Thursday, September 28, 2023

By Mike Sokol

# Q&A’s from my Facebook group:

I’m getting a lot of interesting questions on my RV Electricity Facebook Group. Here’s one with an answer that’s not obvious. Sometimes the seemingly simple things are the most complicated.Â

Q: Would it be possible to combine two 15-amp circuits, on separate breakers, to generate a 30-amp input? Would you run into phase problems? â€”Wilson Ferguson

A1: Please donâ€™t. On a 50-amp rig you might have phase problems. On any rig one breaker will trip first grossly overloading the other’s wire and breaker. Iâ€™ll also bet it violates code.
And someone may shut off one breaker thinking itâ€™s off and get electrocuted as the other is on. â€”Mike Ehlert

A2: Here is another look at it. YES it’s not only possible but if done right, would also be safe. Most residential/ RV pedestals are single phase. As long as they are 2-15A breakers on the same pole AND tied together, itâ€™d work fine. Many 200A main breakers are just 2-100A breakers tied together. Prove me wrong. â€”Ray Hapeman

A3: If it’s a 30A-only pedestal, then you are correct in saying that the pedestal is single phase. However, if it’s a 50A pedestal, then it is most definitely two phase (normally called “poles” by the NEC). Here’s why:

1. A 50A socket has two hots (L1 and L2), a neutral (N) and a ground. There is 120V from L1-N and L2-N. There is also 240V from L1-L2.
2. The reason L1-L2 is 240V is that L1 and L2 are out of phase with each other.
3. The reason it needs to be 240V is simple: wire gauge. With a 240V two phase circuit, each wire (L1, L2, N) can and is the same gauge. E.g., with 50A, the wiring in an RV cord will be three #6 wires and a smaller #8 ground. This means that L1 can handle 50A, L2 can handle 50A, and N can handle 50A.
4. When L1 and L2 are wired to the opposite phases, then the return current on N would always be less than 50A because L1’s current can return either through L2 or N. Same for L2 returning through L1 or N.
5. If L1 and L2 were wired to the same phase, then all of L1 + L2 current must return through N. This would require N to handle up to 100A and the wire would have to be larger to accommodate.
6. When two 100A breakers are tied together in a circuit panel, they are such that they operate on the two opposite phases. When you turn on/off the tied-together 100A breakers, it controls both phases at the same time. That’s also true for any other pair of adjacent tied-together breakers in a panel: that’s because adjacent breakers control opposite phases. And two 100A breakers will give you two 100A 120V feeds (opposite phase) or one 100A 240V feed. You can never get a single 200A feed out of this setup.
7. The National Electrical Code requires it to be wired this way. â€”Paul Pelsinski

A4: Great write-up Paul. So, for all the reasons you describe, you can’t combine circuit breakers as thatâ€™s a big code violation and extremely dangerous. And if you managed to hook them up in-phase and get it all working, and you were to accidentally pull one of the male plugs out of the outlet, it would remain energized by the voltage backfed from the other male plug. â€”Mike Sokol

Email me at mike (at) noshockzone.org with your questions.

Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent bookÂ RV Electrical SafetyÂ is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert,Â click here.

##RVe21

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