KVA calculations on 2 pole breaker.

[mikealvz]

Hello,

i have a 208v 30A power circuit.
I would like to take live readings on circuit breaker to calculate the total KVA usage at any given time.
When taking my readings on both legs I get 11A and 9.4A.
To Calculate the KVA will i need to add both Amp readings together from each leg? So,
11+9.4A = 20.4A?

(208V x 20.4A)/1000 = 4.24KVA?

Thank You for your help.

 

[petersonra]

Hello,

i have a 208v 30A power circuit.
I would like to take live readings on circuit breaker to calculate the total KVA usage at any given time.
When taking my readings on both legs I get 11A and 9.4A.
To Calculate the KVA will i need to add both Amp readings together from each leg? So,
11+9.4A = 20.4A?

(208V x 20.4A)/1000 = 4.24KVA?

Thank You for your help.

my guess is what you have is a MWBC with both lines being L-N loads.

what you would have then is an 11 A 115V load and a 9.4A 115V load.

 

[kwired]

Hello,

i have a 208v 30A power circuit.
I would like to take live readings on circuit breaker to calculate the total KVA usage at any given time.
When taking my readings on both legs I get 11A and 9.4A.
To Calculate the KVA will i need to add both Amp readings together from each leg? So,
11+9.4A = 20.4A?

(208V x 20.4A)/1000 = 4.24KVA?

Thank You for your help.

You obviously must have a neutral in this circuit otherwise you have 1.6 amps going to ground somewhere in the circuit.

If you have an 11 amp @ 120 volt and a 9.4 amp @ 120 volt loads on a multiwire circuit you really have 11x120 and 9.4 x 120 which gives you 2448 VA total. If you have a mix of 120 and 208 volt loads it gets a little more complicated.

Unlike 120/240 single phase the neutral doesn't carry the imbalance in this particular type of circuit. If it were a 120/240 single phase supply the neutral would be carrying the 1.6 amps of imbalance, but on a 120/208 three wire circuit the neutral will be carrying somewhere close to same current as the higher loaded ungrounded conductor.

 

[mikealvz]

You obviously must have a neutral in this circuit otherwise you have 1.6 amps going to ground somewhere in the circuit.

If you have an 11 amp @ 120 volt and a 9.4 amp @ 120 volt loads on a multiwire circuit you really have 11x120 and 9.4 x 120 which gives you 2448 VA total. If you have a mix of 120 and 208 volt loads it gets a little more complicated.

Unlike 120/240 single phase the neutral doesn't carry the imbalance in this particular type of circuit. If it were a 120/240 single phase supply the neutral would be carrying the 1.6 amps of imbalance, but on a 120/208 three wire circuit the neutral will be carrying somewhere close to same current as the higher loaded ungrounded conductor.

Yes there is a neutral in the circuit. So to get the correct KVA i will need to add after calculating the kva on each separate leg. Adding the amps first gives me wrong amount of KVA?
I am new to these calculations. Thanks again for all the help.

 

[kwired]

Yes there is a neutral in the circuit. So to get the correct KVA i will need to add after calculating the kva on each separate leg. Adding the amps first gives me wrong amount of KVA?
I am new to these calculations. Thanks again for all the help.

If all you have is 120 volt loads you can add amps on each leg, but you need to multiply that sum by 120 and not 208 to get the net VA.

 

[gar]

1321029-1410 EDT

mikealvz:

Why does kVA load have any importance? What are you really trying to find out?

The RMS current in a wire will determine the heating within the wire. This will be true for the wire in a transfomer, any interconnecting wire, and the heating element in any fuse or circuit breaker. This has nothing to do with the nominal voltage of the circuit.

If you have more than one circuit sharing a common wire (neutral typically), then you need to know the RMS value of the combined current to determine wire size of that common wire.

Transformers may have a kVA rating rather than a current rating. This is to simplify the description of a transformer that has multiple separate coils that can be combined in various fashions.

Suppose you have a 10 kVA transformer with a center tapped 240 V secondary. If you load 1/2 of the secondary with 10 kVA the transformer secondary half that is loaded will burn out. Probably 4 times the power dissipation it can tolerate, or thereabouts. The internal thermal conductivity will determine the maximum continuous load that can be applied to only half of the secondary. In the 10 kVA transformer it is probably safe to somewhat exceed 5 kVA on the secondary side that is loaded. Note: there is no dissipation in the unloaded side, and the primary is near 1/4 its full load dissipation. Thus, those item don't contribute any, or at least much less heat to the total transformer package, and that means slightly more heat could be generated in the loaded 1/2 secondary than the nominal rating.

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