size service neutral or grounding conductor per 220.61 or 250.24(C)(2)

[sojama]

In reference to the Mike Holt NEC 2008 Commercial Calculation example under Section 11.13 video and as a general question...

Service load was 196,800VA @ 240V (820A). Breaker must be 1000A. Conductors are sized to the breaker with say 3 sets of 400 kcmil.

Neutral was calculated as 147,200VA @ 240V per 220.61 (613A) and adjusted by the demand factor of 70% for loads in excess of 200A yielding 489A. 489A divided by 3 raceways yield 163A, which requires a 2/0 neutral conductor per raceway.

However, 250.24(C)(2) says you can also size the neutral to the largest ungrounded service-entrance conductor per Table 250.66 but no smaller than 1/0 when paralleling. In this case, 400 kcmil phase conductors only requires a 1/0. (This method was used in the Section 11.14 example in the video and similarly here: http://ecmweb.com/grounding/electric_grounding_vs_bonding_3/)

Which method do we use: 220.61 or 250.24(C)(2)? Method 1 (220.61) seems like it could vary based on load, which could change over the life of the building. Method 2 (250.24(C)(2)) seems to be undersized based on calculated neutral loads per Method 1.

As a best practice, would it be better just to make the neutral the same size as the phase conductors?

 

[suemarkp]

I haven't seen the video, but I think I understand your question. There are two independent requirements for neutral sizing. One is that it must be sized for the calculated load. That was your firsr number of 163A.

The other requirement is a minimum size per 250.66. The size you end up with is the larger of the two values. If you had no neutral load, you can't go smaller than 250.66. If you have a huge neutral load, then you must size way over the 250.66 minimum.

You don't need to size per the ungrounded conductors since you always get to apply the 70% factor when over 200A. It may be safer to size it close to the max that could ever occur (100% of load below 200A plus 70% of the load above that).

I like the neutral to be a size or two below the ungrounded conductors. Makes it easier to identify should its white markings disappear.

 

[augie47]

actually, I think (3) points to consider:
Load (220.61), Sized per GEC {250.24(C)}, and, where applicable, parallel conductors (310.4)
In your case we have parallel condcutors so we start by say minmum of 1/0
We have a load requirement of 163 amps so a 2/0
And 250.24(C) would require a 1/0 both based on your 400kcmil and the minimum for paralleled.
You would have to use the highest of the 3 requirements, here a 2/0