Is it ok that an oven circuit's wire is much larger than the oven's wire?

Question

I am replacing a Microwave/Oven combo. The circuit is 50 amps and 6 AWG wire runs to the back of the oven cabinet. When the old oven was installed, I assume they used the wiring that came with the oven which was 10 AWG in flexible conduit, wire-nutted to the #6.

1. Was this proper? It was that way for 23 years without a problem, but if there was one, wouldn't the #10 wire overheat before the breaker knew there was an issue?
2. If this installation wasn't safe, should I run #6 from the back of the cabinet to the oven?

Thanks.

Answer 1

The wiring in your walls and the wiring attached to the appliance are covered by different safety standards, each with their own set of rules and assumptions.

(Assuming you are in the US) The wiring and receptacle are governed by the NEC, whose standards required that particular circuit to be installed with #6 AWG wire and either a NEMA x-50 (x being 10 or 14 depending on the year) receptacle, or in your case a junction box.

Greatly simplifying things, the NEC sets minimum standards for wiring and devices to ensure they do not overheat or fail based on the worst-case scenario. They have to specify wire sizes with a large enough safety margin to account for heating. They don't know what type of walls or insulation exist, how many wires are bundled together in a small space, etc. Essentially, they want that wire and device to survive the worst-case installation and dumbest of intentional overloading without killing anyone or destroying homes, relying on the circuit breaker to open it up before things get spicy.

The appliance wiring, however, is governed by a Nationally Recognized Testing Laboratory (NRTL) such as UL, ETL, CSA, etc. NRTL approvals of appliances specify wire and connectors based on exact knowledge of what the appliance can/does do. This allows more flexibility to specify wire sizes/types/connections for the actual current draw of the appliance. They don't have to design for a wide range of mis-use scenarios because they know how the appliance is constructed and assume that it will be installed and operated as specified.

Bottom line: As long as the appliance is listed by a well-known NRTL such as UL/ETL/CSA, you can safely assume that the appliance wiring is perfectly fine as provided, even though it wouldn't meet the NEC.

Answer 2

The oven is UL Listed. If the oven was made of shiplap siding, 2x4 studs, cellulose insulation and wire with a thermal insulating rating of 60C, then UL would make them use #6 wire also :)

However ovens typically use high-temp wires for obvious reasons, so the ampacity is higher. So UL will allow smaller wire when it knows wire is going in a place where that will be safe, i.e. an equipment chassis of known characteristics. (now if the oven maker said "oh hey, I'll fill these voids with rockwool insulation" they might need to go back to UL and get the oven re-listed.)

And it's even worse in home wiring because for some reason, the favorite is NM-B type cable, which has the lowest insulation rating - only 60C (shared only by UF and the obsolete TW). Other suitable in-home cable types such as SER or NMD90 are allowed 75C (or 90C, but breaker panels are limited to 75C) and can run 50A on #8 copper or #6 aluminum.

Answer 3

Wires you run/place must be the size listed in code.

Wires/cables on a device from the manufacturer/or stated in the instructions, are tested(by labs) to be safe.

Your wires/cables/connections are not tested, except to be the right size for safe use.

The new microwave/oven should also state the size of breaker needed. wire size listed for a breaker is the minimum, larger size/gauge wire is allowed.

Answer 4

The answers focusing on electrical codes are correct and valuable, but let's add an answer about physics

Generally speaking, a short run of thin wire is not a big deal compared to a long run of thick wire. Electrical codes exist because it specifically matters how short and how long and without the necessary training in physics or electronics, those numbers aren't obvious.

But here's the basic issue from a physics point of view: All wires are just fuses waiting for a large enough current to cause them to burn up. The point of breakers is to stop the flow of current before it gets large enough to burn up a wire that wasn't intended to burn up (i.e., it's not meant to be a fuse). But if you use a long-enough, thin-enough wire, the breaker won't trip before the wire burns up (which is why appliance fuses burn out before breakers trip). Conversely, if you use a short-enough thin wire, the breaker will trip before the wire burns up.

I apologize that this is a bit technical, but think of it this way. Sheet resistance is a phrase and a mathematical concept that identifies the resistance of a material to the flow of electrons (aka, current). It's usually expressed in ohm-meters. Copper has a specific sheet resistance (it doesn't matter for this answer what that is) and we'll use the variable ρ to represent it.

The resistance for a length of copper wire is calculated as R = ρ(Length/cross-section Area). A wire will burn when a specific current is passed through it (calculating that is too complex to worry about for this answer) because the wire's length and thickness can handle only so much power (power = Resistance * Current²).

The goal (simplistically, but good for this answer) is to guarantee that two wires will burn up at the same power level and that power level is greater than the breaker rating. The two wires can be any combination of length and thickness (gauge or awg) so long as they are compatible with those two rules. Again for simplicity, let's say that the two wires must have the same resistance. R₁ = ρ(L₁/A₁) = R₂ = ρ(L₂/A₂). This is a long and fancy way of saying that L₁/A₁ must equal L₂/A₂. If A₁ > A₂ (the area of a 6awg cross-section is greater than the area of a 10awg cross-section), then L₁ > L₂ (the length of your 6awg wire can be longer than the length of your 10awg wire). In other words, a short length of thin wire can carry the same load as a long length of thick wire.

Which is why you can plug a crappy 18awg lamp cord into an outlet served by a 20amp breaker and a 12awg wire without burning down your house. The short length of 18awg wire can handle the same power load as the longer 12awg wire.

And that's why it's OK for for your oven to come with 10awg wire. This is actually a pretty common occurrence. Furnaces, air conditioners, and many heavy-load appliances are assembled with light-gauge wires which must then be connected to heavy-gauge wires in the building.

I apologize for the lengthy, technical answer. But I wanted you to understand that the Electrical Code rules exist because of physics, and we follow them because Mother Nature has a nasty habit of not caring who's in the house when she's not respected.

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BTW: Why isn't the Mathjax interpreter working on this Stack? $R=\frac{V}{I}$ should have resolved to an equation....

Answer 5

A thing or two about "Electrical Codes". First, all devices and wiring must comply with the minimums given by the NEC, OSHA and other entities. But, for now, the just NEC. This includes, but is not limited to, wire gauge based on ampacity, spacing, "fill", temperature and hazardous environments.

Second, in general, any electrical modifications, repairs or configurations may also require compliance with the minimums as set forth in "local" electrical codes. Chicago has one of the toughest local codes in the country that causes many contractors to plan carefully when dealing with possible conflicts.

That's where I cut my teeth for several years working as a draftsman and designer for an electrical contractor. Then 15 years working in-house as a service-repair tech (union electrician), working with machines from all over the country and the world.

Answer 6

You state "The circuit is 50 amps". That is unclear. If you mean the circuit breaker is 50 amp rated with a #6 wire connected to it then a #10 wire is connected to that #6 wire further down the line there is a problem, a code violation and a fire risk. The #10 wire is rated at 30 amps so the 50 amp circuit breaker is not protecting that #10 wire as required by code. I suggest you simply replace the 50 amp circuit breaker with a 30 amp circuit breaker which I believe is a 2 pole circuit breaker so you have two #6 wires connected therefore a 2 pole 30 amp circuit breaker should suffice, about $15 at home depot. A circuit breaker is intended to protect the wires in your walls, not the oven or whatever device is connected to it. My experience; I have replaced 4 meter panels which required permits from the city and I have also done extensive electrical work in residential and industry in a 50 year career. I am an unlicensed electrician.