Why does my electric oven present the option of 40A and 50A breakers?

Question

The oven in question states 40A with 120/240 volts.

In the installation manual:

This range is manufactured with the neutral terminal connected to the cabinet. Use a 3-wire, UL Listed. 40- or 50-amp power supply cord (pigtail) (see following Range Rating chart). If local codes do not permit ground through the neutral, use a 4-wire power supply cord rated at 250 volts, 40 or 50 amps and investigated for use with ranges.

To me this is saying a minimum of 40A cable but can use 50A.

If I install a 40A then how much safety buffer is included? Is there a safety factor built into the stated slide in oven 40A rating? i.e. it will only pull up to a certain percent below the 40A rating? I do not imagine running a 40A cable where during operation its reaching the upper limit of the cable and associated breaker.

Also why state: 120/240 volts?

How do i decipher the meaning of 120/240 volts isnt it one or the other??

Answer 1

There are a few different issues here:

• 120 vs. 240

A typical oven for installation in a system like the US (split 120/240) will use 240V for the heating elements (i.e., needs 2 hot wires) and 120V for the controls (i.e., needs 1 hot, 1 neutral). So you can't run it on only 120 or only 240. That is different from, for example, a computer power supply that might list 120 - 240 (or more typically something like 100 - 250) which means that any voltage in that range is OK. That is because a computer uses a power supply that can take "anything" and convert to a single lower voltage. An oven will actually use the power "as is" (at least for heating elements and lights) and has to get what it needs or it won't work properly.

• 40A/50A

An oven is designed with specific power requirements. It will not, unless damaged, exceed those requirements. In addition, as a device with a continuous load (i.e., it can be on for a while), it will actually be designed to use less (I think the normal number is <= 80% of the rated current) power. So there is plenty of safety margin even at 40A, by design. As far as 50A, there are some ovens that require 50A instead of 40A. By designing a 40A oven so that it can operate safely on a 50A circuit - i.e., that it is engineered such that the breaker not tripping until 50A instead of 40A is "OK", that gives a little bit of flexibility on the installation. Otherwise, if you had a 50A circuit (both appropriate wire and a 50A breaker), you would need to replace the 50A breaker with a 40A breaker in order to install this oven - but that is not the case here and you are fine with 40A or 50A.

• ground through the neutral

You didn't ask about this part, but I will tell you anyway. For historical reasons, many ovens used to have no ground and then later ground was piggybacked on the neutral wire. That is not safe (though marginally better than no ground at all) and is not recommended, even if local code permits it. If you have a hardwired oven then you need to connect the oven ground to either a ground wire or (if you have a metal junction box attached to metal conduit going all the way back to the breaker panel) to a metal box. If you have a plug-in installation then you need to use a properly wired 4-wire socket (NEMA 14-50) connected to 2 hots, 1 neutral and 1 ground (ground again can be metal box/conduit). If you have an older 3-wire socket, now is the time to replace it - and yes that may require running a new cable.

Answer 2

3-prong vs 4-prong

3-prong connections lack ground; they bootleg ground off neutral and if neutral breaks, it guarantees the appliance is electrified.

Appliance manufactures are terrified of losing sales (or much worse, getting customer returns!) because the installed power was not right for their appliance. That's why they lobbied to allow new 3-wire bootlegs so late (1989 they were finally outlawed?) and to have old 3-wire connections stay grandfathered. Hence shipping it with a 3-wire, and the sneaky language encouraging 3-wire even though that's usually illegal and certainly unsafe.*

If your wall box doesn't have separate neutral and ground, then either retrofit a #10 ground wire (allowed now), or put a GFCI breaker on it and label the socket "GFCI Protected/No Equipment Ground".

40A vs 50A

Common kitchen range circuits are either 40A or 50A.

If it's wired with #8 cable, it must be breakered for 40A. If it is wired with #6 cable, it can be breakered 50A - though a 40A breaker is also allowed. (that would be a case of using larger wire than is required; that's always allowed).

Either way, they use a 50A receptacle because 40A receptacles do not exist.

This manufacturer chose to pay a little extra to Underwriter's Laboratories (UL) to certify their oven both for a 40A supply and for a 50A supply. So the customer can just "drop it in" to either situation. It's back to avoiding lost sales.

That is legal because UL says it is legal, because it passed UL's battery of tests and design requirements. Given the narrow ampacity range, that's not a high bar.

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* what they fail to mention is it's required in most cases. You're not allowed to downgrade a 4-wire connection to 3-wire (the sneaky instructions sure sound like they're suggesting that, eh?) -- So if the house was built 4-wire (certainly any after the 1989? NEC came to force)... or was previously upgraded for 4-wire, you can't downgrade it now. Metal conduit installations, conduit is the ground. "Local codes" (read: they adopted NEC) do not permit 3-wire in any of those cases, and that's effectively national. Add to it localities who put their foot down and said "ENOUGH!"

In short, if neutral and ground are separate in your box, you must use 4-wire.

Answer 3

The reason they give the option of 40A or 50A is because 50A is a standard circuit size a builder would install for a wall oven or range. However, they also want you to know that if you are running wire specifically for this oven, 40A would suffice. For the sake of future upgrades, you should stick with 50A. You want to choose the most common "standard" size for appliances whenever possible.

As far as the 240/120v specification - this is because the appliance requires a neutral. It will have 3 wires - red, black and white. The voltage from red to black is 240v, and the voltage between red and white and black and white will be 120v. This allows the appliance to use 240v for the heating coils, but can run the electronics or fans with 120v. This is common on ranges and ovens, but something like a water heater has no electronics other than the heating coils so they only need two wires (red and black, typically) for 240v. In modern construction every circuit would also have a ground wire but older homes may not have that for large appliances.

Answer 4

I'm not sure what your specific concern is, but here are a few points that may lend clarity:

• The manufacturer states that a 40A circuit is adequate. Take them at their word that you won't have safety or nuisance trip problem.
• They're offering the option of using a 50A circuit because the circuit protects the wiring leading to the device, not the device itself, and you may want the option to upgrade appliances later.
• There is some degree of protection for the device in not drastically oversizing the circuit, but the device has integral safety mechanisms for current and thermal overload.
• Again, the breaker protects the wiring, so be sure that the wire size you use is up to the task of whatever the breaker will allow. Do not use #8 wire on a 50A breaker, for example.