voltage drop

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sparky59

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I am wireing the second floor of a commercial building. There are two entrances 150 ft. apart. I am installing 3-way switches at each entrance for the hallway lights. The distance through the travelers and back to the farthest lights will be around 450 ft. The voltage is 3-phase 120/208. The lights are 120 volt. The load on each circuit will be around 15 amps on #12 wire. I'm thinking I should use larger wire or maybe run the switch loops to relays to cut the distance. Any helpful comments would be appreciated.
 
Re: voltage drop

Use the following formula to calcualte your wire size:
CM=2KxLxI/Vd

CM - cirular mills of wire
K - ohms per mil foot (copper=12.0 @75degrees)
L - length of conductor
I - amperage
Vd - voltage drop desired (recommended 3% max for branch circuits per NEC)
 
Re: voltage drop

There is another aspect to this problem that is worth some attention. I just did a quick calculation., using an Excel spreadsheet, using the numbers you gave. It told me the voltage drop was 22%. :eek:

However, that presumed that the entire 15 amps of load was located at the end of the 450 foot run, and that is simply not true. You could calculate the VD from the source to the first light at the full 15 amps, and then calculate the VD from the first light to the second light at a slightly lower current, and continue the process until you reach the last light in the string, which will probably have a load of less than one amp. But this is a very messy process.

So I would start by determining how much of the total 450 foot run is traversed by all of the 15 amps. That is, does all 15 amps travel through the 150 foot distance between the doors? Does all 15 amps travel through some additional distance from the source to the first switch? Pick a number that represents the minimum distance from the source to the first light (i.e., after which you can say that the load further downstream is lower than 15 amps). Do a calculation for that distance, using a 15 amp load.

As an example, with a distance of 150 feet and a load of 15 amps, and presuming a single 120 volt circuit, I calculated that you need a minimum of #8 AWG, in order to limit the voltage drop to 3%. With the same distance and current, but with a full boat (i.e., a 3-phase, 208 volt run), I calculated that you need a minimum of #10 AWG, in order to limit the voltage drop to 3%.
 
Re: voltage drop

By sparky59:

The distance through the travelers and back to the farthest lights will be around 450 ft.
I just want to point out that you don't include the distance of both travellers. I don't know whether or not you are, I just thought I'd mention it.

Edit: This is why I bring it up:

By sparky59:

There are two entrances 150 ft. apart. I am installing 3-way switches at each entrance for the hallway lights.

[ April 14, 2005, 03:03 PM: Message edited by: physis ]
 
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