I am studying for an exam that I have already taken. This may be a silly question but here goes. When you are asked to size a feeder conductor for a 225a panel used only for lighting with a total connected load of 90,000va, do you size it to the panel or the connected load? I don't remember the voltage or the type of building it is in.
Lighting panel feeder sizing
- Thread starter Rouse
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jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
You would size it per the connected load, unless speced otherwise.
I believe voltage is irrelevant.
Lighting loads are calculated per T220.42.
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- Illinois
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- retired electrician
It has to be sized to supply the load and to be protected by the supply side OCPD.
So you guys are saying the 225a is just extra info I don't need and just size it to the connected load?
You size the feeder for the load. If you assume the voltage is 120/208 volts
3 phase 90000 va = 250 amps. Lighting is usually considered a continuous load so you would need to multiply 250 x 1.25 = 313 amps. Going back to your origional question, you can size the conductor to match the ampacity of the panel if it is protected by a 225 amp breaker.
Voltage is very relevant! A 225A panel with 90kVA lighting load would be an overloaded panel (by 88 amperes) if supplied by a 208Y/120 source and an occupancy which does not qualify for any lighting load demand, regardless of the feeder being sized to carry the load. Supplied with 480Y/277 would make a world of difference: 90kVA?(480?√3)=108A
I believe it was 277v because several lighting questions were 277v. I think I know were my problem was. Tell me if you think this is right, 90,000va @ 277v = 188 amps. What I think I did wrong was not multiply this amount by 125% for it being a continious load. At that point I was not sure to size it by the load of 188 amps or the panel at 225 amps. But If I had multiplied the lighting load by 125% that would make it 235 amps so I would want to size it by 235 amps? (which if THW would be a 250 kcmil?)
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- Chapel Hill, NC
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- Retired Electrical Contractor
If the total connected load is 188 amps and the load is continuous then you have a load of 235 amps --as you stated. Now article 215. 3 states the feeder OCPD must be not less than the sum of the continuous plus the non continuous load. This is a problem since you would have to protect this 225 amp panel with a 250 amp OCPD. I don't believe you can do this. 408.36
There is an excep.- if the ocpd is rated for 100% of its rating then you do not have to figure the load as continuous.
Not given any more info then I would have to say the feeder could be 188 amps or 3/0 copper at 75C
Now do your formula with 90000/ 480*1.73
Sorry I just caught that mistake.
There is an excep.- if the ocpd is rated for 100% of its rating then you do not have to figure the load as continuous.
Not given any more info then I would have to say the feeder could be 188 amps or 3/0 copper at 75C
Now do your formula with 90000/ 480*1.73
Sorry I just caught that mistake.
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- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
It is not right, as Dennis has pointed out. You are dealing with a three phase system. Current flowing out on any wire will return on one or both of the other two wires. The voltage driving this current is the line-to-line voltage of 480. So the correct calculation of current is 90,000 divided by (480 x 1.732), or about 108 amps. You then multiply that value by 1.25, and get 135 amps. So the MINIMUM feeder size (which is what a test question is likely to be asking for) is based on 135 amps.
That said, in a real world situation, I would at least consider sizing the feeder for the full 225 amps of the panel's rating. I might not choose that feeder size, but I would consider it. Tests, however, are asking for specific things, and that is what you must base your answer upon.
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Let me rephrase what what I meant.
"Voltage is irrelevant in applying the demand factors of T220.42. A higher voltage will result in a lower amperage, yet the demand factor for the occupancy will stay the same."
I did not do the math for the OPs question, I was only trying to make that point.
For some reason I have heard guys around this area think that if the commercial lighting is 277v instead of 120v, they will not have to apply the 100% demand factor. Where this comes from, I do not have a clue.
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