300' 20amp circuit

[byron]

Hi All,
When do I become concerned about the length of the wire? I searched the site and found answers to voltage drop that included that the NEC does not place a limit on the voltage drop. Can anyone refer me to somewhere in the NEC 1999 that addresses this?
Thanks in advance

 

[jeff43222]

Re: 300' 20amp circuit

I doubt it was covered as a rule in the 1999 NEC. The 2002 NEC has recommendations (no more than 3% each for feeders or branch circuits, and no more than 5% total), but they are not enforceable.

If you're planning on feeding a load of 20A with 300' wires (I'm assuming 1?, 120V), voltage drop will be an issue. I don't have the Ch.9 tables in front of me, but I suspect you'd want to use at least #10, and possibly #8 for such a circuit.

[ October 18, 2004, 03:29 PM: Message edited by: jeff43222 ]

 

[charlie]

Re: 300' 20amp circuit

You will only find a couple of FPNs that address voltage drop and they can not be enforced. Normal equipment is designed to operate with as high as a 10% voltage drop but the electric utility is normally permitted to use 1/2 of that. It is best to design for a maximum of 5% voltage drop tot he furthest piece of equipment.

 

[rbalex]

Re: 300' 20amp circuit

Voltage drop is not an inherent safety issue, so the NEC doesn't make mandatory requirements.

The nature of the load determines the actual need; but the NEC "suggestions" are fairly practical for most purposes. Motors and other loads subject to significant "inrush" may take a bit more review.

 

[cpal]

Re: 300' 20amp circuit

Voltage drop is addressed as a fine print note and therefore is not enforceable as such. The NEC does lend a recommendation in 210.19 (A) (1) FPN No.4 for branch circuits and in 215.2 (A) (1) FPN No. 2.for feeders.
One rule of thumb would suggest that conductors installed per their ampacities as listed in 310.16 Table, should provide reasonable service in lenghts not exceeding the applied Voltage. That is a run not further than 125' (250' total Circuit Lenght)at 125V.or 250',(total 500' circuit lenght) at 250V.

The only real way to be sure is to perform a calculation. Vd = Load current X Line Resistance.

There are many ways to determine line resistance based upon lenght of conductor, material, ambient temp, ect.

Charlie

 

[physis]

Re: 300' 20amp circuit

When do I become concerned about the length of the wire?

When it gets long. You kind of have to do the math. Conductor properties are in Chapter 9 tables 8 & 9 of the 2002 NEC. I don't have a 1999.

If you need to know how to do the calculations I recommend anything but the NEC. That's not it's purpose.

 

[electricman2]

Re: 300' 20amp circuit

Here is one voltage drop calculator, there are others. According to it, 5 amps is the max for a 300ft. N0.12 circuit at 120V. to maintain less than 5% loss. It may be a bit on the conservative side.

 

[dereckbc]

Re: 300' 20amp circuit

Originally posted by byron:
When do I become concerned about the length of the wire?

Rule of thumb 100 feet. But this can be misleading because of the varibles involved, such as a combination of feeder and branch lengths. From a design point of view you limit feeders to 3%, and branch ckts to 2%. So if feeders are involved you have to throw out 100 feet and use 40-to-50 feet for a branch assuming the feeder is 3% or less.