Required use of 250 KCMIL for the line side of a main disconect on a 400 amp service

[Angel Electric]

I have been asked to derate my main circuit breakers because I used 4/0 aluminum between the main disconects and the meter. I was told it was a Dept of insurance ruling and it was in the NEC. I use 4/0 Uluminum XHHW for a 200 amp service, which is what the two main disconect are rated 200 Amps . Does anyone know of the ruling as well as the reference to derating a service riser conductor?

 

[augie47]

The 4/0 is commonly used here without question, however, by right the E/C should present load calculations on each set of conductors as they are only rated at 180 amps.
310.15(B)(7) does not apply as there are two sets so neither carries the full house load on its own.

 

[Dennis Alwon]

I will assume this is a residence. If so then you can use 4/0 as long as the calculated load on each panel is less than 180 amps-- assuming 75C. You can use article 240.4(B) to upsize the breaker to 200 amps and be compliant. Now if you are using se cable that may be rated 60C then that may be a different issue. Where are you in NC

 

[jxofaltrds]

View attachment 12327

I have been asked to derate my main circuit breakers because I used 4/0 aluminum between the main disconects and the meter. I was told it was a Dept of insurance ruling and it was in the NEC. I use 4/0 Uluminum XHHW for a 200 amp service, which is what the two main disconect are rated 200 Amps . Does anyone know of the ruling as well as the reference to derating a service riser conductor?

I am guessing that since 'it' is not carrying the entire load is the problem. So Table 310.15(B)(16) 70c = 180 Amps

Why a #4 to the ground rods?

 

[iwire]

The 4/0 is commonly used here without question, however, by right the E/C should present load calculations on each set of conductors as they are only rated at 180 amps.
310.15(B)(7) does not apply as there are two sets so neither carries the full house load on its own.

I agree with Gus

 

[Carultch]

Now if you are using se cable that may be rated 60C then that may be a different issue. Where are you in NC

Is SE cable different than SER and SEU?

Most datasheets I've seen, show SER and SEU cable as being 90C rated. Southwire, Encore and Cerrowire.

They indicate that it is built with THWN-2 or XHHW-2 individual conductors, wrapped in an outdoor-rated PVC jacket.

 

[electrofelon]

Is SE cable different than SER and SEU?

Most datasheets I've seen, show SER and SEU cable as being 90C rated. Southwire, Encore and Cerrowire.

They indicate that it is built with THWN-2 or XHHW-2 individual conductors, wrapped in an outdoor-rated PVC jacket.

Its similar to NM in that the insulation is rated 90, but the NEC requires it to be used at a lower ampacity. The requirement kicks in when it is "used like romex" see 338.10(B)(4). The NEC as waffled back and forth over the last few code cycles on the specific circumstances that require it to be derated.

 

[kwired]

Is SE cable different than SER and SEU?

Most datasheets I've seen, show SER and SEU cable as being 90C rated. Southwire, Encore and Cerrowire.

They indicate that it is built with THWN-2 or XHHW-2 individual conductors, wrapped in an outdoor-rated PVC jacket.

Its similar to NM in that the insulation is rated 90, but the NEC requires it to be used at a lower ampacity. The requirement kicks in when it is "used like romex" see 338.10(B)(4). The NEC as waffled back and forth over the last few code cycles on the specific circumstances that require it to be derated.

Equipment terminals are generally rated 75C so you won't be using 90C conductor ampacity with any conductor - the problem with NM and SE cable (depending on which code edition applies) is the NEC requirement to use them at 60 C even if the terminals or conductors are rated higher.

 

[kwired]

View attachment 12327

I have been asked to derate my main circuit breakers because I used 4/0 aluminum between the main disconects and the meter. I was told it was a Dept of insurance ruling and it was in the NEC. I use 4/0 Uluminum XHHW for a 200 amp service, which is what the two main disconect are rated 200 Amps . Does anyone know of the ruling as well as the reference to derating a service riser conductor?

Outside of any local amendments - NEC generally says a conductor needs to have an ampacity greater or equal to the load served. This is stated in different sections depending on application but remains fairly consistent - primarily is stated in 210, 215, and in 230 to apply to branch circuits, feeders, and to service conductors.

In general we must also protect the conductor at it's ampacity - with exception to go to next higher standard size overcurrent device.

So this means a 4/0 aluminum conductor with an un-adjusted ampacity can supply a 180 amp load and can be protected at next higher overcurrent device which is 200 amps. If your load calculation is 181 amps you still can have a 200 amp overcurrent device but your conductor size needs to be larger then 4/0 aluminum as it is only good for 180.

 

[mwm1752]

View attachment 12327

I have been asked to derate my main circuit breakers because I used 4/0 aluminum between the main disconects and the meter. I was told it was a Dept of insurance ruling and it was in the NEC. I use 4/0 Uluminum XHHW for a 200 amp service, which is what the two main disconect are rated 200 Amps . Does anyone know of the ruling as well as the reference to derating a service riser conductor?

the drawing could be mounted on a single family residence - NEC 2014 requires single family services up to 400 amp to have a min of 83% of the service rating. 400 x 83% = 332 amps- 180 x 2 = 360 amps well within the limits. It also meets the requirements if it is a two family residence . 200 x 83%= 166 amps -- And of course serves the load.

 

[kwired]

the drawing could be mounted on a single family residence - NEC 2014 requires single family services up to 400 amp to have a min of 83% of the service rating. 400 x 83% = 332 amps- 180 x 2 = 360 amps well within the limits. It also meets the requirements if it is a two family residence . 200 x 83%= 166 amps -- And of course serves the load.

I think the conductors between the meter and each 200 amp main were what was in question.

Still even in the absence of 310.15(B)(7)applications a 4/0 aluminum can be protected by a 200 amp overcurrent device as long as the load is 180 or less.

 

[mwm1752]

I think the conductors between the meter and each 200 amp main were what was in question.

Still even in the absence of 310.15(B)(7)applications a 4/0 aluminum can be protected by a 200 amp overcurrent device as long as the load is 180 or less.

agreed