Service drop wire size

[Jsmith84]

I am upgrading a duplex to 200 amp service in each apt. There is one service drop into a dual meter base. I am trying to determine the correct wire size from the drop into the base. I am getting conflicting info from an inspector. I say 700 kcmil. He says only needs to be 500 kcmil aluminum. I am fairly new in the trade and want to make sure I am doing this correctly. Thanks in advance for any tips.

 

[infinity]

How did arrive at the 700 kcmil?

Welcome to the Forum!

 

[Jsmith84]

2 200 amp services would be 400 amps combined and the 700 kcmil is rated for four something I don’t remember off the top of my head was my reasoning...

 

[Jsmith84]

But I may be wrong. Lol I just want the right size wire.

 

[LarryFine]

Do you have load calcs for each unit?

 

[Dennis Alwon]

The inspector is using 240.4(B) which allows you to upsize to the next breaker size however there are no breakers in this case. 500 kcm copper is sufficient would be my guess because the service conductors only need to be as large as the calculated load- if I am not mistaken.

 

[Jsmith84]

Do you have load calcs for each unit?

The calculated load is 120 per unit. Customer wants 200 amp service per unit even though it is not really necessary 125 would be sufficient. But if I’m doing 200 amp service in each unit the way I see it add the two together which would be 400 amps total. I’ve never done a double meter base yet.

 

[Dennis Alwon]

So are you talking copper or aluminum. You cannot use the 90C column for ampacity, you are stuck with the 75C column

 

[Jsmith84]

An inspector told me 350 Kim copper or 500 Kim aluminum. Does this sound correct for this situation is what I’m trying to determine.

 

[Dennis Alwon]

An inspector told me 350 Kim copper or 500 Kim aluminum. Does this sound correct for this situation is what I’m trying to determine.

Yes, it is compliant but that does not give the owner the possibility of using 200 amps each. If the total calculated load is 240 amps totals then 500 kcm aluminum is good for 310 amps and is sufficient

 

[Jsmith84]

Yes, it is compliant but that does not give the owner the possibility of using 200 amps each. If the total calculated load is 240 amps totals then 500 kcm aluminum is good for 310 amps and is sufficient

I want to be able to utilize that if need be. But that would make the wire size 600 copper or 1000 aluminum

 

[Jsmith84]

I want to be able to utilize that if need be. But that would make the wire size 600 copper or 1000 aluminum

Can anyone tell me 310.15 b7 from 2014 nec. Does this mean it can be 83 percent of the total load which would be how the inspector is coming up with his wire sizes?

 

[electrofelon]

The calculated load is 120 per unit. Customer wants 200 amp service per unit even though it is not really necessary 125 would be sufficient. But if I’m doing 200 amp service in each unit the way I see it add the two together which would be 400 amps total. I’ve never done a double meter base yet.

First, are you really supplying the service drop or is the utility? If the former, when you have multiple service disconnects, the common conductors are sized to the load, not the sum of the disconnects. Looking real quick, I think table 310.15(B)(20 is what you would use. You cant use the 83% factor for conductors feeding multiple dwelling units.

 

[infinity]

Can anyone tell me 310.15 b7 from 2014 nec. Does this mean it can be 83 percent of the total load which would be how the inspector is coming up with his wire sizes?

No, the conductors can sized based on the total calculated load of 240 amps. You're free to use any size conductors that are the minimum size required or larger. If you want the full 400 amps then 600 kcmil copper or parallel #3/0 copper will work. Even a single set of 750 kcmil aluminum is too small for the full 400 amps.

 

[kwired]

First, are you really supplying the service drop or is the utility? If the former, when you have multiple service disconnects, the common conductors are sized to the load, not the sum of the disconnects. Looking real quick, I think table 310.15(B)(20 is what you would use. You cant use the 83% factor for conductors feeding multiple dwelling units.

:thumbsup:

service drop is typically POCO supplied and is not subject to NEC.

Here that service drop is likely only 2/0 aluminum. If POCO feels it is justified to have more conductor they will simply parallel another 2/0.

 

[Jsmith84]

:thumbsup:

service drop is typically POCO supplied and is not subject to NEC.

Here that service drop is likely only 2/0 aluminum. If POCO feels it is justified to have more conductor they will simply parallel another 2/0.

Im talking about the service conductors I to the meter. Not the service drop conductors. Sorry fo the confusion

 

[infinity]

Im talking about the service conductors I to the meter. Not the service drop conductors. Sorry fo the confusion

No confusion.

Are you now understanding how the inspector may have arrived at his conductor sizes?

 

[kwired]

Im talking about the service conductors I to the meter. Not the service drop conductors. Sorry fo the confusion

Service drop is a specific term in NEC, but thanks for clearing up what you are asking about.

310.15(B)(7) only applies to conductors supplying a single dwelling.

230.42 only requires these conductors to be sized to the calculated load.

230.90(A) exception 3 will allow overcurrent protection to exceed the conductor ampacity when there is 2-6 service disconnecting means and the calculated load is no more than the conductor ampacity.

Anything that exceeds the calculated load is design decision, NEC is met if that conductor has ampacity of the calculated load or more.

If you want to allow more capacity then calculated load, running parallel 4/0 seems to be a pretty common choice for such a situation, cost of material as well as easier to handle than 500kcmil+ conductors are major players in this decision.

 

[Jsmith84]

No confusion.

Are you now understanding how the inspector may have arrived at his conductor sizes?

Honestly no... He did not ask for my calculations all he said was what size wire are you planning to use I said 700 kcmil aluminum. He said you would think that but you can use 350 copper or 500 aluminum. And I will use that as long as it will be sufficient. I will have two 200 amp disconnects outside and then feeding two 200 amp panels inside.

 

[kwired]

Honestly no... He did not ask for my calculations all he said was what size wire are you planning to use I said 700 kcmil aluminum. He said you would think that but you can use 350 copper or 500 aluminum. And I will use that as long as it will be sufficient. I will have two 200 amp disconnects outside and then feeding two 200 amp panels inside.

Inspector technically is wrong if he doesn't know the load. However sizes he did give you probably are sufficient over 90% of the time. Your NEC calculated load was 120 amps per unit. NEC is overkill most of the time, but they want to assure the conductor is heavy enough for the load. Reality is each unit likely seldom sees more than 60 amps and if it does it is not for very long.