Voltage Drop Wire down sizing

[Pok3r]

Am running 2 circuits for the light poles show on the diagram. On of them its a run of 410' and the other one its a 920' run. For the long run am aware of the calculation show a # 1 AWG for 120v 13 A 920'... my question is can I down sizing the circuit and what will be there correct way of downsizing it
( should I come out of the panel with a # 1 and after surten distance down size it o can it go From a # 4 AWG and jump to a bigger size every 100')



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[winnie]

First as a design issue, consider running a MWBC which provides 240V as a pair of 120V L-N supplies, then split your loads across the pair of circuits. This will greatly reduce voltage drop for a given size wire.

Next: All voltage drop calculations are an approximation. Simply lumping the entire load to the end of the circuit and calculating the voltage drop is very conservative...but as you note gives very large wires for long runs.

More accurate is to generate calculate the voltage drop to each separate load point in the circuit; so you have 13A at the first lamp location with distance X and voltage drop Y, then 11A at the second lamp location with additional distance Y' and voltage drop X+X', then 9A at the third lamp location with..... You will find that for the last lamp you have low current and thus low additional voltage drop and can use thinner wire. You will probably want to make a spreadsheet and simply try different wire sizes out.

Finally: What voltage drop are you designing to? Are you required to design to this level. If you use loads which tolerate greater voltage drops (eg. LED lamps with drivers that compensate for voltage drop) then you might be able to use smaller wire and have a larger voltage drop with no functional issues. (Note that over the long term this uses more electricity...but you have to balance long term and short term costs.)

-Jon

 

[cpinetree]

See this thread: http://forums.mikeholt.com/showthread.php?t=141401&highlight=segmented

If the original poster does not respond to an email, I have a copy I could send you, just drop me a PM.

I have a used it for several pole / post light jobs, it works great :happyyes:

 

[Pok3r]

First as a design issue, consider running a MWBC which provides 240V as a pair of 120V L-N supplies, then split your loads across the pair of circuits. This will greatly reduce voltage drop for a given size wire.

Next: All voltage drop calculations are an approximation. Simply lumping the entire load to the end of the circuit and calculating the voltage drop is very conservative...but as you note gives very large wires for long runs.

More accurate is to generate calculate the voltage drop to each separate load point in the circuit; so you have 13A at the first lamp location with distance X and voltage drop Y, then 11A at the second lamp location with additional distance Y' and voltage drop X+X', then 9A at the third lamp location with..... You will find that for the last lamp you have low current and thus low additional voltage drop and can use thinner wire. You will probably want to make a spreadsheet and simply try different wire sizes out.

Finally: What voltage drop are you designing to? Are you required to design to this level. If you use loads which tolerate greater voltage drops (eg. LED lamps with drivers that compensate for voltage drop) then you might be able to use smaller wire and have a larger voltage drop with no functional issues. (Note that over the long term this uses more electricity...but you have to balance long term and short term costs.)

-Jon

Thank you for your respond, since am using a contractor photo cell to control this lights how will you suggest to uses MWBC, will it make any difference. Also do you know if the NEC have any Requirements for vault to vault distance or separation.?


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[ActionDave]

Thank you for your respond, since am using a contractor photo cell to control this lights how will you suggest to uses MWBC, will it make any difference. Also do you know if the NEC have any Requirements for vault to vault distance or separation.?


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Let the photo cell control a multi pole relay.

 

[Pok3r]

Is there anything where its says that there has to be a Vault every 200'?


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[ActionDave]

Is there anything where its says that there has to be a Vault every 200'?


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No

 

[Strathead]

First as a design issue, consider running a MWBC which provides 240V as a pair of 120V L-N supplies, then split your loads across the pair of circuits. This will greatly reduce voltage drop for a given size wire.

Next: All voltage drop calculations are an approximation. Simply lumping the entire load to the end of the circuit and calculating the voltage drop is very conservative...but as you note gives very large wires for long runs.

More accurate is to generate calculate the voltage drop to each separate load point in the circuit; so you have 13A at the first lamp location with distance X and voltage drop Y, then 11A at the second lamp location with additional distance Y' and voltage drop X+X', then 9A at the third lamp location with..... You will find that for the last lamp you have low current and thus low additional voltage drop and can use thinner wire. You will probably want to make a spreadsheet and simply try different wire sizes out.

Finally: What voltage drop are you designing to? Are you required to design to this level. If you use loads which tolerate greater voltage drops (eg. LED lamps with drivers that compensate for voltage drop) then you might be able to use smaller wire and have a larger voltage drop with no functional issues. (Note that over the long term this uses more electricity...but you have to balance long term and short term costs.)

-Jon

It is generally not worth the detail you cite above. the smaller wire at the end isn't small, just smaller because you still have all the accumulated voltage drops to that point. If the light fixtures are reasonably evenly spaced. +- 30 feet or so, I am very comfortable with a typical method of going to the middle pole in the run and measuring full distance from there, then using that distance for the voltage drop and running all the wire that size, except I usually just pull #12 up the pole. In Florida we are code required to comply with 3% branch voltage drop and this is still the way most Engineers deal with it.

 

[JFletcher]

Keep in mind too that smaller wire is more easily broken... if you're running UF in a trench, no conduit, it has to be done right or you will have a load of failures. We maintain one property that uses 480V lighting, fed with 12/2 UF from vaults. Roughly a third of it has failed in less than a decade. Every time one fails, it's about an 80' line bore to run power from the closest pole, at several thousand dollars per instance.