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First, let me preface this by saying this is entirely theoretical. I don't plan to install anything myself, and I'll contract a licensed electrician to do any of this work, so please don't feel the need to post warnings, disclaimers, or otherwise explain the dangers of mains electricity.

Given that, I have learned that my utility company provides 240V to the meter at my house. There is a breaker box immediately downstream of the meter, and it services 100A at 120V (so it becomes split-phase at some point). After a bit of conduit, I have my current breaker panel, with its circuits going off into the house.

Assuming that I am correct in that there is a center-tapped transformer stepping the meter voltage down to 120V from 240V and, therefore, provides two phases (we'll call them "Phase 1" and "Phase 2"), this means I have 240V, Phase 1 120V, and Phase 2 120V available to me, even though I am only using Phase 1 120V.

I want to have three total breaker panels, one for each Phase 1 and 2, and 240V, but there is also that 100A breaker immediately following the meter to deal with.

My question is, what is the best way (schematically) to install 300A (I can get up to 320 from the meter without an upgrade) of breaker to protect all three circuits?

My current thought is that I would need a 300A breaker on the Phase 1 hot wire and a 300A on the Phase 2 hot wire. Is this correct, or is there a simpler/better way to arrange this?

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  • \$\begingroup\$ I think you'll get a better response on DIY.SE. Though most people here could probably work this out, we're electrical engineers, not electricians. \$\endgroup\$
    – Samuel
    Jan 4, 2017 at 0:18
  • \$\begingroup\$ @Samuel, I am also on the DIY SE, but I wasn't quite sure since I've seen a lot of mains power-related questions here. I am mostly asking this theoretically because I'm curious about breaker arrangement to fail safely. \$\endgroup\$
    – Hari
    Jan 4, 2017 at 0:27
  • \$\begingroup\$ ganged 300A twin breaker that trips Line 1 and Line 2 is what its called. For countries with ungrounded neutral it uses a 3 circuit breaker such that all 3 lines trip if either L1 or L2 trip but N is not sensed. \$\endgroup\$
    – Tony Stewart EE75
    Jan 4, 2017 at 0:28

1 Answer 1

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In North America, the normal domestic breaker panel will carry both "phases" of the 120/240 volt supply. The panel is arranged so that a single pole breaker will deliver 120 volts from one of the two "hot" wires depending on its position in the panel, while a two pole breaker will deliver 240 volts, as it will pick up both "hot" wires.

If the incoming main circuit breaker is 100 Amp, it will deliver 100 amp on each "hot" wire, to allow a total of 200 amps at 120 volts (if evenly split between phases), or some mix of 120 volt and 240 volt, with a maximum load on either phase of 100 Amps.

You should look for web sites or videos covering North American (or just American - most people ignore us Canadians) electrical codes and wiring practices.

As @Samuel suggests, DIY.SE will have lots of Q&A on house wiring that should help you with your proposal.

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  • \$\begingroup\$ If I understand you correctly, this means that each phase's hot wire is connected to a 2-pole breaker, allowing 100A on each, which is similar to how I described my thoughts on how it should be connected, except the breakers are joined, so breaking one phase breaks the other. However, going to the subpanel I only have three wires (L, N, G) so only one phase is actually connected to any circuits. At any rate, I appreciate the answer. I didn't realize the "joined" breaker was a 2-pole breaker for 240V, not just a double ampacity breaker. \$\endgroup\$
    – Hari
    Jan 4, 2017 at 0:34
  • \$\begingroup\$ Yes - the house main breaker, and breakers for 240 volt branch circuits, must be two pole breakers that will break both hot wires. Also, it is usually permissible to use two single-pole breakers with their handles tied so they trip together for a 240 volt circuit. \$\endgroup\$
    – Peter Bennett
    Jan 4, 2017 at 0:39
  • \$\begingroup\$ As I said, commonly-available breaker panels handle both 120 volt and 240 volt circuits - they carry both "hot" wires. You can get some very small panels (4 circuits or so) that are designed to handle a single 120 volt phase. Your suggestion of phase A in one panel, phase B in another, and a third panel for 240 Volts would be extremely unusual, and may be contrary to the Electrical Code. \$\endgroup\$
    – Peter Bennett
    Jan 4, 2017 at 0:43
  • \$\begingroup\$ I am guessing it's against code to leave one phase hot, in case the other trips then. I was hoping to have two separate phases, in case one breaker tripped. I also posted this to DIY SE here: diy.stackexchange.com/q/105694/64100 \$\endgroup\$
    – Hari
    Jan 4, 2017 at 0:45
  • \$\begingroup\$ A normal 15 Amp 120 volt branch circuit will be protected by a single pole breaker. If you overload that circuit, only its 15 amp breaker will trip - other breakers in the same panel, on same or opposite phase will not be affected. \$\endgroup\$
    – Peter Bennett
    Jan 4, 2017 at 0:50

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