3-Phase Transformer, 1-Phase Loads

[zimmerDN]

How many amps of 120V single phase loads can a 3-phase 30KVA, 600V - 120/208V transformer delivery on L1, L2 & L3?

What about 208V single phase loads?

I understand that you can have 30,000/208/1.73 = 83A of 3-phase 208V balanced loads but I want to understand the single phase 120/208 scenario.

 

[dpc]

If the single phase loads are balanced, it can carry 30 kVA. The transformer doesn't know the difference between three-phase and single-phase loads. It only knows about current through the windings.

 

[zimmerDN]

DPC,

If it can carry 30KVA, would that be 30,000/120 = 250A on L1-N, L2-N, L3-N if they are all balanced 1-phase loads?

To the transformer these are the equivalent of 250A / (120/208v) / 1.73 = 83A across L1-L2, L2-L3, L1-3?

 

[waross]

First, a 30 KVA transformer may deliver 10 KVA per phase.
That is 10000VA/120V = 83 amps per phase.
L1, L2, L3 or X1, X2, and X3 are each the end of a single phase winding. The entire current through the terminal must pass through that winding. Although it may be argued that when only one phase is loaded, the transformer may be overloaded somewhat due to the ability of the transformer to reject the heat from more than 10 KVA, this is not good practice and the extra thermal capacity is just a fraction of the apparent 30 KVA, considering I²R losses and the heat conduction path to the unloaded cores.
So, 83 Amps per winding, MAX.
A single phase load across 208 Volts may be 83 Amps for a total of 17.3 KVA. Still 83 Amps per winding.
Where it gets complicated is when 120 Volt loads are combined with 208 Volt loads and unequal 208 Volt loads on more than one phase are connected.
The phase angles of the currents must be considered. The vector sum of the currents of all the loads may not exceed 83 Amps. (Don't forget to consider the power factor of the load currents when determining the phase angles.)

Bill
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"Why not the best?"
Jimmy Carter