mull982 said:
kingpb: Thanks for the example!
I understand this now, if i have a load across two or three poles I use the L-L voltage when calculating the kvA for that phase, and if I have a single phase load on that phase I use the L-N voltage for calculating the kVA for that phase. In other words if you had a single phase load on one of the legs in your example that was a 5A single phase load then you would take 120*5=600VA. You would then add this 600VA to your 1000VA load (using 208V) for a total kVA of 1600VA.?
In the exapmple that you posted, does this mean that there would be 4.808A on each leg?
You mentioned in a previous post that you could add up the kVA in each load individually and then multiply it by (3) for a three phase load to come up with your kVA. How would that same method be done for the two legs used in your example?
If I am not given the individual loads but am just told that the overall three phase load is 40A is it safe to just take 40 * 208 * 1.73 = 14.3kVA? Do you have a three phase example that is similar?
Thanks for the help
Don't forget the sqrt of 3 if you have a 3 pole load.
The sum of the single phase KVA loads is the total 3 phase KVA load. Think of it as using either (3) single phase transformers or (1) 3 phase transformer. So yes, the 1600VA would be the total connected VA.
Yes, in the example the 4.8 A is on each leg.
Summing each leg KVA, taking the largest and multiplying by 3 is useful for sizing a transformer ahead of the panel, and for selecting the main breaker or fuses.
The beauty of using KVA instead of amps, is that you can split the KVA between poles, e,g, in the example 1000VA is the total load and each pole would have 500VA. If you combined it with the 600VA single pole load, you would have 1100VA on one leg, and 500VA on the other, and techincally 0 VA on the third. the total connected VA is 1600VA.
But, since you have to have the capability to feed the 1100VA on leg one, you take 1100VA x 3 = 3300VA, and 3300VA/(208*1.73) = 9.2A. But now say you have 100VA 2 pole, and 600VA single pole. You could have 500VA on leg 1, 500VA on leg 2, and 600VA on leg 3. in this case your largest leg is the 600VA, and therefore your breaker would only be 3 x 600VA = 1800VA, 1800VA/(208*1.73) = 5A (plus 125% factor)
If you are simply told 40A, 3pole total load then you can determine the total VA as you described.