240V, 30A heater on a 240V, 50A circuit?

17 years ago

Hi,

I have a 240V, 50A outlet (NEMA 6-50R) in my garage. I believe the previous owner used this for an arc welder. I would like to use this source for an electric heater. I purchased a 240V heater but it is rated at 30A and has a NEMA 6-30 plug on it. I checked the NEMA wiring diagrams and the 6-30 and 6-50 plugs seem to be wired in the same manner. The outlet is behind some shelving and not real accessible to replace the receptacle with a 6-30. I was thinking that I could create some sort of "extension" cord using a 6-30 receptacle and a 6-50 plug. I understand the danger of trying to pull 50A from a 30A source but is there anything I should be concerned about when trying to pull 240V, 30A from a 240V, 50A circuit?

Any feedback would be most appreciated.

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Comments (9)

jason1083
17 years ago
IF you tried to pull 50 amps from a 30 amp breaker it would trip. If you have a 50 amp circuit and a 30 amp heater on it, the circuit would likely not trip out if the heater malfunctioned.

Personally if it was me I would be cautious, if feasible I would probably match the breaker to the appliance.
kevder
Original Author
17 years ago
Other than increased amperage / voltage, how does this differ from a 120V circuit? There are many household appliances that do not draw the same amount of amps as the circuit is rated for. Theoretically, if the appliance shorts wouldnÂt it try to draw an unlimited amount of current causing the breaker to trip? I admit that IÂm a novice here so I appreciate you straitening me out.

Thanks
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This forum software confuses me, so I'll just reply to everyone at once. "Are there no labels on this compressor?" Yes, it's labeled 15A. I thought I included that but may have forgotten it. Thanks for the info. "If you change the breaker, you need to change the wire." Right, I understand that. What I should've said is install wire capable of carrying 30A and start by installing a 15A breaker, then size up if needed. I'd planned on going a wire size up once I determine what the size I need is anyway, since from what I've gathered there is no reason not to do that, and if I ever need to repurpose the circuit, I can install a larger breaker if needed. As far as the amps question, I think I understand the answers. I guess what I wasn't getting is that measuring the two legs of a 240V circuit independently wouldn't mean anything because without both legs together, it isn't a 240V circuit. What I was asking is akin to if you installed two 20A single pole breakers right above/below one another and connected each to a 120V receptacle, you would have two 20A circuits, so total 40A. If you connected both of those to a 240V receptacle, then you would have a 240V receptacle, but the amperage would still only be 20A since the voltage has changed. I know that wouldn't be safe because 240V circuits breakers need to be tied together so if one trips they both trip, and I'm probably not wording that right, but that's what my original question was about. I just didn't get that the amperage doesn't add when you have two 20A breakers because the voltage also changes. It's my understanding that a 240V breaker is no different than two single pole breakers except for the bar that ties them together so they trip together, so my analogy seemed reasonable to me. As far as your answer regarding wire size being governed by current, I understand that, but what I was saying is that I see multiple different wire sizes for the same voltage, current, distance and conductor material when I look at different charts. When you say a 14A circuit needs 14 gauge wire, I assume that could change depending on if the length of the circuit is 25' vs 250'? Maybe it would be easier if I just tell you exactly what I have. 1) Compressor - labeled 15A on the motor. Wire distance from load center about 35'. 2) Table saw - Delta Unisaw model 36-829. Wire distance to receptacle, approx 45', with a 10' 12 gauge extension cord from receptacle to tool. The manual says "A separate electrical circuit should be used for your machine. This circuit should not be less than #12 wire and should be protected with a 20 Amp time lag fuse" 3) Band saw - Grizzly model # G0513X2. Specifications say it draws 10A at 220V or 20A at 110V. I have mine wired for 220V. The wire distance from the load center is approx 35'. These three items are what I am trying to determine wire size for.
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bigbird_1
17 years ago
You have a potential problem with changing just the receptacle and breaker if you decide to move the shelving. The problem will be if someone other than you looks in the panel and sees a DP 30A breaker on #6 wire and decides to pull the breaker and put a DP 50 or 60A breaker back in, not realizing there is only a 30A recept. So, I would use the extension cord that you suggest, but you MUST also change the breaker to a DP30A.
normel
17 years ago
Two problems: one, you are using an appliance contrary to the manufacturers instructions/design; two, the appliance may draw more than the designed current(30A) before causing the breaker (50A) to trip, causing damage to the insulation and presenting a fire hazard.

It's very easy to swap breakers.
kevder
Original Author
17 years ago
Thanks for all of the feedback. It has been very helpful.

Is there an easy / cheap way to install an in-line breaker / fuse as part of my "extension cord" (i.e. after the outlet but before the heater)? IÂm not sure I want to crack open the breaker box and this would save any future confusion between the outlet type and breaker size.
petey_racer
17 years ago
IMO if someone were to change a breaker simply because of wire size, and not check the receptacle it feeds, it is their problem if something happens.
It is not at ALL uncommon to use wire larger than "typical".

Changing the receptacle and breaker is fine if you want to go that way.

At the same time I agree with kevder. The breaker is NOT intended to protect the appliance. It is to protect the circuit. This is why a circuit breaker is technically called the "branch circuit overcurrent protective device"
It protect the branch circuit from overcurrent, NOT the applaince.
perel
17 years ago
I thought that for continuous loads / single appliance circuits, the breaker sometimes WAS also protecting the equipment. Or is that just for AC hermetic compressors - the MAX FUSE specification on the unit, etc.

In any case, what DO the manufacturer instructions say about breaker sizing, if anything? Is there a max breaker on the nameplate?
brickeyee
17 years ago
"I thought that for continuous loads / single appliance circuits, the breaker sometimes WAS also protecting the equipment."

The OCD is for the permanent wiring only.

"Or is that just for AC hermetic compressors - the MAX FUSE specification on the unit, etc."

Even for a motor load or hermetic compressor the OCD is for the permanent wiring. Larger motors and hermetic compressors contain their own overload protection.
The overload protection is designed to breal the circuit if excessive power is drawn. The OCD in the panel os short circuit protection for the permanent wiring. It needs to be limited to make sure the built in protection can interrupt the circuit under overlaod conditions.
A motor that pulls 20 amps running may have overload protection at around 30 amps (with a time delay 'built in' by the thermal mass). The contacts are designed to work with no more than about 30 amps. A direct short in the motor may pull 10,000 amps. The contancts are not rated to interrupt this kind of current. The OCD in the panel is designed for this and WILL provide the protection to the permanent wiring under short crcuit condions to preven overheating of the branch circuit conductors.
Using equipment on an oversized branch circuit may result in the wiring in the equipment failing before the OCD can be tripped. The equipment wiring then becomes the fuse. Never a good idea and not allowed under the NEC. Even in vehicles it takes special wire and insulation to create a 'fusible link' in the wiring harness. In vehicles even the length of the wire matters for fusible link applications.

Install a smaller breaker and change the receptacle to the correct style.
ionized_gw
17 years ago
If you have to pay an electrician to change the breaker, it might be cheaper to buy a 50 A heater.

240V, 30A heater on a 240V, 50A circuit?

Comments (9)

jason1083

kevder
Original Author

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240V, 30A heater on a 240V, 50A circuit?

Comments (9)

jason1083

kevderOriginal Author

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bigbird_1

normel

kevderOriginal Author

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kevder
Original Author

kevder
Original Author