Voltage Drop

[Selleathy]

I am estimating a job and the distance from the panel to the first light is 400' there are then 8 lights spaced out 35' apart totaling about 240' from the first JB, the engineer has #4 specced out for the lights and i would like to downsize after hitting that first JB but I am not sure how to calculate the voltage drop using the different gauge wires from the first JB. I have more information on fixture specs and voltage if needed thanks.

 

[ActionDave]

Break the whole run down into segments and calculate the voltage drop for each one....total load to the first light, load from that light to next seven and on down the line, then you can size your conductors accordingly.

 

[jumper]

ECM article on the subject. Basically it is just a series circuit.

http://ecmweb.com/basics/calculating-series-voltage-drop

 

[Smart $]

If this is a stamped engineer-of-record project and approved for installation through the AHJ, you cannot alter the installation without approval and revision documentation from the preceding entities.

 

[JFletcher]

If this is a stamped engineer-of-record project and approved for installation through the AHJ, you cannot alter the installation without approval and revision documentation from the preceding entities.

this^

 

[iwire]

If this is a stamped engineer-of-record project and approved for installation through the AHJ, you cannot alter the installation without approval and revision documentation from the preceding entities.

this^

I was thinking the same thing as I read it.

 

[Electric-Light]

One common reason beyond voltage drop and continuous current handling ability is the available fault current. If there's a fault to ground or another phase, the peak let through current is unaffected by the breaker size. This can be tens of thousands of amps and if the wire isn't adequately sized, the let through for 1/120 to 1/60 seconds can heat the wire up enough to make the insulation brittle.

There maybe other valid reasons that's beyond what you or I know.

 

[JFletcher]

I am estimating a job and the distance from the panel to the first light is 400' there are then 8 lights spaced out 35' apart totaling about 240' from the first JB, the engineer has #4 specced out for the lights and i would like to downsize after hitting that first JB but I am not sure how to calculate the voltage drop using the different gauge wires from the first JB. I have more information on fixture specs and voltage if needed thanks.

You'd need those specs and voltage, and how the lights are wired. Everything else being equal, daisy-chained from one to another will have more VD than if they were all wired 'star pattern' from one j-box, 2 runs of 4, etc.

The #4 may be to meet 2 or 3% VD for the circuit, future expansion, boilerplate specs, lazy/over-engineering, or any of a number of other reasons. Obviously the last light will only have its own draw and wouldnt need #4. 8 600W HPS @ 208V would need #4 cu to get 2.5% VD @ 400'. The last light would only need #14 cu (2.88A @ 250', 1.94% VD).

There are online calculators that you can use to figure the VD. You can estimate it two ways; #4 all the way, per the specs, and your way. For a bid tho, you have to go by the plans and not what you think you maybe able to install if you get the job. Considering the cost of trenching, conduit, parts and labor, there isnt going to be an enormous difference materials wise between #4 and #6 or smaller the last 250'.

 

[iwire]

For a bid tho, you have to go by the plans and not what you think you maybe able to install if you get the job.

Unless the EE signs off on the change you can't bid to plans and then install something different. That's shady.

 

[JFletcher]

Unless the EE signs off on the change you can't bid to plans and then install something different. That's shady.

Isnt that more or less what I wrote?