# Thread: I feel stupid..... Derating conductors

1. ## I feel stupid..... Derating conductors

I've been looking for the table or formula in the NEC, but I can't find it.
Circuit homerun 350 feet. 20 amp.... 6 wire?? or 10?? I can't remember if you need to derate every 100 or 300 feet. I know you guys know. Thank you in advance!!

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Are you talking about voltage drop? I don't think the voltage drop formula is directly in the NEC. Its easier to use a voltage drop calculator on the internet because if I pasted it in here it wouldn't make any sense. The ampacity you use is the ampacity of the device or motor, and then the ohms per 1000 feet is found in table 8 in chapter 9.

If your using an internet calculator you wont need table 8 though, it will probably just ask you the wire size and material.

Heres one:

http://www.csgnetwork.com/voltagedropcalc.html

3. It sound like you are trying to correct voltage drop. Instead of using a rule of thumb calculate the actual VD and determine if you chosen size is adequate. Increase it if the VD is too high.

Online calculator http://www.electrician2.com/vd_calculator.htm
Last edited by bob; 05-18-10 at 05:21 PM.

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Yea, definitely no rule of thumb because it all depends on the load.

A 20A breaker may have a 5 amp load on it.

5. You can't find it, because it is not there. But then, you are not asking the question that you think you are asking.

You do not "derate" a wire by virtue of its length. You may use a bigger wire than you normally would, given the breaker rating, but that is not an act of "derating." A #8 wire with THWN insulation has an ampacity of 50 amps, whether the run is 1 foot long or 1000 miles long. Length has nothing to do with ampacity, and the word "derate" means to alter (i.e., reduce) the wire's ampacity.

Separate from the notion of ampacity is whether a given size wire is adequate to handle the load current you plan to put through it, given the wire's insulation type, length, and circumstances of installation (e.g., in open air, or in conduit with lots of other wires, or in a hot room, or underground). If you have to use a larger wire because of the temperature of the room, or because there are several other current-carrying wires in the same conduit, then you have actually changed the allowable ampacity; that is "derating." If on the other hand you use a larger wire because it is a long run, and because you wish to minimize the voltage drop, you are not changing the ampacity, and you are therefore not "derating."

6. I belive you are refering to Voltage Drop. Their are many formulas that will work but you need to list your Voltage, Phase, Amps, Copper or Aluminum.

For example a 350' run of # 10 cu 240 volt single phase can have a load of 8.276amps at a permitted voltage drop of 3%.

or # 6 with the same info can carry 20.922 amps.

Ive attached a list of useful voltage drop info that you can slip into your code book in front of Table 8.

I hope this is what you needed.
Kevin

7. Originally Posted by relochris
I've been looking for the table or formula in the NEC, but I can't find it.
Circuit homerun 350 feet. 20 amp.... 6 wire?? or 10?? I can't remember if you need to derate every 100 or 300 feet. I know you guys know. Thank you in advance!!
More info is needed. Is it 240v or 120v, 3phase or single phase, copper or aluminum????

Here is a online VD table.

If the circuit is 240v single phase and copper than yes, a #6 would be necessary for a full 20 amp load at 350'. If the load is 10 amps then you could use a #8 copper conductor.

8. ## I see.....

I'm waiting on the builder to get the ampacity of the motor. It's a 120v electric gate. Any you all are right. I was more concerned with the voltage drop than the ampacity. So I guess the real question is.. at what point does the voltage become too low to effectively run the motor without causing damage to the motor or simply rendering it inoperable. Thanks to everyone for all the insight. Right ampacity is not the issue, it's voltage.

9. Originally Posted by relochris
Circuit homerun 350 feet. 20 amp....
Run a 240v circuit and use a 240-120v transformer at the load end.

Don't forget about bonding and an electrode, which they'll probably want anyway.

10. Originally Posted by relochris
I'm waiting on the builder to get the ampacity of the motor. It's a 120v electric gate. Any you all are right. I was more concerned with the voltage drop than the ampacity. So I guess the real question is.. at what point does the voltage become too low to effectively run the motor without causing damage to the motor or simply rendering it inoperable. Thanks to everyone for all the insight. Right ampacity is not the issue, it's voltage.
VD is based on the ampacity as well as the distance and voltage. a 120 volt motor that draws 10 amps will need a #6 wire. Not sure what you want to know. The VD formula is based on the FPN in the code of 3%. This would appear to be an acceptable voltage.

Thus a 120v with a VD of 3 % is 120 x .03= 3.6 volts. So you final voltage will be 120-3.6= 116.4 volts. This is within acceptable parameters.