated unbalanced neutral current is 12A. It's the entire 120V A line load or the entire B line load, whichever is greater... not the difference.

A word of caution regarding certain 240/120V MWBC, such as range or dryer. Both the example loads count in full towards the neutral load (half the VA at 120V).
 

Smart $

Esteemed Member
Location
Ohio
A. B. N-A phase
Gen lighting 93750. 93750. 7500
Outlets 3150. 3150. 3150
copiers. 12024. 10020. 12024
computers. 4800. 4800. 3840
point of sale 2880. 2880. 2880
sign. 1500 0
heat 240v. 7500. 7500. 0

Phase A Has the maximum neutral imbalance potential at 29394 va. ...
I clock in A higher at 118,104VA. Only load dismissed was heat. Are you considering any of the other loads as 240V?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Ummm... that is incorrect.

The maximum calculated unbalanced neutral current is 12A. It's the entire 120V A line load or the entire B line load, whichever is greater... not the difference.

What am I missing? If @120 volts there is 2 amps on "phase A" and 12 amps on "phase B" the neutral current is 10 amps.
 
You are not missing anything Rob. Yes that is the current imbalance if both loads are on. If you turn off the 2amp load then the imbalance is now 12amp. That is why 220.61 says the "maximum" unbalanced load between the neutral and any one ungrounded conductor. The question posed is do you divide that one phase worth of neutral current by 240v or 120v?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Ah, I missed the maximum part otherwise the calculation is correct. Sorry for the confusion.
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The only reason that it is complicated is that you are talking about dividing the current by the voltage. What is really happening is dividing *load* in KVA by voltage to get amps.

I am not looking at the
Annex D calculations at the moment, but if you have followed the standard practice of assigning one half of the line-to-line load KVA to each phase and all of the line to neutral load to each phase you really need to ignore all line to line loads which are two wire 240 and divide the total of all single ended loads on that line by 120.
How you handle three wire line, line, neutral loads is at the mercy of how that load works and therefore how unbalanced it can become.
You can use total load on both lines and divide by 240 to get maximum line current and then use that as a basis for calculating an upper limit on neutral load. But that will still not be correct if the remaining loads are unbalanced. One will be more than half and one less.
 
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Smart $

Esteemed Member
Location
Ohio
....
How you handle three wire line, line, neutral loads is at the mercy of how that load works and therefore how unbalanced it can become.
...
Manufacturers of 240/120V MWBC (three wire) equipment rarely publish maximum neutral load information let alone include it on the nameplate. The question is how to handle this in the calculation for maximum unbalanced neutral current... ? Do we assign half the load to each line-neutral connection, or do we assign the entire load to each line to neutral connection? Code does not state explicitly how to do this.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I have taken the supervisors exam in Oregon and the state is failing me because of my neutral calculations. I know they are wrong and I need as many opinions as possible to build my case. 220.61 states "The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor". I was given a 240/120v single phase service supplying primarily 120v loads and a 240v heating load. I totaled my neutral va and divided it by 240. This is a pretty standard method. If you want to get more precise you would balance your 120v loads on A and B phases and which ever one had the slightly larger va would be used for the calculation. This is the method the state used but when the they came up with the va calculated on one phase they still divided it by 240v instead of 120v. I discussed this with the Asst Chief for an hour and a half. He will not pull his head out of the sand and look at the math. The difference in va for sizing the neutral was off by double. Please agree or disagree please and help me pass this test and force the state to reexamine the tests that they are giving. Thanks

While I have not taken this exam, so I am only speculating, you would think that they would use the NEC example method and just take ALL the line to neutral load (plus any other required neutral load per code) and divide by 240. The other method that you used of taking the highest phase, I would agree you would divide by 120. I think if I were taking this I would work it both ways and see if an answer fits, but even better would to know ahead of time their method. It also seems that there is some misunderstanding in your review as obviously the way you explained your method you would use 120, not 240. But I really think they would be looking to use the more standard NEC example method and use 240.
 

kwired

Electron manager
Location
NE Nebraska
The load has to be pretty definite otherwise it is still just a spin of a wheel and see what we land on when there is a lot of load diversity and/or general purpose receptacles when it comes to knowing exactly what the neutral load will be. I honestly don't know how you can ask for such a calculation on a test and expect to see the same answer from everyone taking the test unless dealing with definite loads.
 

Smart $

Esteemed Member
Location
Ohio
... But I really think they would be looking to use the more standard NEC example method and use 240.
I will agree as long as there is no indication in the question that the line-to-neutral loads are unbalanced. Most of the examples make it appear dividing by 240 is standard because they assume a balanced scenario. But what do you do if these loads are not balanced? See Example D3(a) for one such situation...
 

benedetti

Member
Location
Plano, TX USA
Suggestion

Suggestion

Couldn't you just focus on getting the other questions correct and let that be the one question that doesn't matter? I've never seen a test require a 100% to pass other than hands on test in a few places.
 

Glock23gp

Member
Location
United States
I just took a prep class and the way we were shown to draw it out (from the guy who wrote a majority of the tests) is to make 4 columns for your single-phase panel.
A phase B phase A Neutral B Neutral.

This allows you to divide your neutral loads according to phase and use the largest for unbalance (even though irl there is only one path to N)

Also your sign circuit is continuous so the phase conductors calculate at 125% however the Neutral is at 100% so it's 1500 on A and 1200 on Neutral A.

Same goes for all non residential lighting unless otherwise stated on exam. Phase conductors at 125%. Neutral at 100%

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Glock23gp

Member
Location
United States
Also how do u have your lighting at 93750 on A & B but only 7500 on the Neutral assuming it's 120v? I'm assuming that's 75000 which is 80% of 93750 (continuous)?

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