Load calculation of multiple 1ph devices

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mic

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I am reviewing a load calculation from an Engineering consulting firm. There are 8 2kW L-N strip heaters (277V), 3 on each of phase A and B and 2 on Phase C.

The load calculation (for feeder) treats all the heaters as a single 3ph device.
I=P/[480*(3^0.5)]

I have always treated the heaters as 1ph devices and calculated the load on each phase and used the max phase current to size the feeder.
I(A)=P(A)/277
I(B)=P(B)/277
I(C)=P(C)/277

Imaxph=MAX{I(A),I(B),I(C)}

Are there any thoughts on the best/correct way to do this.

C
 
Re: Load calculation of multiple 1ph devices

Either way is correct except that in this case you do not have a balanced load. Phases A and B
have 3 heaters and C has 2. To get the amps on each phase I would use the wattage and divide by the 277 volts which give the amps per phase.
Obviously phases A and B have 3 x 2000 /277 = 21.7
amps and C has 2 x 2000/277 = 14.4 amps. If you had 9 heaters and 3 heaters per phase you could
caculate as follows 9 x 2000/(480 x 1.73)= 21.7 amps.

[ January 12, 2005, 04:33 PM: Message edited by: bob ]
 
Re: Load calculation of multiple 1ph devices

For the purposes of calculating the feeder load, I would agree with the Engineering Firm. But to make sure we are ?on the same page,? I would calculate the total load in terms of VA, and then divide by 831 (i.e. 480 times the square root of three).

8 times 2000 divided by 831 equals 19.25 amps.

Is that their answer? If so, that is the way I would submit the design. I recognize that the load is not balanced, though it is as nearly balanced as can be achieved with only these 8 loads. I agree that the actual currents are 21.7, 21.7, and 14.4. Nevertheless, I believe it is acceptable, even appropriate, to continue the design process with the value of 19.25 amps as the average phase current.
 
Re: Load calculation of multiple 1ph devices

So it is not necessary to use the max phase current to size feeders, only the average phase current? Does this only apply when the loads are "reasonably" balanced?

C

[ January 13, 2005, 02:02 PM: Message edited by: mic ]
 
Re: Load calculation of multiple 1ph devices

So it is not necessary to use the max phase current to size feeders, only the average phase current? Does this only apply when the loads are "reasonably" balanced?
Click to expand...
Pretty much, but be careful. You still need to make sure the conductor is not loaded beyond its ampacity as you know. If your loads are perfectly balanced then the average phase current is equal to all of the phase currents. But I do wonder from Charlie b's statement if he would size his conductors for the max phase current which in this case, would be 21.7 Amps and not size the wires for the average phase current of 19.25 Amps. In the real world, would one just install a #10 AWG anyways?

[ January 13, 2005, 02:34 PM: Message edited by: wirenut1980 ]
 
Re: Load calculation of multiple 1ph devices

Originally posted by wirenut1980: But I do wonder from Charlie b's statement if he would size his conductors for the max phase current which in this case, would be 21.7 Amps and not size the wires for the average phase current of 19.25 Amps. In the real world, would one just install a #10 AWG anyways?
Click to expand...
I would size it for the 19.25 amps, and not bother trying to figure out what the maximum current might be on any phase. Whether I were to design the phase loading, or whether that would be left to the installer, there is an inherent presumption that the loads will, indeed, be balanced as nearly evenly as is possible.

That being said, I would select a #10 for this circuit. It is not because I think the NEC would require it (it doesn?t), nor because of a concern that imbalanced loading might cause one phase to be loaded beyond 20 amps (not a design consideration). Rather, I would want a bit of a margin on this circuit. If the owner wanted to install the ninth heater, it would drive the total current beyond the 20 amp limit of a #12. This is a design consideration, not a code issue.
 
Re: Load calculation of multiple 1ph devices

Thanks to all for their input. Much appreciated.

Just a little background info... We design a product that is classified more as equipment than building. The owner would be taking on the liability for the equipment if they made any modifications. As such, we usually size wires based on the engineered requirement and state that no changes should be made without consulting us or a qualified engineering firm.

C
 
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