Light Pole Feed Calc

[Alwayslearningelec]

Morning. So there is another contactor bringing the feed from the panel inside the building to the two j-boxes(100' from panel to j-box) in the upper right hand corner. We are to extend the feed from there to the S1 & S2 fixtures(from j-box to last fixture 250'). Not able to get(right now) what size feeder they are running from panel. How can I calculate approximate size of main feeder to keep VD under 2.5%? Do I just convert the watts to ampos and total the amperage for 1 circuit and but that in VD calculator or just use the 1P-20A breaker. Do I calculate all of them on same circuit? After I hit underground "homeruns" boxes I can branch off with smaller wire to light poles correct? It's 120v. Thank you very much.

 

[augie47]

Initially I would calculate the load on each circuit (input amps or input watts converted to amps) for the total number of fixtures on that circuit.
Even though your load will change as the run progresses I would use that total number at full length at first just to see what the drop would be.
Those are very low wattage fixtures and you may find that initial step (full load at the end) results in a small enough conductor that it's not worth breaking it down in segments.
Most of those accept a decent range in voltage so I don't know if I'd worry if it was over your 2.5%

 

[Alwayslearningelec]

Initially I would calculate the load on each circuit (input amps or input watts converted to amps) for the total number of fixtures on that circuit.
Even though your load will change as the run progresses I would use that total number at full length at first just to see what the drop would be.
Those are very low wattage fixtures and you may find that initial step (full load at the end) results in a small enough conductor that it's not worth breaking it down in segments.
Most of those accept a decent range in voltage so I don't know if I'd worry if it was over your 2.5%

That was exacty what I was thinking. The load change as I go further out along the circuit(becomes less).
So as an example:

1P-20A 120v circuit feeding (20) 46W fixtures.
Last fixture is 250' away.
I get .38 amps per fixture. That's 7.6Amps.
Plugging 7.6 amps into VD calculator using no more then 2.5% VD I get # 8 copper.

Did I go about it correctly?

 

[augie47]

Your methodology is correct but to be accurate the "46 watt" fixture likely is referencing a 46 watt lamp and the input watts will be greater so that number would change.
You can decide if it's worth your time to break the load down in segments and figure each one and reduce the wire size as your progress.
(I'd be comfortable using a #10 throughout unless specs are holding you to the 2.5% but let's see what others say)

 

[Alwayslearningelec]

Your methodology is correct but to be accurate the "46 watt" fixture likely is referencing a 46 watt lamp and the input watts will be greater so that number would change.
You can decide if it's worth your time to break the load down in segments and figure each one and reduce the wire size as your progress.
(I'd be comfortable using a #10 throughout unless specs are holding you to the 2.5% but let's see what others say)

Thank you.