panel feeder

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shaw0486

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Am i doing this correct,if i had a 3 phase 480 volt lighting panel with 50 kva connected loads of 277 volt lighting, continuous duty and i want to find out what size feeder i needer for thsi panel. 50kva x 1000=50000va,

50000va/480v=104.16666/1.732=60.142413 x 1.25= 76 amp

So i would need #4 copper for this installation?

So even though you have single phase loads in the panel you calculate the feeder as a three phase load?
 
shaw0486 said:
yes lighting loads does that make a differance on the feeder
Click to expand...

It _might_ make a difference, but only if the characteristics of the lighting system are such that you have a 'majority of non-linear loads' on the feeder. If this is the case, then the feeder will have to be treated as having 4 current carrying conductors, and derating will be required.

One can't even say for certain that this is an issue with discharge lighting, since it depends upon the detailed design of the ballasts being used. Some ballasts are designed to prevent harmonics problems, other cause significant harmonics problems.

-Jon
 
shaw0486 said:
right i got 200% neutral to the panel for that problem
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This will help resolve any problems with harmonic-induced neutral current. But it will not address the requirement to derate the ampacity of the ungrounded conductors to 80% of their tabulated value, given that the conduit will enclose four current-carrying conductors.
 
shaw0486 said:
Am i doing this correct,if i had a 3 phase 480 volt lighting panel with 50 kva connected loads of 277 volt lighting, continuous duty and i want to find out what size feeder i needer for thsi panel. 50kva x 1000=50000va,
50000va/480v=104.16666/1.732=60.142413 x 1.25= 76 amp
So i would need #4 copper for this installation?
Click to expand...

...don't forget to check that the cable size you come up with is properly protected by the upstream breaker or fuse.
 
Sorry but I didn't see anything that stated if the 50kVA was balanced or unbalanced. What is the phase breakout of the 50kVA?
 
This will be a new panel?
Might want to base the feeder size on the panel size, not the load.
(Although in this case you might get the same answer...)

Is distance an issue?
Might want to take voltage drop into consideration...
 
JLann said:
Sorry but I didn't see anything that stated if the 50kVA was balanced or unbalanced. What is the phase breakout of the 50kVA?
Click to expand...
Wouldn't that be taken care of by the usual neutral installation, and not in itself affect anything else?
 
LarryFine said:
JLann said:
Sorry but I didn't see anything that stated if the 50kVA was balanced or unbalanced.
Click to expand...
Wouldn't that be taken care of by the usual neutral installation, and not in itself affect anything else?
Click to expand...

No. Minimum feeder size is to be based on the calculated current of each individual conductor. This is typically not addressed in the Annex D 3? examples because most assume a balanced load. However, Example D1(b) does depict, even though it is for a 1? service, how a service rating is adjusted for an unbalanced load.

In addition to adjusting size for a load imbalance, overcurrent protection, derating (both already mentioned above) and ambient temperature correction also have to be address.
 
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