When the neutral serves both functions

[George Stolz]

I was reading a recently borrowed copy of Mike Holt's Grounding versus Bonding book, and came across a peculiarity I found...peculiar.

Editted in italics

[ September 19, 2005, 12:10 AM: Message edited by: georgestolz ]

 

[benaround]

Re: When the neutral serves both functions

George,

All this said is make sure the neutral is large enought to trip the OCPD.
For example,if the feeder was 200a 3ph 4w and all the branch circuits were 50a 3ph except for one 20a 1p needed for a light,if you size the neutral for the unbalanced load then #12 would be good.
This is why it said PLUS the size needed for fault current. Make sense?

frank

 

[George Stolz]

Re: When the neutral serves both functions

Frank,

'02's 250.32(B)(2) states (at the end):

The size of the grounded conductor shall not be smaller
than the larger of
(1) That required by 220.22
(2) That required by 250.122

'05 says essentially the same thing, but the min. neutral spot in 220 moved a bit.

This section is saying "pick one", not "add these two together". The G&B book says you must add them together, not compare them and pick the larger.

 

[don_resqcapt19]

Re: When the neutral serves both functions

George,
The is no reason for the "plus". The grounded conductor is sized based on 220.61 or 250.24(C)(1) for services and 220.61 or 215.2(A)(1) for feeders. In either case you use the larger conductor size.
Don

 

[bphgravity]

Re: When the neutral serves both functions

I think Mike's logic stems from the Note to Table 250.122. If the EGC, or feeder grounded neutral in this case, is not sized large enough for both the normal current carrying load and for the maximum potential fault current, the withstand rating of the conductor may be exceeded and could fail.

In essence, instead of worrying about a service neutral or feeder neutral under 250.32 being sized large enough to carry the unbalanced load, it is more important to consider the conductor as being the effective fault path and thus should be sized accordianly.

 

[George Stolz]

Re: When the neutral serves both functions

I had the book in my van when I originally posted (and the second time too.