A Question about Electrical Breakers and Amps

[JHZR2]

Originally Posted By: jrustles
That is true, but since these are simple resistive loads, they will simply draw less current with less voltage, thus less work (output) and the current derating on 208v. if this were another type of (non-resistive) load that required a certain amount of work output, then yes it would draw more amps with lower voltage to equal the same work.


V=I*R

Thus for a given resistance, voltage and current have to directly scale.

Just for simplicity, in a 1 ohm load, 240V would yield 240A, while in that same load, 208V would only take 208A. Voltage is a driving force in that variant of the equation, such that the higher potential,the more current you can flow across that resistor in theory.


For the OP, chances are practically you'll be fine if you're not talking continuous use/duty, and particularly if you preheat one oven first then the other. Often the breaker ratings are based upon adding up all loads operating fully at once.

Still, breakers can only handle some fraction of full rating (there are usually curves for this, they have to allow some amount over their rating for some time to allow for inrush currents, but the balance is that vs opening too slow in other cases), as can wire so a high enough insulation temperature rating. Doesn't mean I'd chance it at all under any circumstance. Let's say there is an electrical fire, or any fire in the kitchen, and the house isn't a total loss (parts can be studied), I'm sure it would be sop to look at wiring and breakers. I wouldn't want to take that chance. No doubt things get under wired or done poorly to save cost... And you're right on the cusp... But I'd not chance it...

 

[A_Harman]

Just throwing this out as a suggestion, those who really know electrical codes can "YAY" or "NAY" it:

If another #10 wire was run to share the current with the existing wire, and a 40-amp breaker was connected to both wires, would that be acceptable?

 

[bullwinkle]

If you are installing a range rated a 208V, and are going to run it at 240 (in my experience 240 can be as high as 248-250V)-you need to increase your wire size, and breaker size, by 240/208, or multiply the amps by 1.154, to get the actual amp rating. Usually breakers & wires are rated 25% higher than the piece of equipment.

 

[bullwinkle]

Originally Posted By: A_Harman
Just throwing this out as a suggestion, those who really know electrical codes can "YAY" or "NAY" it:

If another #10 wire was run to share the current with the existing wire, and a 40-amp breaker was connected to both wires, would that be acceptable?
Nay, need 8 gauge wire for 40 amps, either 8/3 or 8/4 (depending on whether a neutral is required or not).

 

[SrDriver]

You will need #8 copper wire. No way around it!

 

[AandPDan]

Originally Posted By: A_Harman
Just throwing this out as a suggestion, those who really know electrical codes can "YAY" or "NAY" it:

If another #10 wire was run to share the current with the existing wire, and a 40-amp breaker was connected to both wires, would that be acceptable?


Since he'd have to run a wire he may as well pull #8 cable.
Parallel conductors gets into a whole other can of worms and impractical in this scenario.

 

[threeputtpar]

Ugh, this is what I hoped wouldn't be the concensus. My wife will likely be disappointed with this.

I guess next step would be to have the situation surveyed by an electrician and see just how difficult and costly it would be to run #8 wires. It looked to be just a red, black, and copper ground in the breaker box and it's not inside any conduit.

 

[JHZR2]

Originally Posted By: AandPDan
Originally Posted By: A_Harman
Just throwing this out as a suggestion, those who really know electrical codes can "YAY" or "NAY" it:

If another #10 wire was run to share the current with the existing wire, and a 40-amp breaker was connected to both wires, would that be acceptable?


Since he'd have to run a wire he may as well pull #8 cable.
Parallel conductors gets into a whole other can of worms and impractical in this scenario.


I agree. I'm not sure if the ampacity of two 10ga, or the thermal mass or copper mass of the two vs one is very different, but it adds risk, and is really two, if not three conductors pulled, which just makes it more complex again.

 

[racin4ds]

I have a hard time seeing how that little regular kitchen double oven needs 40amps and 8awg wire...

for instance I have a large powder coating oven in my garage, it has (2) 3500W elements in it on 240V so ~29amps max draw. It has a smaller 2500W element that kicks on to speed the warm-up time so:

All together I have 9500w of power (9500/240V=39.5A)

I can't see how a kitchen duty double oven is going to pull anywhere near that...

 

[AandPDan]

Originally Posted By: threeputtpar
I guess next step would be to have the situation surveyed by an electrician and see just how difficult and costly it would be to run #8 wires. It looked to be just a red, black, and copper ground in the breaker box and it's not inside any conduit.


It's not consensus, it's Code. It gets better (or worse)...

Your NEW circuit needs 4 conductors. Red/Black/White and ground. The old 3 wire circuit, Red/Black/White would have been fine if you weren't replacing it. It was grandfathered.

Your electrician can discuss this with you.

 

[440Magnum]

Originally Posted By: Rand
Originally Posted By: eljefino
Sometimes you find 208v single phase on farms and in spots down South (?). Being a resistive load, the less volts you stuff through, the less amps it takes-- though it'll warm up slower.


Can you explain why it would use less amps at a lower voltage?

I understand why it would use less Watts.






Because an oven is a resistive load. It presents a resistance, call it "R" ohms, to the line. The line applies a voltage (208-240), and then the current that flows depends on Ohm's law: V=IR, where I= current, V= voltage, and R=resistance. So the current that flows is I=V/R. IF the voltage goes down, so does the current.

 

[eljefino]

Originally Posted By: SrDriver
You will need #8 copper wire. No way around it!


#6 aluminum is used quite a bit and can be cheaper but of course is even uglier.

 

[threeputtpar]

Originally Posted By: AandPDan
Originally Posted By: threeputtpar
I guess next step would be to have the situation surveyed by an electrician and see just how difficult and costly it would be to run #8 wires. It looked to be just a red, black, and copper ground in the breaker box and it's not inside any conduit.


It's not consensus, it's Code. It gets better (or worse)...

Your NEW circuit needs 4 conductors. Red/Black/White and ground. The old 3 wire circuit, Red/Black/White would have been fine if you weren't replacing it. It was grandfathered.

Your electrician can discuss this with you.


Dude, quit spitting in my Cheerios!

J/K, I appreciate the help.

 

[Bladecutter]

Originally Posted By: threeputtpar
So, after the microwave fire that my son created I think we're just going to replace the microwave and stove built ins and upgrade. We're looking at putting in a double oven in its place and going with a new coutertop microwave.

The oven that we are eyeing up has electrical specs of 30.8 amp draw at 240v and 26.9 amps at 208v and recomends a 40 amp breaker. The outlet that it would be using and corresponding breaker is 30 amps and wired with 10 ga wire. Talked with an electrician and he said that 10 ga wire would not support a 40 amp breaker, and since the basement is finished running new wires across the house is not a practial option.

What would be the likelyhood of the 30 amp breaker being a problem?


I take it that your current oven is a standard rangetop/oven unit?
Is it currently electric, or gas?
If it's electric, you're saying that the current socket and wiring that it is plugged into is a 3 wire 30 amp setup? That seems pretty odd for a dedicated oven plug to not already have 40 amp service to it. How old is your house/apartment?

But yes, you will need to replace the current wires with the correct wires for that 40 amp service. Since it's a dedicated oven circuit you're upgrading, it really should be an issue for a good electrician to hook the new 8 gauge wire to the current 10 gauge wire, and then physically pull it through from the breaker box to the kitchen. There shouldn't be any need to tear into the finished basement at all.

Well, that is if the wire is going through conduit.
If it's wire that has been stapled to the joists in your walls, then the electrician will have to cut holes into the house in order to pull it through, but it shouldn't be a completely destructive project. It's not like you will have to replace the entire ceiling, and a whole wall in your basement, just a small patch job after he opens up the ceiling enough to run the wire towards the kitchen.

BC.

 

[Wolf359]

Originally Posted By: A_Harman
Just throwing this out as a suggestion, those who really know electrical codes can "YAY" or "NAY" it:

If another #10 wire was run to share the current with the existing wire, and a 40-amp breaker was connected to both wires, would that be acceptable?


The simple answer would be no, again another code violation. Now you're going to have what's commonly known as a double tap where there's two wires on one breaker. Most breakers are only rated for one wire. Probably same issue at the outlet end.

If you're being cheap, there are a couple of other options which is to use wire molding to cover up the wire, then you don't have to rip into walls. Looks ugly though. Then you can always run the wire outside in conduit until you get to a point where you're close to the oven. Again looks ugly though. In terms of what you were trying to achieve initially, the difference is probably in the outlet, 30 amp is different from a 40 amp. If you just changed the outlet and plugged in the oven, things would probably be fine until you ran both ovens at the same time. Also while a circuit might be rated for 30 amps, from a design standpoint, you don't really like to have it pull a full 30 amps, normally only 75% of max load. That's why most ovens today are at least 40 amps if not 50 amps. The longer the run from the panel to the outlet, the less current you can pull from the outlet. All that would be a code violation. If the house burned down and it was due to the oven, you run the risk of insurance not paying because of code violations.

 

[pcoxe]

You could step up the voltage at the main panel to 480v with a transformer and reduce it back down to 240v at the appliance and the current would be halved on the existing line. Then your problem would be the voltage rating of the existing wire and still would not address the missing neutral conductor. And transformers can get expen$ive too. No cheap way to do this right.

 

[threeputtpar]

Well after all the thourough research, we've decided to just replace the built-in microwave and keep the single oven. For the 3 or four times a year that we would need a second oven for family gatherings, we'll just make due with what we have.

Bladecutter, this was a microwave over an oven that is built into the wall. The wire leaves the breaker box and is not inside conduit; boy I wish it was as that would be easy to run new wire. The house was built in 1994 and the original oven/microwave was a smaller 27" unit that we upgraded to 30" when we bought the house 2 years ago. We got a great deal on a used unit and install that we couldn't pass up.

Even the #8 wire that goes out to the A/C unit is not in conduit. I found that odd.

 

[tom slick]

Residences generally don't require EMT, just NM or MC cable for most wiring needs.

 

[threeputtpar]

When my wife picked up the new microwave yesterday, she mentioned to the salesguy that we were passing on the double oven idea because of the 40A requirement.

He said that he's had many people buy an appliance that required 40A service but ran it on 30A 10 gauge wire and the majority of them encountered issues. He didn't go into detail, but he said that the appliance just never worked right because of the powering issues.

I'm surprised that he didn't mention anything about it the first time we were in there and told him that it would be replacing a combo unit. I know he's trying to make a sale, but we've worked with him in the past and never found him to be the unscrupulous type of salesman. Our first experience with him is the reason that we went back.