Can a 40amp breaker be used safely and without issue with a 40amp device on 6AWG wire?

Question

We are installing a Cooktop that calls for a 40amp circuit. I was given 6AWG wire (instead of the 8AWG wire I asked for) by mistake. The electrician saw the 6AWG wire (that I had already run) and used a 50amp breaker. The Cooktop installer requires that the breaker be 40amp. My electrician was concerned that using the larger 6AWG wire would lower the resistance therefore affecting the amps; he cited Ohm's law. He has refused to change the breaker from 50amps to 40amps for fear of damaging something or affecting safety. I thought the amp requirement was dictated by the device (it asks for 40amps) and the 40amp breaker would ensure that 40amps is all that it would get. Does anyone know of any dangers/issues if using a smaller breaker with 6AWG wire?

Answer 1

Your "electrician" is not one of the brighter bulbs in the pack.

The 40A is to protect the wiring and the device.

If the wiring is AT LEAST 8Ga then it's adequate to protect the wiring. It also protects 6Ga, (or 500 MCM for that matter) just fine, and it properly protects the device at the end of the wire just fine.

"Ohms law" has squat to do with this. You could have a cooktop located 3 feet from the breaker panel and connected with 8 Ga or one located 100 feet from the panel and connected with 6 Ga - the 8 Ga would have (much, about 20 times) lower resistance, because of the wire length. Upsizing wire for longer runs on heavy circuits is actually quite normal. As stated, not a particularly knowledgeable electrician you have there.

Answer 2

The breaker needs to be sized to protect the wire and the device.

Wire

Larger wire (which is a lower # due to the way wire sizes are named) can use a larger breaker. But a smaller breaker is always safe. 55A is the largest breaker you can normally use for 6 AWG copper. 40A is the largest breaker you can normally use for 8 AWG copper. But you can always use a smaller breaker - it will be 100% safe. That includes the very typical 50A (instead of 55A) for 6 AWG. But it can include lots of different things. For example, a 30A breaker on 8 AWG wire, a 15A breaker with 12 AWG wire (which can also use a 20A breaker), etc. You could even use a 15A breaker on 6 AWG wire - strange but nothing unsafe about it.

Device

The device needs to be protected by an appropriate size breaker which is determined by the design of the device and is part of the UL (or equivalent) listing for the device. So if the cooktop calls for a 40A breaker then you must use a 40A breaker. You can't use a smaller breaker (probably safe, but you would get frequent nuisance trips which are inconvenient at best and lead to unsafe operation at worst if you (or a future owner) ends up "fixing" it later in an unsafe manner). And you can't use a larger breaker because the device is not rated for that - i.e., it expects to have the protection provided by a 40A breaker in order to handle any faults in a safe manner.

It is possible to have multiple valid breaker sizes. For example, a circuit consisting of 12 AWG wire and 15A duplex receptacles can use a 15A breaker (perfect match for the individual receptacles) or 20A breaker - OK because of the wire size (15A would only need 14 AWG) and a special exception for 20A circuits that allows for multiple 15A receptacles instead of 1-or-more 20A receptacles, and the 15A receptacles are designed to allow 20A passing through. Any normal plug-in 15A device can use a 20A receptacle. But that is not necessarily the case for 40A vs. 50A - and unless the cooktop instructions actually say it is OK to do so, you need to stick with 40A, even if the wire can handle 50A.

Answer 3

You're always allowed to upsize wire

What you have there is a 40A circuit, because it is breakered 40A per instructions.

On a 40A circuit you are allowed to use any cable 8 AWG or larger.

It's that simple.

6 AWG is larger than 8 AWG, so you are ducky-doo with the #6. Good call, since some better stoves/ranges want 50A or even 60A, and #6 is good for all that.

The only speedbump with the "any size or larger" is a very much larger wire may not physically fit on the breaker or panel lugs. In that case you need to simply pigtail to an intermediate size or metallurgy. For instance if you wisely chose 4 AWG Aluminum for your 400' long-run 30A dryer circuit, neither the 30A breaker nor socket will accept #4 nor aluminum. So you use Al-rated Polaris connectors to pigtail to #10 Cu, which will fit without trouble. #10Cu is good on a 30A circuit.

As for the electrician's "mistake" I don't see the problem. If he wasn't aware of the range specs, he made absolutely correct assumptions based on facts at hand. Many 40A ranges are dual-listed for 40 or 50A breakers, and both use the same socket. If wrong, it's a $9 change. No big.

Answer 4

I have a similar issue (or would have had 7 years ago after i bought my fixer upper by the lake and discovered that only one electrician lived in this area that had not yet become part of metro Charleston, SC) with wiring for which I'd paid $500 that I now see amounted to robbery, which by God's grace didn't end up as a 1,900 square foot mound of embers. The 49-year stick-built rambler I bought had been abandoned for three years following a caustic divorce between two narcissists comprising a residential contractor/drug dealer and his grossly drug addicted wife. (The ex-husband acquired the home nextdoor, and thinks I'm living in his house despite the deed recording otherwise. It's been exhaustive living nextdoor to a OCD-driven bully who's evil orchestrations included compromising every handyman I hired to sabotage whatever I'd paid them to fix, install or upgrade. This included wiring my new stove and wall oven, and another, a master electrician and fraudulently licensed contractor, to finish my garage creating two rooms and a laundry closet. He'd installed my metal roof (atop the old shingles and two busted joists, all without an inch of flashing). After i paid him $18k plus as much for materials in 2014, he immediately bought a pile of cocaine, then later boasted in a local bar that he'd wired my house to burn. I provide this background to help explain some of the curios comments, questions and whatever offered 2-cent suggestions i may have. Juxtapose, after pulling my rusted out GE cooktop I'd bought to replace a working old cooktop (the only thing wrong with it was that awful '70s mustard color). The electrician i hired was the neighborhood gossip who relished defaming me with flatly fabricated tales about me all while i was paying him four times what anyone paid anybody for work here. Based on my plumber's training, the electrician may have acquired his skills in prison. He had a gift for code-breaking workarounds. I recall hearing him "hmm," as he began installing my new 40 amp oven next to my new drop in stove. He used the existing original fat sheathed black cable that was either 4 or 6 awg stranded. He created a junction box in which he twisted and taped together the two appliance line sets to the twisted heavy gauge copper stranded cables from a 60-amp breaker. So, somehow he connected super thick copper cables to bundles of 10 awg stranded aluminum wire sets from the two appliances in a 5" metal junction box and everything worked just fine... Until I over-boiled peanuts in salt water a few too many times that accelerated rusting of the cheap GE stove top. After my life here prompted me to move an hour away, crack-addicted burglar became a squatter of my lake house who must have turned my place into a cat house.literally, because a feral tomcat sprayed my stove as evidenced by the distinctive pungent cat spray stench when i tried to cook something. Cat urine also is highly acidic, corrosive, which was worse than overboiling peanuts in salt water based on the accelerated rust that rendered an otherwise useful stovetop totally corroded from elusive, adherent cat spray and resulting toxic plume. So disgusted, I pulled out the whole thing, stovetop and underlying drip pan. This revealed sections of swollen pressed wood beneath the 70s yellow formica countertop, which I trimmed away. Roughly 5" below the counter was the 5" metal junction box the electrician set when he installed my stove and new wall oven. Upon uncovering that revealed thickly twisted copper and black electrical tape and two metal conduits, a 1" cable leading to the oven, a 3/4" metal conduit leading to the stove and the 3/4" black-sleeved cable fished under the house via crawlspace from the 60-amp breaker in the Square D circuit panel ~18' away in the wall of the finished garage behind the den and fireplace. Since the stove is gone, I wondered if that junction box could be used to power a 20-amp outlet for my two new pluggable countertop stoves. One is conduction and the other is a glass top electric stove unit for ferrous cookware. I also plan to install a two-eye LPG fueled cooktop (I'd prefer a single eye one, but they all look like cheap little camp stoves). Each of my electric units are rated 1800 watt capacity, and neither would be on max heat simultaneously. Either way, I wondered why two 20 amp outlets couldn't be fashioned to work off of what had also powered a four-eye drop-in stovetop. Yet, my assumption was flawed, and I've been advised to install dedicated 20-amp circuits for each single burner. Problem remains what to do with the heavy duty cable, a 60 amp breaker on the single wall oven that was meant for a 40 amp breaker. The reminder of why God gave us electricians was indicated by my decision to cap the cables and leave the junction to serve the stove. It was because I misjudged that the red coated wire as neutral so that after i capped the stove's white and red cables together, and then bound together the two fat service black wires and the oven's black (hot) lines, the 60 amp breaker seemed to moan when the breaker i hit also snap-crackle-popped loudly back idf when i flipped it on. As I paused, retreating from my kitchen and re-imagined cooktop project, flashbacks and an ah-ha moment emerged when i realized that the red wire was hot not neutral, so binding a hot to a neutral is forbidden. I'm now left with the questions: Are all of the hot lines, black and the red, supposed to be twisted together, or one of the fat black cables bound to the 10 awg hot lines separately, the two black service cables separately bound to the thinner appliance hot lines so at least i could securely and more neatly cap each of the two bound sets? The effect would be the same, right? And what do i do with the straggling white line from the oven? Just cap it? Finally, does that thick twisted mass of copper ground wires pose a threat to the unwitting soul who'd get a jolt from touching it by extending a long metal utensil to the metal box grounded by a mass of ground copper? Ah, the consequences of moving out of the city to the country where the slow pace is backward mentality by provincial local opposed to progress and anyone new.

Answer 5

The breaker now matches the wire rating (so you are protected against wall fires from overheating wires) and exceeds the cooktop rating (so it will not trigger accidentally). You are fine, but you'd have been fine with a 40A breaker as well. You state that the cooktop calls for a 40A breaker but that just means that the breaker (and wire) should be able to deliver at least 40A. The makers of the cooktop cannot delegate operating security to the wire breaker. Arguably without additional circuitry it would be a bad idea to mount the cooktop on a 5000A rail since it might melt into slag in case of a shortcircuit. The difference between 50A and 40A for the fault modes to be expected, however, are negligible. There just isn't a failure mode where the cooktop would continually draw, say, 48A through its internal wiring.

Unless, of course, some terminals have been mixed up and parts are running between different phases rather than between phase and ground, leading to a consistent too high load but not a short circuit. While the overall competence of your electrician does not seem overwhelming, his insistence on sticking with what he knows makes that unlikely. If you find that some of your plates heat water significantly faster than expected, you probably should have that double-checked.

Answer 6

You are lucky you got a 50amp circuit for your commercial kitchen. Enjoy. You can also plug a waffle iron in at the same time! Think of this as an upgrade.

OP is wrong electrician is correct using a 50amp circuit. It won't damage anything.n

Edit 20220314 ahh you lost in court by now but the down votes are really hurting the question. The correct answer was the license electrician you got an upgrade for free they will not come back if they did come back be very afraid. Hahahahahahahaha sass.

thanks continued down votes I know what is right and wrong.

you can charge a Tesla on 6awg circuit. You could charge an ev on your kitchen plugs how safe is that. Paid for a 40 got a 50 that's really a bonus.

I have sperated lines in my kitchen dishwasher a separate line refrigerator a separate line counter plugs a separate line. Guess what kids there all different amperages and wire sizes. I know the dishwasher is spec because I installed the dishwasher after inspection of the circuit nice big wire direct to box is good.

Happy pie day