Article 230.79(C)

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Davisteam

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Does this article preclude the neutral being de-rated since it reads, “3-wire”? On the Master Electrician exam if my calculated amps for a given service is 78amps, does this automatically mean I should choose a #1AWG conductor since it’s rated for 100amps?
 
Not sure what you mean by derating the neutral? That section is for the service disconnecting means size, in this case 100 amps in a dwelling.
 
You would have to do a calculation for the neutral conductor. There are examples in annex D in the back of the NEC
 
Dennis Alwon said:
You would have to do a calculation for the neutral conductor. There are examples in annex D in the back of the NEC
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Yes, and the first example is the one I’m wondering about. It doesn’t give us the wire size in the example. But it does say that “Sections 230.42(B) and 230.79 require service conductors and disconnecting means rated not less than 100 amperes.” So I believe I cannot have my wire size smaller than #1AWG for any dwelling unit — ever. Is this correct? If so, does this also mean the neutral cannot be smaller than #1AWG, also?
 
No, the neutral can be as small as the grounding electrode conductor if calculations allow or if it is a feeder then it can be as small as the equipment grounding conductor.
 
So the minimum size of a service for a single family dwelling is 100 amps. Then we have a section in the code that allows us to take 83% of the service size for our service conductors. (310.15(B)(7)) So a residence can have service conductors sized at 83 amps for a dwelling. The neutral then can be sized as small as the grounding electrode conductor.
 
So we are allowed to use a100 amp breaker and size the wire smaller than 100 amps for the service even though 230.42(B) says they “shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D)”? I must be missing something.
 
Davisteam said:
So we are allowed to use a100 amp breaker and size the wire smaller than 100 amps for the service even though 230.42(B) says they “shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D)”? I must be missing something.
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Yes, what Dennis stated was correct take a look at 310.15(B)(7).

310.15(B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.15(B)
(16) through Table 310.15(B)(19), and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21) as modified by
310.15(B)(1) through (B)(7).
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Ok. I believe I am understanding now. But what in the world does “specific installations” mean with regards to 230.42 (B)? This is the one that confused me in the first place. By the way, thank you for your time.
 
Dennis Alwon said:
The neutral then can be sized as small as the grounding electrode conductor.
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Yes, the neutral can be reduced if the load calcs permit but the minimum size would be based on 250.102-not the GEC size. It is true that these sizes are one and the same until you you get to 1100 CU or 1750 AL but they are technically serving different purposes. See 250.24(C).
 
texie said:
Yes, the neutral can be reduced if the load calcs permit but the minimum size would be based on 250.102-not the GEC size. It is true that these sizes are one and the same until you you get to 1100 CU or 1750 AL but they are technically serving different purposes. See 250.24(C).
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You are correct Texie, I still haven't gotten that through my skull. The only difference in the tables are when you have very large services, as you stated.
 
If you use the 60° C column you end up with a #3 AWG copper conductor (85 amps) as the minimum size for 100 amps. Using the 75° C column you get to use a #4 (85 amps) which is the same conductor size that's in T310.12 in the 2020 NEC or earlier versions that had the table.
 
Thank you, Sirs! Now if someone could please clear up the meaning of “specific installations” with regard to 230.42 (B) I think I'll be straightened out. This is the article that caused me all the confusion about the sizing of service conductors in the first place.
 
Davisteam said:
Thank you, Sirs! Now if someone could please clear up the meaning of “specific installations” with regard to 230.42 (B) I think I'll be straightened out. This is the article that caused me all the confusion about the sizing of service conductors in the first place.
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Specific installations are the things mentioned in 230.79(A)-(D).
 
texie said:
Specific installations are the things mentioned in 230.79(A)-(D).
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230.79(C) says we cannot use a disconnect smaller than 100A. 230.42(B) says we have to size our service conductors the same as the 100A disconnect. Just want to make sure I'm reading this correctly. There seems to be a contradiction here as it seems to negate our 83% rule.
 
Davisteam said:
230.79(C) says we cannot use a disconnect smaller than 100A. 230.42(B) says we have to size our service conductors the same as the 100A disconnect. Just want to make sure I'm reading this correctly. There seems to be a contradiction here as it seems to negate our 83% rule.
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Ah, I see your confusion I think. Yes the service conductors are sized to the disconnect under the 83% rule. When you qualify for the 83% rule those reduced size conductors have the full ampacity rating under those conditions.
 
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