sizing service conductors for 400A service?

[Whale Rage Forever]

If I have a 400A underground fed residential service which feeds a meter can that feeds 2 200A service disconnects immediately adjacent-one on either side-fed from double lugged terminals...do the service conductors from the meter to the disconnects still have to meet the rating requirements for 400A, even though they're immediately separating into two 200A disconnects, or would this be like using the tap rules and I could use two sets of wires individually suited for 200A service disconnects?

Obviously, having to size the wires and associated system changes for 400A service conductors is more difficult than for 200A and one would be much more convenient than the other, but I want to learn the answer beforehand instead of in front of an inspector.

 

[LarryFine]

If I have a 400A underground fed residential service which feeds a meter can that feeds 2 200A service disconnects immediately adjacent-one on either side-fed from double lugged terminals...do the service conductors from the meter to the disconnects still have to meet the rating requirements for 400A, even though they're immediately separating into two 200A disconnects, or would this be like using the tap rules and I could use two sets of wires individually suited for 200A service disconnects?

The latter.

 

[Whale Rage Forever]

The latter.

Heck yes.

Thank you.

 

[Whale Rage Forever]

I suppose while I'm here I may as well ask another question.

Would the two sets of service conductors in this scenario still be applicable for the 83% ampacity adjustment of 310.15.7?

 

[LarryFine]

I suppose while I'm here I may as well ask another question.

Would the two sets of service conductors in this scenario still be applicable for the 83% ampacity adjustment of 310.15.7?

No, but, as an example: as long as each panel's load is within its ampacity, 4-0 al. is still large enough for a 200a main breaker.

 

[infinity]

do the service conductors from the meter to the disconnects still have to meet the rating requirements for 400A, even though they're immediately separating into two 200A disconnects, or would this be like using the tap rules

They would be sized according to the 200 amp OCPD's. As Larry stated #4/0 Al=180 amps next size up rule 200 amp OCPD. There are no tap condcutors ahead of the service disconnects.

 

[shortcircuit2]

For an underground dwelling service fed to 1-320 meter on into 2-200-amp service disconnect switches.

My understating is that it is not really a "400-amp" service although it is typically referred as such. It is a service sized per the load calculation with 2- 200-amp switches...

310.12(A) does not apply to the service conductors to each 200-amp service disconnect because you have 2 switches...

But, 4/0 into each 200-amp switch is OK as long as the load is not over 180-amps...tap rules do not apply to service conductors, but next size up rule is permitted...

The underground lateral to the meter must be sized for the calculated load. It could be 1-set of 350MCM aluminum as long as the dwelling load calculation doesn't exceed 300-amps per 310.12(A)

 

[Strathead]

I suppose while I'm here I may as well ask another question.

Would the two sets of service conductors in this scenario still be applicable for the 83% ampacity adjustment of 310.15.7?

I think Larry's answer is too simple without an if or a but... Here is one thing to keep in mind always when sizing wire. IN no case, ever can the wire be smaller than the load it is intended to carry. In most cases that is the calculated load, but you can do a 30 day study to determine the load also. Keep this in mind for ANY wire sizing. Note this isn't associated with the overcurrent protection. Those rules are different, but again, IN no case, ever can the wire be smaller than the load it is intended to carry.

 

[LarryFine]

The underground lateral to the meter must be sized for the calculated load. It could be 1-set of 350MCM aluminum as long as the dwelling load calculation doesn't exceed 300-amps per 310.12(A)

In most places, the POCO has control and responsibility over everything on the line side of the meter.

 

[LarryFine]

I think Larry's answer is too simple without an if or a but... Here is one thing to keep in mind always when sizing wire. IN no case, ever can the wire be smaller than the load it is intended to carry.

I did mention that, perhaps less than clearly:

No, but, as an example: as long as each panel's load is within its ampacity, 4-0 al. is still large enough for a 200a main breaker.

"Its ampacity" referring to the 4-0 al. wire.

 

[wwhitney]

IN no case, ever can the wire be smaller than the load it is intended to carry.

Except per 310.12(A).

Cheers, Wayne

 

[Whale Rage Forever]

For an underground dwelling service fed to 1-320 meter on into 2-200-amp service disconnect switches.

My understating is that it is not really a "400-amp" service although it is typically referred as such. It is a service sized per the load calculation with 2- 200-amp switches...

310.12(A) does not apply to the service conductors to each 200-amp service disconnect because you have 2 switches...

But, 4/0 into each 200-amp switch is OK as long as the load is not over 180-amps...tap rules do not apply to service conductors, but next size up rule is permitted...

The underground lateral to the meter must be sized for the calculated load. It could be 1-set of 350MCM aluminum as long as the dwelling load calculation doesn't exceed 300-amps per 310.12(A)

I'll just use 3/0 copper and be done with it. It's only going a few feet over to either side.

The poco has already told me I need to install 3" conduit to the transformer pedestal.

Which code cycle are you referring to 310.12. I'm in 2017 and don't see it?

 

[don_resqcapt19]

I'll just use 3/0 copper and be done with it. It's only going a few feet over to either side.

The poco has already told me I need to install 3" conduit to the transformer pedestal.

Which code cycle are you referring to 310.12. I'm in 2017 and don't see it?

It is 310.15(B)(7) in the 2017. Things moved in the 2020.

 

[don_resqcapt19]

Except per 310.12(A).

Cheers, Wayne

The only reason 310.12 even exists is because they know that the load calculation in Article 220 and very very conservative. Could probably change the 83% to 60% without any problems.

 

[electrofelon]

If I have a 400A underground fed residential service which feeds a meter can that feeds 2 200A service disconnects immediately adjacent-one on either side-fed from double lugged terminals...do the service conductors from the meter to the disconnects still have to meet the rating requirements for 400A, even though they're immediately separating into two 200A disconnects, or would this be like using the tap rules and I could use two sets of wires individually suited for 200A service disconnects?

Obviously, having to size the wires and associated system changes for 400A service conductors is more difficult than for 200A and one would be much more convenient than the other, but I want to learn the answer beforehand instead of in front of an inspector.

Just want to expand on one thing that was mentioned. The conductors betwixt the transformer and the meter. Note those are serving 2 service disconnects. 230.90 something allows service conductors that are serving more than one service disconnect to be sized to the load. You don't have to necessarily size to the sum of the disconnects. This is handy because I will often "cheat" a little bit on residential 400 amp services because I know I pretty much never need the full 400 amps. I typically use what is most convenient. For example two sets of 4/0 aluminum, even in the same raceway with the 80% derating, saves quite a bit of money and hassle over going up to say 250 and/or two separate raceways. Or one set of 500 AL is a good one too.

 

[kwired]

They would be sized according to the 200 amp OCPD's. As Larry stated #4/0 Al=180 amps next size up rule 200 amp OCPD. There are no tap condcutors ahead of the service disconnects.

But- if load calculation is 181 to 200 then you still would need to go one size larger with the conductor.

 

[infinity]

But- if load calculation is 181 to 200 then you still would need to go one size larger with the conductor.

Correct. As I stated Larry had mentioned that in post #5.

 

[Whale Rage Forever]

Just want to expand on one thing that was mentioned. The conductors betwixt the transformer and the meter. Note those are serving 2 service disconnects. 230.90 something allows service conductors that are serving more than one service disconnect to be sized to the load. You don't have to necessarily size to the sum of the disconnects. This is handy because I will often "cheat" a little bit on residential 400 amp services because I know I pretty much never need the full 400 amps. I typically use what is most convenient. For example two sets of 4/0 aluminum, even in the same raceway with the 80% derating, saves quite a bit of money and hassle over going up to say 250 and/or two separate raceways. Or one set of 500 AL is a good one too.

My understanding is that the POCO where I am supplies the wiring from the transformer to the meter.

 

[electrofelon]

My understanding is that the POCO where I am supplies the wiring from the transformer to the meter.

Could be. That is common in some places, not common in other places.