Roadway lighting 347 or 480?

[mbrooke]

Basically Im in a economics calculation dilemma.

I have the option of ordering either 480 volt lighting ballasts and running 3 wire plus earth in underground feeders or ordering 347 volt ballasts running 4 wires plus ground. I have can get either a 480Y/277 or a 600Y/347 volt service for the control pedestal.

While 600 volts would require an extra conductor, I could theoretically reduce the phase conductor size and that will come out more economical?

 

[petersonra]

crunch the numbers but my guess is the 480V solution works out better.

 

[mbrooke]

From 480 to 600 volts about a 20% reduction in current, which looks good.

 

[winnie]

Assuming same %voltage drop, the cross section of conductors scales at about 1/V^2 (If you double the voltage, you halve the current, but are allowed _double_ the voltage drop, thus conductors of 1/4 cross section for the same power and same %VD)

If you balance things carefully, does the neutral need to be upsized for voltage drop? The EGC needs to be increased in size, and it would seem strange to have the EGC larger than the neutral...I don't know if there is code for minimum neutral this case.

-Jon

 

[mbrooke]

Assuming same %voltage drop, the cross section of conductors scales at about 1/V^2 (If you double the voltage, you halve the current, but are allowed _double_ the voltage drop, thus conductors of 1/4 cross section for the same power and same %VD)

If you balance things carefully, does the neutral need to be upsized for voltage drop? The EGC needs to be increased in size, and it would seem strange to have the EGC larger than the neutral...I don't know if there is code for minimum neutral this case.

-Jon

You bring up a good point. A reduced neutral would help but at this point Im calculating a full size (only because Im not seeing a code allowance), as large as the phase conductors.

 

[petersonra]

You bring up a good point. A reduced neutral would help but at this point Im calculating a full size (only because Im not seeing a code allowance), as large as the phase conductors.

I think this a reasonable approach, especially if you are using single pole CBs where there is a chance of having the entire load of a single phase to neutral.

IMO you are only required to provide a minimum size neutral that accommodates the maximum current it is likely to see, just like any other conductor. However, there are some minimum size neutrals in the code in various places that might apply along with some minimum size OCPD requirements that might have some bearing.