Determining ampacity for 3 phase panel with single phase loads

Question

I am an electrician seeking clarification on determining the ampacity for a 3-phase electrical panel with balanced single phase 2P loads. I have a specific scenario and would appreciate the expertise of the community to guide me in the right direction.

Scenario: I am connecting three equal balanced single-phase 208 V loads to a 3-phase 208 V wye panel. Each load is on a 100 A 2P breaker, and that is the minimum breaker size for the load. One breaker goes on L1+L2, one goes on L2+L3, and one goes on L1+L3. My question revolves around determining the correct ampacity required for the conductors feeding the 3-phase panel.

My understanding is that the square root of three (√3) should be applied to calculate the required ampacity (100 A × 208 V) × 3 / √3 / 208 V = 173.2 A). However, I don't understand why because each 2P breaker is essentially two 1P breakers, leading me to think that each pole should receive 200 A, thus requiring the conductors to have an ampacity of 200 A.

Could someone please clarify the correct approach for determining the ampacity of the feeder conductors in this scenario? I would greatly appreciate it if the explanation could be presented in plain terms suitable for an electrician.

Answer 1

For a delta connected load which you have described. The incoming line current will be √3*phase current which for 100 amps would be 173.2 amps. It comes down to trigonometry. The line current is a combination of the currents from the two phases. Not sure if you want a more in depth math class. I've haven't thought of these proofs in a long time.

Answer 2

Since the loads are symmetrical, just use VA.

So one load is a 100A / 208V breaker, or 20,800 VA.

All 3 loads are 62,400 VA.

Now what would happen if loads of the exact same VA were 120V single-phase loads that were connected "Wye"? They would be 20,800 VA each, or 173 amps each.

Since a balanced load has functionally zero current on neutral, switching the connection from wye to delta does not change the amount of power required for the same VA.