Service Load/Feeder Sizing Confusion

[Revous]

I feel like after reading the code for a bit I've confused myself a bit.

For Underground and Overhead Conductors (230.31 and 230.23 respectively): The ampacity shall be sized based on the calculations from 220.

But then in Part 4 of Section 230, 230.42 says that service entrance conductors should be size to the larger of 230.42(A)(1) or 230.42(A)(2).

230.42(A)(1): This section mentions the the size shall be not less than 125% of the continuous + 100% of the continuous (Unless you have a 100% rated breaker). Just like you would a regular feeder in section 215.

230.42(A)(2): Doesn't call out 220 specifically but....where else would you get your "maximum load" other than a calculation with demand factors from 220. Or do they just mean the "Total Connected Load"? In which case.... every service I've ever seen sized has been woefully undersized.

The diagram at the start of the 230 implies that section 230.31 and 231.23 apply to the "service drop" conductors which kind of makes sense but in typical commercial services the underground feeder tend to terminate directly into the service disconnect switch or CT cabinet.

Here is where I am confused:

1. 230.42(A)(1) and 230.42(A)(2) implies that you perform (2) separate load calculations. One based on 220 only and the other based on considering the continuous and noncontinuous loads. Unless I misunderstood and it means calc#1 is all of the demand factors+125% of any continuous loads or just full load amps of the service?
2. 230.90(A) says that Service OCP shall not be sized greater than the ampacity of the conductors but if I have a service with total connected load of 3000A but after demanded loads based on 220 I come up with 2000A of load then I still have to go with 3000A even if the demand loads come in lower.

I've seen the Annex D exampled for service calculations but they just left me more confused because the feeders are size to the "actual" load but the sections above are asking me to size the feeder for 100% of non-continuous load and 125% of continuous loads.

 

[augie47]

230.42 Calculate your load (125% of continuous + 100% non-continuous)
Select your conductor to supply that load applying adjustment factors (conduit fill, ampacity, etc)
Select your overcurrent device to protect those conductors per 230.90 (normally based on Art 240.4)\

The conductors must be sized to carry the calculated load and they must be protected against overcurrent. In your case, with a single 3000 amp service protection device, 240.4 would require conductors rated at 3000 amps per 240.4(B).

 

[wwhitney]

If "total connected load" means the simple procedure of adding up all the loads, without reference to Article 220, the NEC never cares about that calculation. Article 220 calculations are always used.

As to the whole "125% of continuous loads plus 100% of non-continuous loads," that is a bit confusing, because the language only occurs outside of Article 220, and Article 220 basically doesn't mention continuous loads. So the conclusion is that when you do your Article 220 calculations, each load that is an input to the calculations has an extra little tag on it that is either Continuous or Non-Continuous, and you track those tags all the way through the calculations to the end. See Example D3 in the Appendix.

Cheers, Wayne