Neutral Load Calculations - Stallcup's

[erickench]

Okay, let's start again. I am looking at two examples in Stallcup's "Electrical Calculations Simplified". Stallcup uses a method of calculating the neutral loads that I find confusing and inconsistent. Here is the first:

Office Building

120 Volt, Single Phase Loads
Office lighting load 525,000VA (before 125% factor applied)
Hall lighting load 1500VA (before 125% factor applied)
Track lighting load 4500VA
Outside lighting load 3600VA (before 125% factor applied)
Sign lighting load 4800VA
Noncontinuous Receptacle 68,760VA
Continuous Receptacle 21,780 (before 125% factor applied)

There are three-phase loads that are not applied to the calculation. I'm okay with that. This is how Stallcup performs the neutral load calculation:

Office Lighting Load 525,000VA
Hall Lighting Load 1,500VA
Total Load 526,500VA

Question: Why aren't the other 120 Volt loads used in the calculation? The track lighting, outside lighting, sign lighting, and receptacle lighting are not taken into account even though they are single phase loads that employ the neutral as a current carrying conductor. Why is this?

 

[marti smith]

Seems like the items left out would be those which would be on at night, and the rest is considered as 220.60 ('08).

 

[erickench]

Seems like the items left out would be those which would be on at night, and the rest is considered as 220.60 ('08).

I would think that NEC 220.60 would apply to A/C and heating loads. The higher of the two would be chosen.

 

[marti smith]

So would I, but that was the only reasoning I could think to apply...Hopefully you can get charlie b to put in his two cents.

 

[iwire]

I would think that NEC 220.60 would apply to A/C and heating loads.

It surely does apply to those loads but it is not limited to those loads.

220.60 Noncoincident Loads. Where it is unlikely that
two or more noncoincident loads will be in use simultaneously,
it shall be permissible to use only the largest
load(s) that will be used at one time for calculating the total
load of a feeder or service.

Notice it also says 'unlikely' not 'will not'.

That said I do not know if this is the answer to the question asked.

 

[erickench]

Okay here's another strange inconsistency with Stallcups. It has been determined that the ungrounded conductors per phase are four 3/0 AWG in parallel. Using Table 8 in NEC Ch.9 we find that 3/0 AWG is 167,800 CMs. We multiply this by four and we get 671,200 CMs. According to NEC 250.66 the minimum size of the neutral is 2/0 AWG. Now this would be fine if we were running one conductor per phase but we're not. We're running four. According to Stallcup we are required to run four 2/0 AWG in parallel for the neutral.
Now here is how I would calculate the neutral size: If we look again at Table 8 of ch.9 we find that 2/0 AWG is 133,100 CMs. If we divide this by four we get 33,275 CMs which would give us a wire size of No, 4 AWG. However, in accordance with NEC 310.4(A) the minimum size for conductors in parallel would be 1/0 AWG. So the correct answer I get is four 1/0 AWG conductors in parallel for the neutral.

Question: Which method do you guys think is correct?