NEC 2017 - 240.4(B)(2) for residential

[fandi]

Hello All,
Can #2/0 CU wires be protected by a 200A main breaker?
#2/0 wires are OK for 200A service (ampacity wise) per 83% rule but per 240.4(B)(2), next size breaker is permitted only if the ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker. And per table 240.6(A), 175A is a standard breaker so it seems that #2/0 wires can't be protect by a 200A breaker.
What do you think?

 

[tom baker]

Yes thats a common arrangement for wire size and OCPD. The table in 2017 NEC was removed but was in the back of the NEC. Now for the 2020 NEC its back as Table 310.12 and lists 2/0 Cu for a 200 A service or feeder, but this is only allowed if the service or feeder carries the entire load of the dwelling unit. Start with section 310.12, read all the parrts

 

[fandi]

Yes thats a common arrangement for wire size and OCPD. The table in 2017 NEC was removed but was in the back of the NEC. Now for the 2020 NEC its back as Table 310.12 and lists 2/0 Cu for a 200 A service or feeder, but this is only allowed if the service or feeder carries the entire load of the dwelling unit. Start with section 310.12, read all the parrts

In NEC 2017, that residential table was in text (see 310.15(B)(7)) so table or not, it's essentially the same 83% rule. What I don't get it is how can #2/0 wires are OK to be protected by a 200A breaker when a combination of 240.4(B)(2) and table 240.6(A) states otherwise. Also 310.12 doesn't exist in NEC 2017 and the OP post pertains only to NEC 2017.

 

[david luchini]

In NEC 2017, that residential table was in text (see 310.15(B)(7)) so table or not, it's essentially the same 83% rule. What I don't get it is how can #2/0 wires are OK to be protected by a 200A breaker when a combination of 240.4(B)(2) and table 240.6(A) states otherwise. Also 310.12 doesn't exist in NEC 2017 and the OP post pertains only to NEC 2017.

Because 240.4 says (before you even get to 240.4(B)(2) or T240.6(A)) conductors shall be protected against overcurrent in accordance with their ampacities specified in 310.15.

310.15(B)(7) is part of 310.15.

 

[david luchini]

If you have #2/0 Cu which is rated for 175A, which is a standard size... you would could not upsize it. From an example 2020 NEC handbook, if you have 500 mcm Cu, which is rated for 380A, it would be allowed to be protected by an OCP of 400A.

The OP is asking about residential. You can protect #2/0 Cu service or feeder conductors with a 200A OCPD in a residential application per 310.15(B)(7) (or 310.12 in the 2020 NEC.)

 

[fandi]

Because 240.4 says (before you even get to 240.4(B)(2) or T240.6(A)) conductors shall be protected against overcurrent in accordance with their ampacities specified in 310.15.

310.15(B)(7) is part of 310.15.

Hi Dave,
The general statement in 240.4 basically points to 310.15. And 310.15(B)(7) only talks about 83% rule ampacity wise but it does not talk about how to size the breaker protecting the conductors. It's 240.4(B)(2) that talks in details about breaker sizing.

 

[Rock86]

IMO, 310.15(B)(7) is telling you that your #2/0 Cu is acceptable for a 200A service/feeder and therefore is acceptable under a 200A OCPD. And 240.4 states "...shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G)." and to me that says if 310.15 tells me I can use 200A OCPD, than I don't need to look into A through G. I'm not saying to omit those sections... it is a case by case situation.

 

[david luchini]

Hi Dave,
The general statement in 240.4 basically points to 310.15. And 310.15(B)(7) only talks about 83% rule ampacity wise but it does not talk about how to size the breaker protecting the conductors. It's 240.4(B)(2) that talks in details about breaker sizing.

Read 310.15(B)

 

[augie47]

When you asked the question at first thought: Wow how did we overlook that" but then realized you have to approach it from a different standpoint. Once we determine a 200 amp service is adequate for the load, 310.15(B)(7) allows us to use a conductor with 83% of the 200 amps (166 amp) thus a 2/0.
It does require one to look at that rule independent of 240.4(B).

 

[jaggedben]

If 310.15(B)(7) didn't allow you to put 2/0 on a 200A service or feeder (in a dwelling) then you'd have to use a smaller breaker, and then your supply wouldn't be 200A anymore so you wouldn't have a 200A service or feeder and 310.15(B)(7) would be contradicting itself.

Did I say that clearly?