I'm not an electrician (thus the question), but I'd like to know what is possible from a specification standpoint.

Let's say we have a standard 3 phase power supply for a commercial building. We want to supply a sub-panel with 3 phase power and the sub-panel is supplied from a 3 phase 100A breaker.

We want to supply 3 dedicated single phase 208V loads, so we'll be building effectively a "load triangle."

What is the maximum ampacity for each load?

It's confusing to me, because each leg is supplying two loads "half way." So does that mean it's "ok" to use 3 100A two-phase breakers in the sub-panel (assuming the loads are correctly distributed)?

Don't worry too much about the nature of the loads. In fact, they will be EVSEs (electric car chargers). This means they'll have a fairly reasonable impedance match, but be continuous draw (which means they'll actually be derated from 100A by 20%).

  • \$\begingroup\$ When you say "two-phase load" do you mean a load that is between two of the three legs of your three-phase AC? If so, that would typically be called a "single-phase load". Two-phase is not a common concept. \$\endgroup\$ Mar 11, 2014 at 21:14
  • 1
    \$\begingroup\$ Also, I'm a little concerned that you're talking about building a hundred-amp three-phase panel, and you're not an electrician. Be careful. \$\endgroup\$ Mar 11, 2014 at 21:16
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    \$\begingroup\$ I've got a hose and water pump at the ready just in case. \$\endgroup\$
    – Andy aka
    Mar 11, 2014 at 21:18
  • \$\begingroup\$ @StephenCollings To me, when I hear "single phase" in this context, I think hot-neutral. Perhaps that's my own stupidity. And I am in no way contemplating building anything. I'm trying to converse coherently with experts about specifications. \$\endgroup\$
    – nsayer
    Mar 11, 2014 at 22:36
  • \$\begingroup\$ Don't confuse ignorance and stupidity. Ignorance can be fixed, which is why we're all here! Stupid is forever. \$\endgroup\$ Mar 11, 2014 at 22:51

1 Answer 1


3 times 100A at 120V means you've got 36KW coming into your fuse board.

3 times 57.7A at 208V means you've got 36KW going out of your fuse board.

Assuming your load is balanced, anything more than 57.7A on the load side will be more than 100A on the supply side.

Where I work, we step down the wiring and the circuit breakers so the end points break before the supply points, so for 208V circuits we'd be looking at a 3 phase (3 pole) 50A circuit breaker to break all three circuits if there is an overload on one, or 3 single phase 50A circuit breakers if you can allow single phase loads to fail independantly

  • \$\begingroup\$ The single phase loads can fail independently. The conversion into total power makes complete sense. I'll consider 3 50A two pole breakers for the design. Thanks. \$\endgroup\$
    – nsayer
    Mar 12, 2014 at 5:22

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