ECN Forum Forums General Topics of the Trade General Discussion Area Long wire runs

Bjarney's chart is close. While I agree the
voltage drop is load dependent you have to
consider the fact that if you install a 20 amp circuit someone is going to try and plug in a 20 amp load. VD=2k*L*I/CM
where: VD= voltage drop
L= length of conductor
I= current in circuit
CM= area of conductor in circular mill
K= resistivity of the conductor- 11
for copper
Thus in this case:
VD=2(11)*150*20/16510=
a voltage drop of 3.99 volts for no.8 wire
A little over 3% for 120 volts or 3.19 volts
for an 80% load.

Thanks for the input everyone. The loads on the screens and cameras will not be much, but the loads on the stage lighting will be high. I will run everything in # 10 wire to be on the safe side.

10 awg. is a lot better than 12 awg. but you
will still have a 5 volt drop with an 80% load. If that is acceptable then you are good
to go.

Bjarney - I apologize for sounding harsh. I did not intend it that way, but after reading it again, I realize I was. (Crusty, grumpy electrical guy, sound familiar?) I guess my point was this: Doing something because you read it somewhere can be really dangerous. If it goes wrong, it is your butt that is exposed, not some anonymous author. No one knows the exact particulars of your installation except you. Always check information, and perform your own calc's, and read the N.E.C.

again - my apologies for being rude.

Article 210.19(A)(1) requires branch circuit conductors to "have an ampacity not less than the maximum load to be served." A little later in the same article it requires branch circuits to "have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."

(As an aside: What load is there if not continuous, noncontinuous or none? Or any combination of these?)

I deduce from this that the intent is to require the electrician to consider voltage drop. This is supported by the FPN that comes out with the familiar 3%, 5% overall rule of thumb.
Same thing in 215.2 for feeders.

That said, the NEC has not left us in a vaccum about the formulas either. They have provided us with Tables 8 and 9 in Chapter 8.

Use ohms law: E=IxR, or with Table 9: E=IxZ.
The tables provide resistance of the wires per 1,000 feet. To compute the resistance of your wire run, simply divide the total length by 1,000.

For a single phase circuit, there and back, the allowable length from the load to the supply to limit the VD to 3% would be equal to: 15xE/IxR. Obtain the R from the value given by Table 8 across from your chosen wire size.

Have you considered running 240V circuits, either by using multiwire branch circuits, or by installing a subpanel? To supply a given number of watts at 240V rather than 120V means that your current is halved at the same time that the headroom for voltage drop is doubled.

seeks, no offense taken, and none intended from this end. Many organizations publish their CSI-format specs on the web, and while not statutory, give an idea of what works in their area. That was my point.