Voltage Drop calculations

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Jim33063

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Is it manditory to use the recommended wire size. The wire size given seems un realistic. example 500' 120 volts 20 amps( small 13 watt cfl lamps used ) 1" pvc for
small pole lights.
I came up with #1 thhn copper
 
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Jim33063 said:
Is it manditory to use the recommended wire size. The wire size given seems un realistic. example 500' 120 volts 20 amps( small 13 watt cfl lamps used ) 1" pvc for
small pole lights.
I came up with #1 thhn copper
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Calulate the wire size to the actual amperes for voltage drop, but do not go below the NEC breaker size/wire size results. If it is a lighting load then you should not exceed 16A an a 20A CB. If you calculate to the actual current load future additions to the circuit will invalidate the results, eg. you will have larger voltage drop. CFL's may not be able to light up at reduced voltages.
 
Jim33063 said:
I came up with #1 thhn copper
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So did I. But that accounts for all 20 amps of load, which as Laszlo pointed out, you don't have to do. Calculating for a 16 amp load, I came up with #3. If you actual load (0.108 amps per lamp times however many lamps are installed) is less than 16 amps, you can use an even smaller wire.
 
weressl said:
If it is a lighting load then you should not exceed 16A an a 20A CB.
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That a boy, keep the myth alive. :roll: :grin:

Lighting loads can be 100% of the breaker rating not a great design, but legal ...... unless the maximum load is expected for 3 or more hours.
 
He said you "should not"; he did not say you "must not." Sounds like a design decision to me; I agree with that design decision. ;)
 
Charlie b said:
He said you "should not"; he did not say you "must not."
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Your right and I saw that as well

To the careful readers like EEs it is clear, to the passive readied like many electricians it sounds like the same old myth. A myth that can cause a missed question on a licensing exam.

You may think that I am being picky but considering the many electricians I work with believe that no breaker can ever be loaded to 100% at any time I think I am being prudent.

On the rare ocasion I get to deign ligting circuits I shoot for 10 to 12 amps as a max on a 20 amp circuit. I like leaving room to add. :smile:
 
iwire said:
You may think that I am being picky but considering the many electricians I work with believe that no breaker can ever be loaded to 100% at any time I think I am being prudent.
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I'm OK with that. What you are describing is a part of the reason I came up with "Charlie's Rule" in the first place. ;)
 
charlie b said:
So did I. But that accounts for all 20 amps of load, which as Laszlo pointed out, you don't have to do. Calculating for a 16 amp load, I came up with #3. If you actual load (0.108 amps per lamp times however many lamps are installed) is less than 16 amps, you can use an even smaller wire.
Click to expand...

An additional practice that I have used on lighting up a bridge is to gradually reduce the conductor size on the lighting run as the sum of the loads to be carried by the wire segment diminishes. Toward the end you can be down to "breaker size".
 
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