NEC Single Phase Conductor Ampacity

[kingtutley]

I am doing conductor voltage drop calculations for a new subdivision. The secondary from my pad mount transformer is -probably- going to wind up at 350kcmil, but I need justification. I have 2 pedestals downstream from my xfmr. Each is feeding 2 single family dwellings with an estimated 10kW each. The first pedesdtal is 35' from the xfmr. Between the pedestals is 110'.

I have a max VD of 5% per NEC -- I understand that the NEC does not specify VD for service conductors, but since it specifies it for all others, I'm just going with that.

Now, the fun part. My question is not really about the calcs, but rather what values for R to use. The NEC specifies R in Table Section 9 Table 9 for AC conductors, but it also stipulates that these values are for the specified parameters ONLY. So, the parameters specify 3-phase. I am working on single phase 2-wire in schedule 80 pvc conduit. I have not been able to locate any derating factors for this, so I have used the values in the table. Is this justified?

 

[dpc]

The resistance per unit length of conductor isn't going to change between three-phase and single-phase, but with single-phase, you have to take into account the resistance of the return as well. With balanced three-phase, you only have to worry about one-way distance. The reactance is another story.

There are some tables available (The Cutler Hammer Consulting Application guide comes to mind) that deal with this.

One other point - the 5% voltage drop in the NEC includes everything - feeders and branch circuits. There are ANSI requirements for the acceptable voltage range delivered at the service point.

 

[rbulsara]

Also what is single phase, 2 wire? I hope it is not 120V. Your reference to 350 kcmil makes me suspicious.

NEC minimum is 100A, single phase, 3 wire for a single family dwelling unit service. You do not have to use 100A for the voltage drop though.

Also see Table 8.





Rafiq Bulsara

http://www.srengineersct.com

 

[kingtutley]

The installation is to be single phase, 120/240. The load I am currently looking at is 50kW (10 kW per dwelling, 5 dwellings) in a new subdivision. The power feed is from a 100kva xfmr (there are 5 other dwellings on the load, but I am not looking at those at this point).

The cable I was told late yesterday is triplex (3 conductors - 2 current carrying 350kcmil AAC conductors and the neutral). I believe my problem was that I was looking at a pair of conductors (hot and neutral) carrying the entire load as opposed to a 240/120 distribution type. This should effectively split the current in the 2 legs, or effectively split the impedence.

Does this make more sense?

 

[rbulsara]

For the purpose of your VD calculations, the single phase voltage will be 240V and still two wires (hot wires). Calculate your load current using 240V. No, the impedance does not get split. Single phase circuit is still a single phase circuit.

This assumes that loads will be reasonably split between two hots and N or major loads such as ranges and dryers are 240V.

Rafiq Bulsara

http://www.srengineersct.com

 

[BJC]

Wher's the meter(s)?
Everything up stream of the meter is not covered by the NEC.
See if you local utility will share their standards with you. Most utilitys have volt drop tables and diversity factors they us for distribution installation. It's not serious engineering, it's look -up.