Sizing Neutral 220.61

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Cletis

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Can you check my calc's and let me know if this was wrong.

New service upgrade. 100 -150 amp

Calculated load on premise 72 amps @ 240V = 17,280 va

Therefore, I used 2-1/0 Cu thw conductors and 1 - #2 Cu thw conductor for service neutral

Sound ok per 220.61 ?

On a side note: whats the smallest neutral I could have used? I seem to recall #4 Cu for some odd reason somewhere in book
 
Cletis said:
Can you check my calc's and let me know if this was wrong.

New service upgrade. 100 -150 amp

Calculated load on premise 72 amps @ 240V = 17,280 va

Therefore, I used 2-1/0 Cu thw conductors and 1 - #2 Cu thw conductor for service neutral

Sound ok per 220.61 ?

On a side note: whats the smallest neutral I could have used? I seem to recall #4 Cu for some odd reason somewhere in book
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Well, with a calculated load at 72A, and #2 THW-CU rated 115A, you're likely okay... leaves a good margin for any unmentioned snafus ;)

Smallest permitted service neutral is the minimum size GEC under Table 250.66. The smallest you can use on this installation cannot be determined from the information provided. It's based on the maximum unbalanced load, i.e. largest sum of respective-line-to-neutral loads. 220.61(B) throws in some further reduction for specific conditions.
 
Absolute smallest permitted is as Smart says based on 250.66, otherwise all you presented is total VA, you didn't mention anything about maximum possible load that could be connected to the neutral, which is what you must have at a minimum, but never smaller then 250.66.

Should you happen to have a lot of 240 volt loads - your possible neutral load is decreased because of loads that don't even use the neutral.

Seems to be somewhat of an unwritten rule in many jurisdictions (maybe written in some) that you can have a neutral that is two sizes smaller then the ungrounded conductors where it is supplying a significant amount of line to neutral loads and the AHJ is not going to ask you for any calculations to determine if the neutral is large enough. Note that this seems to be a common practice and is not mentioned anywhere in NEC.
 
I really don't know max exactly but it should be between 30-50 amps at most. This is just a little piddly $ 1700 service upgrade in a cracker jack little house mostly gas 1,000sq ft so
 
As Smart mentioned you have a 115 amp conductor - chances are you will not see that kind of neutral load, but if you went any smaller many AHJ will want you to prove what you used is large enough.
 
Smart $ said:
Well, with a calculated load at 72A, and #2 THW-CU rated 115A, you're likely okay... leaves a good margin for any unmentioned snafus ;)

Smallest permitted service neutral is the minimum size GEC under Table 250.66. The smallest you can use on this installation cannot be determined from the information provided. It's based on the maximum unbalanced load, i.e. largest sum of respective-line-to-neutral loads. 220.61(B) throws in some further reduction for specific conditions.
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I really am going to have to buy you a 2014 NEC...lol....the minimum permitted grounded conductor size is now under Table 250.102(C)(1).:angel:
 
MasterTheNEC said:
I really am going to have to buy you a 2014 NEC...lol....the minimum permitted grounded conductor size is now under Table 250.102(C)(1).:angel:
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I have the 2014. I cited 250.66 out of habit. Perhaps I'll remember the change when the 2017 is rolled out. :D
 
ActionDave said:
I'll ignore it until someone can give me a compelling reason to use the new chart.
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I just carefully looked at the two charts - they are identical other then the last row and the notes that follow. How did we ever manage this before with only one chart? (been mentioned before, we need a sarcasm emoticon)
 
I think they added the new chart to help with the confusion about the grounding electrode conductor and bonding. Although manyof us know the sizing is the same many people were confused because the table 250.66 did not mention bonding, etc like the new Table 250.102(C)(1) does in the heading
 
Dennis Alwon said:
I think they added the new chart to help with the confusion about the grounding electrode conductor and bonding. Although manyof us know the sizing is the same many people were confused because the table 250.66 did not mention bonding, etc like the new Table 250.102(C)(1) does in the heading
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Why not change the heading and maybe notes to the table then?
 
ActionDave said:
I'll ignore it until someone can give me a compelling reason to use the new chart.
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WHAT...this is not good enough for you:roll:

Analysis of Change

"A new Table 250.102(C)(1) was added with corresponding revisions to related
sections in Article 250 to provide simplification to the sizing requirements for
fault-return carrying conductors that are not sized using Table 250.122.
Previous Code language for multiple Code sections such as sizing requirements
for main bonding jumpers, system bonding jumpers, supply-side bonding jumpers
and grounded conductors referred to Table 250.66 for sizing these conductors
or bonding jumpers. Table 250.66 is titled for grounding electrode conductors
and has a maximum required conductor of 3/0 copper or 250 kcmil aluminum.
Each Code reference for sizing these conductors or bonding jumpers contains
similar language for sizing requirements above those Table 250.66 maximum sizing
requirements, but the Code text was not always identical. Table 250.66 was
designed for grounding electrode conductors and bonding conductors used to
connect multiple electrodes."

from the IAEI 2014 Analysis of Change Book
 
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