Calculating load?

[Polarcat]

why do some Engineered drawing calculate load for two and three pole ckts carrying the KVA for the device on each of the phases and some only carry it on one of the phases???? starting to confuse me as to why the diff. approach is taken.

for example to try to explain myself.

if you have a 2.3kw 2pole load 208v (on a three phase 120/208v) in the load calculation summery and on the panel schedule A=2.3 and B=2.3 for that one item.

Vrs.

Same load but with just one phase being calculate at 2.3kw.

Why the diff? whats correct? It would make a big difference either way, lets say calculating a mech panel service size.

Hope I explained my thought correctly, sometimes my mind and my hands don't communicate well!!!!

 

[charlie b]

The correct way is to account for the load on each phase. If there is a 2-phase load of 10 KVA on Phases A and B, then Phase A should be assigned 5 KVA, and Phase B should be assigned 5 KVA. If there is a 3-phase load of 15 KVA, then a load of 5 KVA should be assigned to each Phase.

But sometimes engineers like to create Panel Schedules using a spreadsheet that they developed themselves. It is possible that somewhere inside the spreadsheet is a calculation process that adds up the loads. I have seen one such spreadsheet that appeared to assign all the KVA for a 3-phase load to one of the phases. However, a column that did not appear in print had a math formula that noticed it was a 3-phase load, and internally did the division into three parts, before adding the load on each individual phase. So the math was right, but that fact was not obvious from the paper printout.

 

[Polarcat]

So let me see if I have this straight . some Engineers (just to calculate total connected load and size) will just put one total KW load for an item either three phase or 2pole but the correct way would be to, lets say:

given a cut sheet for a 208v single phase HVAC load and it is listed for 5kw @208v 1ph. you would carry 2.5KW on Aphase & 2.5KW on Bphase

correct?

But why the two ways of looking at it, is there not a standard to follow.

 

[petersonra]

There is a huge difference between a calculated load as it is referred to in the NEC and a calculation that shows what the real load is likely to be.

Often both calculations are made. The former is used for sizing the electrical service and feeders as required by the NEC.

The latter is used for determining ones utility costs.

Many times both are required but for different audiences.

 

[charlie b]

. . . given a cut sheet for a 208v single phase HVAC load and it is listed for 5kw @208v 1ph. you would carry 2.5KW on A phase & 2.5KW on B phase. Correct?

That is how I would do it. But I think you misunderstood my attempt to describe the "other way" that I have seen used.

The "other way" would also have assigned 2.5 KW each to Phase A and Phase B. But it would have done it without making the user first manually divide the 5 by 2, before manually entering the number 2.5 in to the spreadsheet. Rather, it would let the user input the number 5, and it would automatically recognize that this was a 2-pole breaker. So it would do the math of dividing 5 by 2, and it would have automatically assigned 2.5 to each phase.