Calculating 3 Phase Equipment Loads

[LeeB]

When determining the correct kva to insert on each phase of a panel schedule for a three phase load should the total KW be inserted at each phase or is the total divided by 3 and then inserted on each line? In other words if an HVAC unit is a 5KW load at 208v three phase is 5000 inserted on each line or is 5000 divided by 3 (1666) and then inserted? Of course I guess the PF would need to be figured to convert from KW to KVA at probably .80

And on the same line would a motor load be converted to kva from table 430.250 as the FLA x 120v for each phase? Or would the FLA be multiplied by 208v, then divided by three.

I would appreciate any thoughts on the correct methods!

LeeB

 

[mivey]

When determining the correct kva to insert on each phase of a panel schedule for a three phase load should the total KW be inserted at each phase or is the total divided by 3 and then inserted on each line? In other words if an HVAC unit is a 5KW load at 208v three phase is 5000 inserted on each line No
or is 5000 divided by 3 (1666) and then inserted? Yes
Of course I guess the PF would need to be figured to convert from KW to KVA at probably .80 Yes

And on the same line would a motor load be converted to kva from table 430.250 as the FLA x 120v for each phase? Or would the FLA be multiplied by 208v, then divided by three.
kVA = FLA * 120 * 3 / 1000
or
kVA = FLA * 208 * sqrt(3) /1000
I would appreciate any thoughts on the correct methods!

LeeB

Here are the formulas:

3 phase kVA = amps * voltage line-line * sqrt(3) / 1000 for 3 phase loads
3 phase kVA = amps * voltage line-neutral * 3 / 1000 for 3 phase loads

kVA = kW / PF for any load

1 phase kVA_L-N = amps * voltage line-neutral / 1000 for line-neutral loads
1 phase kVA_L-L = amps * voltage line-line / 1000 for line-line loads

For each ungrounded conductor (UC) of the panel schedule, you may have some pairs of UCs sharing different line-line loads, and some UCs with different line-neutral loads, but all UCs will share the 3 phase loads:

kVA for a UC = 1/3*3_ph_kVA + 1_ph_kVA_L-N + 1/2*1_ph_kVA_L-L
kW for a UC = 1/3*3_ph_kW + 1_ph_kW_L-N + 1/2*1_ph_kW_L-L

 

[LeeB]

Thanks

Thanks

Thanks Mivey,

Very clear and concise answer, much appreciated!!!

LeeB

 

[Smart $]

Here are the formulas:

3 phase kVA = amps * voltage line-line * sqrt(3) / 1000 for 3 phase loads
3 phase kVA = amps * voltage line-neutral * 3 / 1000 for 3 phase loads

kVA = kW / PF for any load

1 phase kVA_L-N = amps * voltage line-neutral / 1000 for line-neutral loads
1 phase kVA_L-L = amps * voltage line-line / 1000 for line-line loads

For each ungrounded conductor (UC) of the panel schedule, you may have some pairs of UCs sharing different line-line loads, and some UCs with different line-neutral loads, but all UCs will share the 3 phase loads:

kVA for a UC = 1/3*3_ph_kVA + 1_ph_kVA_L-N + 1/2*1_ph_kVA_L-L
kW for a UC = 1/3*3_ph_kW + 1_ph_kW_L-N + 1/2*1_ph_kW_L-L

It should be noted that the formula portion I highlighted in red is only a vague approximation. Vector math or diagramming is the only way to get a nearer-to-accurate result.