Optimal Breaker Configuration for Split Phase Mains Power

Originally asked here on EE SE

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?

• Speculative "Imagine if I did this..." questions are't a good fit here, where questions should be about actual problems that you face. You can only get 100A @ 240V or 100A+100A @ 120V (or some combination), but you can't get 100A + 100A + 100A = 300A. The most practical solution is likely to just install a standard 100A 240V split bus panel which will let you use any combination of circuits that fit within the current limitations. Jan 4, 2017 at 1:26
• Your profile does say you're from USA and 300 or 320 would be a very odd sized service. I would suspect service sizes 100, 200, and 400 amp for residential. Jan 4, 2017 at 2:26
• Also USA split phase is 240 phase to phase and 120 phase to neutral. It doesn't become "split" after it gets to you, it's split at the transformer before it arrives at your residence. Jan 4, 2017 at 2:29
• The OP says he has a 100-A panel of only 120-V single-pole breakers. He seems to think that they are all the same side of the split phase, i.e., the voltage between any two hots is zero. But are panels ever wired from the meter base with only the neutral and one side of the split phase? Could it be that there are 240-V 2-pole breakers in a panel under the meter and that the panel he has seen is a sub-panel with only 120-V breakers, but half are on one side and half on the side of the split phase? He could see what the situation is with an extension cord and a voltmeter. Jan 4, 2017 at 3:34

This is not 2-phase, it is split-phase. Think through how that works.

Imagine if you will, two 100A transformers separately service two completely separate 120V systems.

What's the capacity of these together? 200A @ 120V, obviously. So now let's bridge them together so we have 240V end to end.

What's the capacity of these together? Still the same 200A@120V, obviously, since all we've done is bridge them. But what's its capacity @240V?

Now here's the 100-amp question. Do these capacities share or stack? Or does one take away from the other? If the upper transformer is supplying 120V at 70 amps, how much 240V can it also supply without exceeding limits of the transformer?

Think it through.

In practice

Now in practice, those two transformers are constructed on a single frame. But they are indeed separate secondaries, just like the diagram shows. They can be wired with the secondaries in parallel, in which case 200A@120V is what you get.

And, in practice, you'll notice that in the following drawing, the transformer is up on a pole and shared by several houses. The only way they'll sell it to you is 240/120 split-phase.

Now, you asked how you hook up 3 subpanels of 240V, 120 leg A, and 120 leg B, respectively. Assuming we're dealing with a 100A main supply, you can do this:

Mind you, this is exactly what you asked for, not what would be commonly done. Because of that choice, you wouldn't be able to support MWBCs or split-phase appliances (NEMA 10 or 14) out of the sub-panels. That may not matter if you don't have any split-phase loads, e.g. split-phase appliances are not sold in the Philippines since half the island has Euro style 240-only power. There may be other reasons for wanting to do this, e.g. putting whole-panel GFCI or AFCI on the sub-panel, sub-metering for tenants, a central shut-off for all electric heaters, switching part of household load onto a small generator or 120V solar, etc.

As you gathered from the earlier exercise, you cannot load all 3 panels to 100A. Either 100A panel can be loaded to 100 - (240A panel load).

You want 300A

Right off the bat, the power company's pole line from transformer to meter must be able to handle 300A, and that's not what they usually install, because they're not in the business of filling the sky with useless aluminum, and pulling another power drop is not that expensive for the rare customer who needs it.

Depending on your panel, upgrading to 300A is not as simple as swapping your main breaker for a 300A one. Very few service panels are listed for 300A. The chance of you having one is remote.

Typically, in this case, power companies feed two main panels from a single meter, and so you could slap a common 200A or 225A panel right next to your existing 100A panel without having to rewire it. However in that case you would not be able to feed 300A to each of three subpanels.

Getting split-phase out of the above

As drawn, you can't get split-phase out of any of the sub-panels because they are not 120/240 panels. That would require a fourth feed wire.

That's because hots, neutral and ground must source out of the same panel. You cannot feed out of 120V "A" and steal a hot from 120V "B". (we actually had a landlord ask to do that, to force tenants to split the cost of a water heater). You cannot feed out of the 240V panel and steal a neutral or use ground as neutral.

In that case, you'd either feed the split-phase load out of the main panel, or add the fourth wire and reconfigure any of the panels to 120/240 service. As a practical thing they probably started as 120/240 panels anyway, since straight 120 or straight 240 panels are rare.

• I understood the thought process you tried to lead me through. My question was about the specific placement of the breakers, which, as I stated in a previous comment, is handled by the 2-pole breaker. I assumed only one hot was wired, which was my mistake. I never doubted that a 300A (or 100A, or whatever size) breaker would ever serve more than 300A on each hot. And yes, that allows a theoretical maximum of 600A @ 120V (if circuits are divided equally) or 300A @ 240V. That's just conservation of energy... Also, pretty sure the upper transformer has 30A headroom in your scenario.
– Hari
Jan 5, 2017 at 1:18
• Right on the button. The main breakers is on the "hots" and not the neutral. In the above scenario with 100A transformers, the breaker would be on the very top and bottom wire, none on neutrals, and it would take a 100A main breaker where either side would trip if that's side current exceeded 100A. Either side tripping knocks out both sides. Jan 5, 2017 at 1:23