Service entrance conductor size for a duplex

[doje]

A friend owns a duplex that currently has two weatherhead‘s right next to each other fed by a single service drop from the pole. PG&E (local power company) no longer allows this scenario. In order to do a service change and install two new 200 amp panels, we have to do a single weatherhead with a 400 amp drop. Then it goes into a hot gutter which then feeds the new 200A meters and panels.

I know that if this was a single dwelling, I could use the 83% rule which would allow me to use 400 kcmil copper for the 400A service.

Am I able to still use the 400 kcmil and feed two services by splicing to 2/0 copper in the hot gutter? Or does the drop from the weather head now have to be fully rated at 600A @ 75 degrees?

My first post here so let me also add a big thanks for all the good info I’ve gleamed from this site over the years.

 

[don_resqcapt19]

310.15(B)(7) does not apply to the conductors between the weatherhead and the wireway.

 

[doje]

Can you explain why it wouldn’t apply? And does that mean I’d need to run 600 kcmil from the weatherhead?

 

[augie47]

It doesn't apply because that's not the way it's worded. The 83% rule is for single dwelling units.
If you are feeding a duplex you can use Art 220 to size your service and install accordingly.

 

[doje]

It doesn't apply because that's not the way it's worded. The 83% rule is for single dwelling units.
If you are feeding a duplex you can use Art 220 to size your service and install accordingly.

Well in this case the services are already sized at 200A each to account for all future (A/C, EV, etc.). So would I be correct that I’d need 600 kcmil copper to feed the hot gutter?

 

[wwhitney]

If you want to support the future case where the Art 220 service load calculation for each dwelling unit comes out to the full 200A, yes.

But if each service calced out to 50A currently (exaggerated example), I believe you could use #3 Cu from the weatherhead to the gutter. You'd need to upsize it per the Art 220 calculation each time you add load to one of the units.

Cheers, Wayne

 

[david luchini]

Well in this case the services are already sized at 200A each to account for all future (A/C, EV, etc.). So would I be correct that I’d need 600 kcmil copper to feed the hot gutter?

You would need a conductor with sufficient ampacity for the calculated load for both units.

Either from Art 220 Part III, or 220.85.

 

[doje]

So (for arguments sake) if the calculated loads is 200A per service (400A total), I’d just refer to 310.15(B)(16) 75 degree table and would require 600 kcmil. Correct or am I missing something? Because that was my initial thought. I definitely wouldn’t want to have to come back and upsize in the future. Especially when two units are involved.

 

[david luchini]

So (for arguments sake) if the calculated loads is 200A per service (400A total), I’d just refer to 310.15(B)(16) 75 degree table and would require 600 kcmil. Correct or am I missing something? Because that was my initial thought. I definitely wouldn’t want to have to come back and upsize in the future. Especially when two units are involved.

If each unit had a calculated load of 200A, the total calculated load could be lower than 400A because of the demand factors.

 

[Dennis Alwon]

If each unit had a calculated load of 200A, the total calculated load could be lower than 400A because of the demand factors.

Doesn't the demand factor only come into play with 3 or more units?

 

[david luchini]

Doesn't the demand factor only come into play with 3 or more units?

Not exactly. The optional demand factor for Two dwelling units is under 220.85.

But the standard demand factors would apply to any number of dwelling units on the same service or feeder.

 

[Dennis Alwon]

Not exactly. The optional demand factor for Two dwelling units is under 220.85.

But the standard demand factors would apply to any number of dwelling units on the same service or feeder.

But that says if the two units are supplied by a single feeder.... I don't see that here

 

[Dennis Alwon]

Or does that mean each unit is fed by a single feeder? I don't think so.

 

[david luchini]

But that says if the two units are supplied by a single feeder.... I don't see that here

Never noticed that...it doesn't make sense, as 220.84 says "feeder or service."

But I was originally thinking of the standard demand factors in Part III. The demand for both units would be lower than the demand for the separate units added together.

 

[Dennis Alwon]

Regardless I think the op was just trying to figure out if he had a full 400 amp calculation could he use 600kcm.

600 kcm al is only good for 340 amps so It is not big enough

 

[augie47]

So (for arguments sake) if the calculated loads is 200A per service (400A total), I’d just refer to 310.15(B)(16) 75 degree table and would require 600 kcmil. Correct or am I missing something? Because that was my initial thought. I definitely wouldn’t want to have to come back and upsize in the future. Especially when two units are involved.

For arguments sake, yes.
From a practical standpoint, finding a duplex where the calculated load for the two units is 400 amps would be as rare as hen's teeth.

 

[doje]

Regardless I think the op was just trying to figure out if he had a full 400 amp calculation could he use 600kcm.

600 kcm al is only good for 340 amps so It is not big enough

I was suggesting 600 kcmil copper.

Also, would my GEC sizing be based on the wire coming down from the 600 weatherhead to the hot gutter? Or the 3/0 that’s actually feeding each individual service?

 

[david luchini]

I was suggesting 600 kcmil copper.

Also, would my GEC sizing be based on the wire coming down from the 600 weatherhead to the hot gutter? Or the 3/0 that’s actually feeding each individual service?

I would imagine 500 kcmil copper would be more than large enough.

I believe you could run a common GEC (based pm the 600kcmil) with taps to each disconnecting means, or individual GECs to each disconnect (based on the 3/0.)

 

[Dennis Alwon]

I was suggesting 600 kcmil copper.

Also, would my GEC sizing be based on the wire coming down from the 600 weatherhead to the hot gutter? Or the 3/0 that’s actually feeding each individual service?

IMO you would have to size it to the largest service conductor. Either 600 kcm or 2 x (167800) for 3/0 cm-- obviously the 600 kcm is larger

 

[electrofelon]

. Then it goes into a hot gutter which then feeds the new 200A meters and panels.

Any reason not to use a 2 gang meter socket?

I usually run parallel 4/0 Al in the same riser for 2 200's and call it plenty. Or 2/0 CU if you insist on copper.