Annex D Example D3(a)

[gunpowder]

First, thanks to forum for providing real world examples to supplement nec while studying for MD masters exam. This process would have been harder and much less interesting w/o y'all. Probably should have moved on, but sometimes can't get past details:dunce:. I've searched the question, and one person asked a couple years ago here but the replies got stuck on another issue and op never got directly answered.

Annex d example d3(a): Subtotal actual load 95.5kVa. Directly adjacent, actual amperage is calculated using actual load of 99kVa. Where did the 99kVa come from? One reply to op in other thread gave power factor as the reason. I haven't seen pf used in any load adjustments in nec.

thanks in advance

 

[jumper]

Best I can figure is that is an adjusted total ampacity of 90C conductors because of the 35C ambient temperature.

The way it is just stuck in right there is confusing.

95.5 kVA x .96 = 99kVA rounded down.

Then it explains how if you had a 100% assembly one could use a 125A OCPD instead of a 150A of course, it goes on to say 100% rated assemblies are rare in that size, so it is a bit silly to even insert the 99kVA at this point.



I like this part of the example better, a lot clearer IMO:

Minimum size conductors in the raceway based on actual load [see
Article 100, Ampacity, and 310.15(B)(3)(a) and correction factors to Table
310.15(B)(16)]:

95,500 VA / 0.7 / 0.96 = 142,000 VA
[70% = 310.15(B)(3)(a)] & [0.96 = Correction factors to Table
310.15(B)(16)]

Conversion to amperes:
142,000 VA / (480V × √3) = 171 A

 

[gunpowder]

Thanks Jumper, silly is right and still unclear, to me. To go with your thought, they applied .96 temperature correction factor from 310.15B2a; why not continue with B3a? Further, 95,500VA/.96=99,479, or 99,500 using 3 significant digits as specified in the example. They have 99,000:?. Am I obsessing?

It's not exactly confusing, but the correcting factors from 310.15 are ampacity adjustments, why are they being applied to the load calcs before amperage calcs?

 

[Smart $]

FWIW, the 99,000VA was corrected in the 2014 edition to 95,500VA and the result of correcting the 42,400VA noncontinuous load to 38,900VA.

 

[gunpowder]

Thanks smart, so I wasn't obsessing for nothing. Do I have a special edition 2014 w/o correction$?

Now I'm trying to figure out how they came up with range VA for feeder neutral loads from service eq to meter for 20 & 40 unit calcs in example D4(b). Individual unit seems ok, but range demand factor doesn't jibe with what I'd expect. Edit: answer found here: http://forums.mikeholt.com/showthread.php?t=185251