Voltage Drop Conductor size

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DJFNEC2005

Member
Location
NJ
Hi,
I have a customer who will be utilizing a temporary lighting setup that will have a load of around 30,000 watts. They will be using these lights on a frequent basis and want a permanant line ran to the location of the temp light truck. The run will be about 500 foot distance from the building to temp lights. What size conductor would you recommend?

Load 30,000 Watts
Volts 240V 1 Phase
Distance 500ft One way.
Conductor Copper in PVC conduit underground

I come up with a load of 125 amps and a conductor size of 3/0. What do you think? would you increase size to 4/0? Is the voltage drop still too much with 3/0 conductor.

Thanks in advance for replies.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
HERE is a link to an online voltage drop calculator. Using this calculator and a voltage drop of 3% I come up with 250 kcmil copper conductors.

Mike also has a free voltage drop calculator HERE.

Chris
 

DJFNEC2005

Member
Location
NJ
Thanks Raider, I knew there was a VD calculator on this site somewhere I just couldn't remember where. According to mikes calculator 250kcmil looks correct.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I disagree with those results.

They are based upon the assumption that the wire will be at 75C. As copper wire warms up, its resistance increases. These conductors are operating at well below their 75C ampacity, and will most likely be only slightly above ambient temperature. The resistance change from 25C to 75C is about 20%, which is petty darn significant. IMHO 3/0 is just shy of giving the desired 3% drop.

I would also like to suggest a design consideration: if the lighting includes voltage regulation of some sort (eg. electronic ballasts that adjust for supply voltage), then you can tolerate greater voltage drop (at the cost of poorer efficiency). If this application is only used a small fraction of the time (you did say temporary), then it might be more effective to pay less for the installation and more for the electricity.

-Jon
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Using a temperature of 25?C the calculator on the bottom of this page shows a 3.5% drop for 3/0 and a 2.8% drop for 4/0. If you raise the temperature to 75?C the drops are 4.2% and 3.3% respectively.
 

chaterpilar

Senior Member
Location
Saudi Arabia
You would require 150 sq mm of copper to carry 125 amps to 500 feet with 3 % voltage drop.

In AWG land it would be two runs of 3/0.

The catch here is the length (500 feet) and hence the increase in cross sectional area of copper.


Cheers.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
You would require 150 sq mm of copper to carry 125 amps to 500 feet with 3 % voltage drop.

In AWG land it would be two runs of 3/0.

The catch here is the length (500 feet) and hence the increase in cross sectional area of copper.


Cheers.
Parallel 3/0s would be ~335kcmil...the voltage drop calculator that I linked to in my previous post shows that the drop would be 1.9% with a single 300kcmil conductor and 1.6% with a single 350kcmil. The use of parallel 3/0s would have a drop about 1.78%.
 

hurk27

Senior Member
Or lower for some ballast types.

True, Pulse Starts draw only half there rated wattage when striking, looks like next year they will be the only available MH lighting. some manufactures have already stopped supplying regular MH fixtures, mercury vapor have already been pulled from the shelves here.

Myself I like the pulse start because the lamps last so long, and the extra amount of light they output per watt.
 
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