Example D1(a) - Calculated Load for Neutral

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Do You Like Ham

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In the Appendix D, sample calculation D1(a)... Can someone tease apart for me why the neutral calculation is essentially putting 50% of the demand receptacle load on the neutral + 70% of Range and Dryer?

I understand the reference to 220.61 for the 70% reduction.. but since the range/drier are 240V... why do they have any relation to the neutral current calculation?

This is my 'mental model' of the "Service or Feeder" vs. the "Neutral Conductor"... as the sample calculation shows...


1681399517488.png




Seems to me the worst case for the neutral would be that half the loads in the residence are on, and all on the same phase, while the other half are off... like this.. (the drier/range being irrelevant). what am I misunderstanding?
1681399676233.png
 
The range and dryer do have and use the neutral. Most dryers only have a 120V motor and I suspect the range may also have a 120V motor but I am not certain.
 
@Dennis Alwon - under what condition would there be that much current on the neutral? if one phase completely dropped out, and the phase with the motors in the range and drier remained on, 70% of the total current being for motor vs. heat seems unrealistic... especially for the range.
 
doing a little digging, I see that drier motors are likely about 1/3HP, and per 430.248 (120v * 7.2A = 864).. only about 15% of the load in the drier in its example (8000VA)... so why does code have such a large current on the neutral?
 
Do You Like Ham said:
so why does code have such a large current on the neutral?
Click to expand...
It's a conservative upper bound picked out of thin air? The code writers wanted a figure where you could be sure than basically no range or dryer would draw more than that on its neutral.

If you wanted to improve the situation, you could try to get the UL listing standard changed so that all 120V/240V ranges and dryers need to list the maximum neutral current on their nameplates, and then get the NEC changed to allow that nameplate data to be used instead of the 70% figure. But that's a lot of work.

Cheers, Wayne
 
@wwhitney - fair enough.

Out of curiosity... do designers/engineers/contractors even bother with this calculation? or do they just specify / install a neutral to match the line conductors? Does the the overhead in trying to maintain stock, etc, of an 'optimal' conductor size outweigh the simplicity in just stocking / pulling all the same size?

I think the answer is here: https://images.thdstatic.com/catalog/pdfImages/4f/4ffa926e-8e5d-47ca-842f-c8fa3766b684.pdf
all these service entrance cables indicate phase and neutral are the same size.
 
Ranges have not used dual (120/240) heating elements for heating in at least 50 years since infinite switches came out.
Some manufacturers of residential ranges still use the same electronics as they do for gas ranges instead of making a dual voltage board.
Its also not uncommon to see nothing on the neutral terminal block of a range or oven.
 
@tortuga - by dual (120/240) heating elements, are you saying, they used to have two elements, one at 120 and one at 240, and thus, in such cases, it would have been more common to see a more significant current on the neutral?
 
Old 1950's ranges would actually switch the voltage to the heating element, so on hi setting they would run on 240V and a low setting used 120V.
 
Do You Like Ham said:
In the Appendix D, sample calculation D1(a)... Can someone tease apart for me why the neutral calculation is essentially putting 50% of the demand receptacle load on the neutral + 70% of Range and Dryer?

I understand the reference to 220.61 for the 70% reduction.. but since the range/drier are 240V... why do they have any relation to the neutral current calculation?

This is my 'mental model' of the "Service or Feeder" vs. the "Neutral Conductor"... as the sample calculation shows...


View attachment 2564821




Seems to me the worst case for the neutral would be that half the loads in the residence are on, and all on the same phase, while the other half are off... like this.. (the drier/range being irrelevant). what am I misunderstanding?
View attachment 2564822
Click to expand...
You have looked at it incorrectly.
The net load of general lighting,small appliance and laundry is 5100 va.

Do You Like Ham said:
doing a little digging, I see that drier motors are likely about 1/3HP, and per 430.248 (120v * 7.2A = 864).. only about 15% of the load in the drier in its example (8000VA)... so why does code have such a large current on the neutral?
Click to expand...
Also the dryer is at 5500 VA not 8000.
The range was listed at 8000 VA.

Now if you look at 220.61 you can see where this reduction came from. They even IFN you to the example you mentioned.
As you read through that section be sure to read A,B and C.

A key words in 220.61 is maximum unbalanced load and maximum unbalanced load shall be the maximum net calculated.
So the 5100 is the maximum unbalanced load of that section so it's at 100% of the net calculated.
 
JoeStillman said:
In my experience, reduced-neutral calculations are not worth the trouble. You can spend $100 of engineering time to save $50 on wire.
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In typical residential yes. Which is why nobody undersizes neutrals in residential. (That said, I've also seen people waste thousands of dollars on services because their contractor didn't do a load calculation. Residential contractor skills in this area a typically pathetic.)

Also in branch circuits 30A and smaller you're not allowed to undersize neutrals.

For commercial, the money savings probably comes out different and could be worth while.
 
If a branch circuit has more than one receptacle then 210.19(B)
For the 20-30 Amp single receptacle on a multiwire branch circuit I am not sure.
 
wwhitney said:
What NEC references do you have on the topic of reducing branch circuit neutrals? I wasn't aware of any.

Cheers, Wayne
Click to expand...
Actually I should have left the 'branch' out of it as it mostly applies to feeders.

Through a combination of various sections the code effectively never allows a neutral to be smaller than the line conductors when the OCPD is 30A or less.
See:
240.4(D)
215.2(A)(2)
210.19(3) Exception 2
 
jaggedben said:
Actually I should have left the 'branch' out of it as it mostly applies to feeders.
Click to expand...
Yes, that was my understanding.

The presence of 210.19(3) Exception 2 suggests that in the general case, branch circuit neutrals can't be reduced in size.

Also, while 220.61 tells you that the neutral load on a feeder may be smaller than the load on the ungrounded conductors, 240.4 doesn't have any exception for feeder neutrals. Seems like given this conflict, a strict reading of 240.4 requires that the neutral conductor be protected by the OCPD, which would disallow downsizing a feeder neutral.

As this is obviously in conflict with industry practice, maybe 240.4 needs to be updated? Or am I missing something?

Cheers, Wayne
 
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