1. ## Duplex service conductor

Based on 2014:

I was reviewing my code book and got to thinking about duplex service conductors and I did my own practice load calcs and here is what I came up with.

1.Unit A & B each calculated load is 67A and 62A
2.Minimum service rating for each is 100Amps based on 230.79(C)
3.Single overhead riser conduit with single set service conductors is based on 129 amps
4.Based 310.15(b)(16) my over head service conductors are based on the calculated load of both units 129 amps = #1 copper conductors

Am I correct?

Or size of the service conductors is based on the main over current protection times two (so 200Amp and use 3/0)

Thank you

2. If 310.15(16) applies you are correct.

But 310.15(16) is for not more then 3 current carrying conductors in raceway cable or earth with ambient temp of 30C.

conductors in free air get to use a different table, which one applies to your conductors?

Quite often this is POCO's service drop cable and they don't have to comply with NEC either. Many of them around here would drop a #2 aluminum to this service.

3. Originally Posted by kwired
If 310.15(16) applies you are correct.

But 310.15(16) is for not more then 3 current carrying conductors in raceway cable or earth with ambient temp of 30C.

conductors in free air get to use a different table, which one applies to your conductors?

Quite often this is POCO's service drop cable and they don't have to comply with NEC either. Many of them around here would drop a #2 aluminum to this service.

The service conductors in my question is not the service drop. I know the service drop is under the POCO jurisdiction. The 3 CCC does not apply because I have two hots and one neutral in the service riser conduit.

4. Originally Posted by edward
The 3 CCC does not apply because I have two hots and one neutral in the service riser conduit.
You are looking at that aspect the wrong way, it says "not more then" 3 current carrying conductors, but still if they are in free air another table may apply and actually allow for higher ampacity from same conductors.

Are you selecting size of the "overhead" conductors that are in free air or are you selecting size of the conductors in the riser conduit? Maybe I misunderstanding what you were asking about?

5. Originally Posted by kwired
...
Are you selecting size of the "overhead" conductors that are in free air or are you selecting size of the conductors in the riser conduit? Maybe I misunderstanding what you were asking about?
#3 in OP. The conductors will not be in free air.
Originally Posted by edward
3.Single overhead riser conduit with single set service conductors is based on 129 amps

6. Originally Posted by edward
The service conductors in my question...
You did not post your load calculation. For combined service entrance conductors you may be able to take advantage of 75% demand for four or more fixed-in-place appliances if the individual units could not.

Is this new construction? ...one meter or two? If one meter I have to question here whether or not the duplex units can legally be under one meter and billed as one service by the power company.

You will not be able to take advantage of 310.15(B)(7) the way it is worded.

7. Originally Posted by kwired
You are looking at that aspect the wrong way, it says "not more then" 3 current carrying conductors, but still if they are in free air another table may apply and actually allow for higher ampacity from same conductors.

Are you selecting size of the "overhead" conductors that are in free air or are you selecting size of the conductors in the riser conduit? Maybe I misunderstanding what you were asking about?
Which aspect I have the wrong way?

8. Edward, to answer your question the risers can be based on the calculated load however if you want to leave room for growth then you may want to up the size of those conductors

9. Originally Posted by Dennis Alwon
Edward, to answer your question the risers can be based on the calculated load however if you want to leave room for growth then you may want to up the size of those conductors

That is all i wanted to learn if in this case the service conductors are based on calculated load or based on the main OCPD.

10. Originally Posted by edward
That is all i wanted to learn if in this case the service conductors are based on calculated load or based on the main OCPD.
The minimum is based on the calculated load. However, service ocpd rating has a mutual relationship with the service conductor ampacity... and vice versa.
Last edited by Smart \$; 04-22-17 at 10:00 AM.

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