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# Safe maximum distance for wiring?

#1
02-14-06, 07:07 AM
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Safe maximum distance for wiring?

I'm sure the NEC mentions this somewhere, but since I do not have a reference guide this is why I'm asking.

What is the maximum distance you can run #14 copper wire and have it still be able to safely carry 15A?

Same question, but for #12 wire carrying 20A?

Same question, but for #10 wire carrying 30A?

Thanks

#2
02-14-06, 07:49 AM
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You need to rephrase your question.

Those cables can safely carry those respective currents any distance. However the voltage will drop. Whether the voltage drop is too much or not depends on what the load is. For example, an incandescent light bulb doesn't care what the voltage is, it will simply light up less brightly with a lower voltage. However, a motor may be damaged or may not run at all with a lower voltage.

To determine the voltage drop you need to know the distance and whether the initial voltage is 120 or 240 volts.

Try searching the Internet for "Voltage Calculator" You will find numerous ones. You can then plug in values all day long and come up with the answers to your questions.

#3
02-14-06, 07:57 AM
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Originally Posted by txnoob
I'm sure the NEC mentions this somewhere
Why do you say this?

What is the maximum distance you can run #14 copper wire and have it still be able to safely carry 15A?
You left a lot of terms undefined.
The answer could be anywhere from 0 to a quarter-mile depending on how you defined the words "able", "safely", and "carry".

Same question, but for #12 wire carrying 20A?
Same question, but for #10 wire carrying 30A?
Perhaps what you are asking is how far until you have more than a 3% or 5% voltage drop.

So what is your starting voltage and ambient temperature?
How much voltage drop will you accept?

#14 = 65'
#12 = 63'
#10 = 67'

A lot of energy is lost to transmission (heat) when wires are near the minimum size for the load on them.
The 15A load could be hauled 165' with a 5% by #10 instead of #14.

Of course, you should not run a circuit breaker continously at 100% of its rating.
So if you really have a 15A load, you should be using a 20A breaker with #12 conductors, etc.

Last edited by bolide; 02-17-06 at 02:14 PM.
#4
02-14-06, 08:29 AM
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Thanks for the input. I didn't quite understand enough I suppose about the electrical principles in phrasing my question. I was under the assumption that if, say, a #14 wire was carrying 12-15A that if that load was run over a long distance, perhaps 100 ft., that heat would begin to play a more significant factor in the energy transfer.

However voltage drop now makes more sense. I am mainly trying to determine what size of wire should be used for a dedicated circuit or two in the house. I plan to eventually hook up an amplifier for radio communications and I anticipate that the output of the amp will draw a significant amount of current.

#5
02-14-06, 08:36 AM
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The longer the wire, the more energy lost in it, and the more heat it dissipates. But it still dissipates the same heat per linear foot, so a run of 10 miles is no more of a hazard than a run of 10 feet. In fact, the run of 10 miles will generally be less of a hazard, since the resistance of the wire will reduce the current.

As a rough rule of thumb, try to keep the wiring of your 120-volt general purpose circuits under 75 feet in length. When you go over than, consider upping the wire size by one. And you need to be more conservative for motor loads than for resistive loads, not for safety reasons, but for proper operation of the motor. In some cases, a long run could reduce the voltage enough so that a motor could overheat. But the motor should have thermal protection on it to prevent a hazard.

#6
02-17-06, 04:00 AM
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a run of 10 miles is no more of a hazard than a run of 10 feet. In fact, the run of 10 miles will generally be less of a hazard, since the resistance of the wire will reduce the current.
At 200 miles, I suppose one could hold the #12 wires in his teeth.

Not really to argue your point, but the limit on current could be a hazard since even a bolted fault a quarter-mile into the run isn't going to operate the OCPD -- at least not quickly.

The conductors themselves won't be harmed. But I can't say that it is always at least as safe as if it were 10' into the run.

txnoob, do keep in mind that a circuit must have a breaker that is at least 125% of the continuous load. So a load calculation is definitely needed. A heavily loaded circuit has a greater voltage drop than a lightly loaded circuit. 10/3 cable will deliver far more power to the far end than will 12/2.

#7
02-17-06, 11:54 AM
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Of course, you should never run wiring continuously at 100% of the capacity. So if you really have a 15A load, you should be using #12, etc.
This may sound quarrelsome but the actual ampacity of 14 gage wire is twenty amperes. The code making panel derated 14, 12, & 10 gage conductors in most uses by requiring that the overcurrent protection be limited to 15, 20, & 30 amperes respectively. Accept for the number ten AWG those values are less than 80% of the actual ampacity of the conductors.

240.4 Protection of Conductors.
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
(D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.
The point I'm trying to make is that there is no need in most uses to derate these smaller conductors.

#8
02-17-06, 02:10 PM
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Originally Posted by hornetd
Except for the number ten AWG those values are less than 80% of the actual ampacity of the conductors.
The rule that the OCPD must be at least 125% of continuous load is, I believe, to protect circuit breakers, not conductors.

But once you upsize the OCPD to match the load, you must use conductors that the NEC says you may use.

So 15A continuous requires at least 20A circuit breaker.
Then 20A circuit breaker requires at least #12 conductors (for the type of circuit under discussion here).

At least that's how I understand it. My choice of words in post #3 was misleading (post #6 was more clear).

#9
02-17-06, 02:58 PM
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I don't know how this thread got off onto continuous loads. But we're off in the weeds now. I'm going to close this thread before we forget how to get back.

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