How to size service conductors

[patricknola]

Good morning, I'm replacing the panel and meter socket on 1/2 of a duplex. They're getting an upgrade to a 200 amp service, the other half will stay 100 amps. There is a single service mast common to both meters that will have to be replaced. Do I add 200 + 100 and then multiply times .87? What other debating factors could I consider? TIA

 

[infinity]

Since you have a single mast you would need a load calculation (not adding 200+100) for both dwellings. Even though your upgrading the service technically if you're not adding any load the existing conductors can likely remain. If load were added in the future then the conductor size would need to be addressed at that point.

 

[Fred B]

Since you have a single mast you would need a load calculation (not adding 200+100) for both dwellings. Even though your upgrading the service technically if you're not adding any load the existing conductors can likely remain. If load were added in the future then the conductor size would need to be addressed at that point.

But you still need to provide wire that is capable of handling the potential maximum apply by to each. Thus wouldn't you need to upgrade the wire to the load potential? (this case 200+100).

The change by the OP would likely result in 2- 200A meter installation. (Not aware of a 200/100A meter stack.) And if already doing it, the Labor side wouldn't change much to provide the capacity for 2-200A, and materials only slightly different between 300A and 400A. Also If upgrade of equipment is being sought to set up for need to future corrections with a relatively small change has no advantage and can lead to HO hard feelings when it is later needed. Seen this sort of issue many times where someone saw 2-200A and felt they could up the 100 to a 200 not even looking at the service wire. (before you even say it I know we don't and can't always, and not required to "future proof" an installation)

The math involved would only realistically be helpful if you are trying to fit in a lessor size wire and then limiting the max current on the wiring, one means to do this is to have a lower back feed breaker on each. No real advantage on the small resi systems. Most homes here until you start adding a lot of EVSE and full electrification are only using about 80A max, even on a full boat 200A panel. But still wire to the 83% 200A.

 

[infinity]

But you still need to provide wire that is capable of handling the potential maximum apply by to each. Thus wouldn't you need to upgrade the wire to the load potential?

No you do not. The service conductors in the riser can be sized according to the load calculation of the two dwelling units. Say the present conductors are rated for 150 amps and the load calculation says that they're adequate in size. Changing the one service from 100 to 200 amps without adding any load does not change the 150 amp conductor size because the load calculation has already determined that they are adequate for the load.

 

[Dennis Alwon]

I agree with Trevor but IMO changing the service is worthless without taking that into consideration for the riser conductors. I would size then for 300 amps x .83 not .87

 

[infinity]

I agree with Trevor but IMO changing the service is worthless without taking that into consideration for the riser conductors.

I agree and I would upgrade the riser conductors too. I was just providing the code complaint way of sizing the riser conductors which is a load calculation for the two dwellings. If no additional load is added the existing conductors may already be adequately sized.

 

[Dennis Alwon]

I agree and I would upgrade the riser conductors too. I was just providing the code complaint way of sizing the riser conductors which is a load calculation for the two dwellings. If no additional load is added the existing conductors may already be adequately sized.

I don't understand why that rule is allowed. If it is a single service you couldn't do that. In fact, a single service of 200 amps requires 2/0 copper but the load calc for a 200 and a 100 amp service may actually require a conductor smaller than 2/0 or the same size. Unless I am missing an exception somewhere

 

[infinity]

I don't understand why that rule is allowed. If it is a single service you couldn't do that. In fact, a single service of 200 amps requires 2/0 copper but the load calc for a 200 and a 100 amp service may actually require a conductor smaller than 2/0 or the same size. Unless I am missing an exception somewhere

I don't understand it either but it's part of the code.

 

[augie47]

I agree with Rob. 230.42 & 230.90 only requires the service conductors be sized to the calculated load when supplying multiple OCP devices.
Although it would be prudent to upsize them if you are upsizing the equipment.
Just a sidenote also: when supplying multiple dwellings, I don't think the .83 multiplier comes into play