1-Phase panel fed from 3-Ph 120/208V main panel

[Eclectic engineer]

Hello Guys,
I have a 1-Ph 100A 120/240V sub-panel being fed from a 3-Ph 120/208V service.
What will be the line to line voltage on the 1-Ph sub-panel?
Will it be 208V or 240V?

Also, what will be the power formula for all loads branching out from the sub-panel?
Do I need to multiply 1.732?

Thank you.

 

[infinity]

Since your feeder to the panel originates at a 208Y/120 volt service the line to line voltage at the panel will be 208 volts. For 1Ø calculations you do not use 1.73 with you voltage that would only be for 3Ø calcs.

Welcome to the Forum.

 

[Eclectic engineer]

Got it!
So a 15A draw would mean 15 x 208V = 3120Watts..?

 

[infinity]

Got it!
So a 15A draw would mean 15 x 208V = 3120Watts..?

Yes, you would not use 1.73 in your calculation because it's single phase. Let's see what others say but I like VA instead of Watts.

 

[Eclectic engineer]

Ohh okay. I assumed a power factor of 1. Hence I used Watts and not VA.

 

[LarryFine]

So a 15A draw would mean 15 x 208V = 3120Watts..?

Depends on the load's design voltage.

If the device is using 15a while supplied with 208v, then, of course, yes.

If the device is designed to use 15a on 240v, then no.

Resistive 240v loads will use less current on 208v, motor loads will use more.

 

[LarryFine]

Ohh okay. I assumed a power factor of 1. Hence I used Watts and not VA.

For theoretical discussions, we can set PF aside for now.

 

[Eclectic engineer]

Okay Thanks guys.
I wanted to confirm if 208V can be used for 1-Ph Power formula. And it appears it can if a 1-ph panel is connected on a 3-Ph 120/208V service..

 

[kwired]

But two 15 amp 120 volt loads on each phase with a multiwire branch circuit will draw 15 amps on both lines as well as on the neutral, and will be 3600 Va

 

[Carultch]

Got it!
So a 15A draw would mean 15 x 208V = 3120Watts..?

Example 1: A 208V 2-pole breaker, feeding a 208V load that doesn't touch the neutral would be 3120 Watts.
Example 2: A 120V 1-pole breaker, feeding a phase-to-neutral load, would be 1800 Watts
Example 3: A 208V, 2-pole breaker, feeding identical 120V loads wired phase-to-neutral on each phase (which we call a multiwire branch circuit), would be 3600 Watts.
All of the above assuming unity power factor, and neglecting any load-specific safety factors that may apply.

The reason example 1 can be different than example 3, is that the neutral will have to carry the full current in example 3, even if both phases are equal. The two phases are 120 degrees out of phase, instead of opposite. A 3-phase circuit would enable current on all three phases alone to add up to zero, but an MWBC on a 2-pole breaker doesn't have enough phases to close the circuit. The neutral has to carry the full load to complete the circuit.