Combining wires from two breakers to one appliance?

Question

I have a wall oven and a microwave oven, connected to 30A and 20A breakers respectively, and both are 240V.

Now I'm thinking of replacing it with another double-oven model that runs a single wire powering both ovens. According to the specs, the maximum possible power is 7800W.

The official website says that 32.5A is the maximum load. I assume that it should require a 40A breaker. However, the installation manual says to use a 50A breaker.

My questions are:

1. Is it safe if I combine the wires that are currently connected to the oven (30A) and the microwave oven (20A) to the new double oven?
   • I assume that the diameter of the wires shouldn't be the problem, because they're joined at the very end of the circuit.
2. According to all references I can find, a 40A breaker is sufficient to power 7800W at 240V. Can I assume this, or do I need to strictly follow the manufacture's 50A guideline?

Thanks!

Update: attached image.

Answer 1

As far as 40 vs. 50 - that depends on the manufacturer's requirements. The installation instructions clearly state:

A 50 Amp circuit breaker with wire gauge #8 AWG must be used.

So there you have it. 50 Amp - not 40 Amp. 8 AWG - not smaller. You can, of course, use larger wire - e.g., 6 AWG - that is always OK. But you can't use a smaller breaker - my guess is you would get nuisance trips if you use the microwave + convection in the top at the same time as you use broil in the bottom oven. And you can't use larger because it would not provide the necessary protection in certain situations.

As to why the oven needs a 50 Amp breaker if it seems to only use 7.8 kW, only the manufacturer can say for sure. But a couple of possibilities:

• 7,800 W/240 V = 32.5 A. 32.5 x 1.25 = 40.625. That is just over the magic 40.0, so goes to the next breaker size. But that would seem like a very poor design to be "so close".
• 7,800 W/208 V = 37.5 A. Of course, if the reason to need a 50 Amp breaker is for 208 V installation, they could have listed 40 A for 240, 50 A for 208. But they didn't.
• 7,800 W is the "usual" continuous rating, fitting OK (or close) with 40 Amp, but maybe the surge (startup of top & bottom ovens at the same time, part of self cleaning cycle, etc.) is significantly > 40 A, so 50 A is needed to avoid problems.

In any case, you must NOT combine two wires to get more capacity. It is against code and, aside from any other technical reasons, the basic problem is if one wire broke (or more likely, one end came off a connector) then 50A would be flowing over a wire that can only handle 20A or 30A. You can't parallel, as logical as it might seem.

Answer 2

Nope

Nope, nope, nope. This is called paralleling and there are several sections of NEC dedicated to it.

It has at least a half dozen requirements. This reuse scheme violates nearly all of them:

• 1/0 or larger wire
• supply-side distribution cabinet specifically designed for paralleling
• supplying cabinet from one source
• same size conductors
• equal length conductors
• cross-sections must add up to a big enough wire

Instead, pick your appliances for the wires

Given that you have a 20A/240V and 30A/240V circuit in there, it shouldn't be too hard to find ovens capable of accepting that power, possibly as separates. After all, you have that now.