Calculating Load Currents of Lighting Panel Boards

[bim802000]

Hi,


I'm currrently checking the lighting panel board schedules of pass projects and noticed a "minor error" in the calculation of the load current.


In it, the designer averaged the currents of the three phases (current phase "a" + current phase "b" + current of phase "c") and multiply this with the demand factor. What should have been done was to choose the HIGHEST phase current and multiply that with the demand factor.


I've tried looking at NEC for the exact article that directly talks about this but i couldn't find it. Can anyone give me leads, in NEC or other references, that directly addresses this issue?


Thanks,

 

[Smart $]

As far as the NEC goes, there is no exacting method. The closest it comes to such, which is not enforceable, is the Annex D Examples... in particular Example D1(b).

 

[Smart $]

PS: Welcome

 

[iwire]

I thought we used VA for load calculations?

 

[jtester]

I believe that the original designer is correct.

Bob also has a good point. If your panel feeds L-n and L-L loads, adding up the amps on a phase will give you an incorrect load number. You should turn each load into va, add them up by phase and then you are fine. If all of your loads are L-n then you won't have a problem using amps.

If you use the highest phase load, turn it into va, and calculate from there, your demand calculation will yield a number larger than the actual demand load should be.
If you apply the demand factor to each phase load, then add the three up, you will get the correct number.

If all your loads are L-n as I pointed out above, then if you average the amps, and do your calculation as though you have a balanced 3 phase load, you'll get the correct answer also. A bit of arithmetic manipulation will show you that, if your amps are correct, you'll get the same answer whether you calculate individual va's, or average the amps.

Jim T

 

[MIEngineer]

When working with load calculations is it correct the take a load kVA, divide by # of phases and allocate that to each phase of the system?

Example 1: 240V, 7.2 kVA load. I would allocate 3.6 kVA to phase A and 3.6 kVA to Phase B.
Example 2: 480V, 3 phase, 33 kVA. I would allocate 11 kVA to phase A, 11 kVA to Phase B and 11 kVA to Phase B.

 

[Smart $]

I thought we used VA for load calculations?

We should. But in the end, we always have to determine the service or feeder ampacity (i.e. wire size, type, method, adjustments...) as part of or extension to the load calculations. I believe how to correctly determine minimum ampacity is the main OP question. I also believe most don't bother adjusting for how well (or not) their panels are balanced.

 

[Smart $]

When working with load calculations is it correct the take a load kVA, divide by # of phases and allocate that to each phase of the system?

Example 1: 240V, 7.2 kVA load. I would allocate 3.6 kVA to phase A and 3.6 kVA to Phase B.
Example 2: 480V, 3 phase, 33 kVA. I would allocate 11 kVA to phase A, 11 kVA to Phase B and 11 kVA to Phase B.

Ideally, yes. But, for one, what do you do if your example loads don't split equally into 3.6 and 11kVA respectively???

 

[MIEngineer]

I performed a more extensive search of the site and found this thread.

http://forums.mikeholt.com/showthread.php?t=81200

Without rehashing an entire thread I believe this answers my question.

Thanks,

 

[bim802000]

Smart $,
Yep, thanks.

Smart $, jtester,

I think the Annex D of the NEC is not the best thing to refer to in my case. It?s actually an Oil & Gas plant lighting that I?m working on. The demand factor will be equal to 1 since all of the plant lighting is on at night, and yes the system is L-N (they will be fed from a 480-400/230V lighting transformers).

The reason why we usually use the ampere as the starting point is that, the actual load per lighting fixture is quoted in FLA. Not in VA. (e.g. say 250W HPS, you wouldn?t compute the FLA as 250W/230V, since the actual current drawn will be bigger than that, usually 1.5 times). The FLA is given in the data sheets from the vendors.

Second reason why we do it from load going to the panel is that, we want to check if the lighting transformer size that was given during the FEED is still correct after all the changes (addition/deletion) of the lighting fixtures during the detailed design.

 

[Smart $]

...

I think the Annex D of the NEC is not the best thing to refer to in my case. It’s actually an Oil & Gas plant lighting that I’m working on. The demand factor will be equal to 1 since all of the plant lighting is on at night, and yes the system is L-N (they will be fed from a 480-400/230V lighting transformers).

Annex D examples do leave some questions, but having the examples does provide some answers, too.

400/230V is not a standard system voltage here in the US. Is this installation in another country? Is this installation subject to NEC compliance?

The reason why we usually use the ampere as the starting point is that, the actual load per lighting fixture is quoted in FLA. Not in VA. (e.g. say 250W HPS, you wouldn’t compute the FLA as 250W/230V, since the actual current drawn will be bigger than that, usually 1.5 times). The FLA is given in the data sheets from the vendors.

Second reason why we do it from load going to the panel is that, we want to check if the lighting transformer size that was given during the FEED is still correct after all the changes (addition/deletion) of the lighting fixtures during the detailed design.

Having only L-N loads makes it easy to use ampere values... it's just that calculation requirements of the NEC are spec'd in VA, making conversion necessary one way or the other. Additionally, here in the US, our transformers are mostly rated in kVA units.