310.12 (83% rule) and 230.90(A) Exception 3 (Multiple service OCPD)

[wwhitney]

Let's say we have a residential service going to a Class 320 meter/main with (2) 200A OCPD. 230.90(A) Exception 3 says that the SEC ampacity only needs to be at least the calculated load, not necessarily a full 400A. And 310.12(A) says the SEC ampacity only has to be 83% of the "service rating".

Further, let's say the load calculation comes out to 360A non-continuous, no ampacity adjustment or correction is required, and the meter socket has 75C line side terminals. May we choose to install 350 kcmil Cu SECs? Possible answers below.

Cheers, Wayne

Answer (1): 350 kcmil Cu SECs have a 75C ampacity of 310A. 310/0.83 = 373A, so we have a 373A "service rating". No problem with 360A calculated load.

Answer (2): "Service rating" has to be a standard OCPD size. So we can just look at Table 310.12, and see that 350 kcmil Cu SECs correspond to a 350A service rating. Too small for a 360A calculated load.

Answer (3): Other?

 

[shortcircuit2]

Good question.
For this service with multiple switches, I have interpreted the "service rating" to be that of the allowable ampacity of the SECs

(3) Other- If you install 1-set of copper SECs for 360A load, I would say 400kcmil per table 310.12(A)

 

[augie47]

Likely not the correct way but I would have taken my (2) 200s to equal a 400 amp service and 83% at 332 amps and used a 400 kcmil

 

[jaggedben]

I go with (1).

The table is permitted to used, not required to be used. Key point.

For a service that didn't use 310.12, your method (1) (without the 83% factor) would clearly be correct.

The concept of the table, I believe, is to simplify the thinking process for a single service OCPD. The table checks out if the OCPD is the 'weakest link', i.e. lowest rated component.

However nothing in 310.12 prohibits the otherwise applicable parts of 230.90 or 240.4(B). I fact, I would say that if a proper load calc is done, it doesn't matter whether your scenario has (2) 200A OCPDs or (1) 400A OCPD.

The thing is, residential electricians almost never ever do proper load calculations in my experience. The table eliminates the possibility of an improper application of 240.4(B) when a load calc is not done. So it's also a key point that you gave a load calc as a given.

 

[wwhitney]

However nothing in 310.12 prohibits the otherwise applicable parts of 230.90 or 240.4(B). I fact, I would say that if a proper load calc is done, it doesn't matter whether your scenario has (2) 200A OCPDs or (1) 400A OCPD.

When there's only one service OCPD, I think it is reasonable to call the "service rating" the rating of the service OCPD. So then with a 400A "service rating" we'd need 400 kcmil Cu, not 350 kcmil Cu, as per Table 310.12.

There's actually a conflict between 240.4 and 310.12. Since 310.12 doesn't raise the conductor ampacity, just the "service rating", compliance with 240.4 is only possible through 240.4(B). But 400 kcmil Cu has a 75C ampacity of 335A, so 240.4(B) limits us to a 350A OCPD. The conclusion is that Table 240.4(G) needs an extra entry pointing to 310.12 for residential services/feeders.

In fact, Table 240.4(G) also needs an entry pointing to 230.90 Exception 3 for services.

Cheers, Wayne

 

[jaggedben]

When there's only one service OCPD, I think it is reasonable to call the "service rating" the rating of the service OCPD.

That's no more reasonable than using the conductor rating. Logically, the rating of the service is the lowest rated component in series, the 'weakest link.'

That said, I was partially wrong. Because 230.90 does *not refer to Article 240*. So in the case of a single OCPD, the conductor ampacity (as allowed by 310.12, see exception) must meet or exceed the OCPD rating, unless you are using the exception for 2-6 disconnects.

...In fact, Table 240.4(G) also needs an entry pointing to 230.90 Exception 3 for services.

Did you notice 240.4(H)? Already addresses this.

 

[wwhitney]

Did you notice 240.4(H)? Already addresses this.

Yes, I overlooked it, expecting an entry in 240.4(G), rather than a new subsection. However, 2023 NEC 240.4(H) and 230.90(A) Exception 5 address 310.12, while 230.90(A) Exception 3 (multiple service OCPDs) is still unaddressed in 240.4, which is what my quoted comment was about.

But 240.4(H) says "Dwelling unit service and feeder conductors shall be permitted to be protected against overcurrent at the ampacity values in 310.12." Which is very odd, as 310.12 doesn't change ampacity values, it just assigns a higher "service rating" or "feeder rating" to some services or feeders. So taken at face value, it is of no use.

What they meant to say was obviously "shall be permitted to be protected against overcurrent at the service or feeder rating in 310.12." With that obviously intended amendment, then yes, 240.4(H) would require 400 kcmil Cu for a single 400A residential service OCPD.

The language in 230.90(A) Exception 5 is slightly better: "Overload protection in accordance with the conductor ampacities of 310.12 shall be permitted for single-phase dwelling services." Again it would be clearer if it said "overload protection matching the service ratings of 310.12(A)". But if you infer that "service rating" = "rating of service OCPD", then it works fine as is, unlike 240.4(H).

Cheers, Wayne

 

[tortuga]

I go with #1 and 230.90(A) Exception 3 pretty often. However there are many inspectors that dont' like it, for the reasons you all mentioned, load calcs are often ignored, wrong or just forgotten.
400kcmil CU strikes me as an odd size wire in general.
I have used 500 kcmil Al to a 320A meter then split to two 200A disco's.
I have also done 350 AL to 320A meter then split off to two 150A panels or a 125A and a 200A.
all based on
I don't think I have ever used 400 kcmil I just checked and my supply house has 16 feet of 400 CU and 2300 Feet of 500 kcmil CU in stock so i suppose people do use it.

 

[tortuga]

In my last post the all based on sentence should have been "all based on 230.90(A) Exception 3".