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First off, I wear many Hats (jack of many, master of...some) I refer to the 2017 NEC often and able to answer my questions most of the time. This one bounces my brain too much. Providing this answer takes a deep knowledge of the NEC gained by years in the trade (or a book geek) therefore I humbly and respectfully thank you for sharing your knowledge.

I'm in Colorado, USA. ( Machavity) (my fluke* says 122v... so 244 volts (Harper)?:)

The heater is a CADET (cadetheat.com Vancouver, Washington) Its tested to UL standards. They cover their a** on installation by saying

  • *1. All electrical work and materials must comply with the National Electric Code (NEC), the Occupational Safety and Health Act (OSHA), and all state and local codes, without giving specifics other than...
  • The maximum amperage load you can put on one circuit breaker is limited to either 80% of the circuit breaker capacity, or the maximum amperage rating of the thermostat, whichever is lower. & some other reference regarding breaker being ...20% larger than load(or something to that effect, not quoted)

I'm installing 4 baseboard heaters of different sizes, each in a separate room, each on a separate breaker, with the smallest being a 240/208 Volt, 500/375 watts, 2.1/1.8 amp, with a wall mount thermostat (mechanical double pole 120/208/240 volts, 22 amps).

My Question is, If I run 12-2 wire with a 15 amp breaker that has only the one above mentioned heater, am I within NEC? My concern is if the breaker is too large to protect the appliance.

Take this one step farther...240-V baseboard heater, 2.1 amp draw, 10-2 wire with a 30 A breaker total run length 40 ft. Can you "overprotect" or defeat the breakers ability to trip/disconnect by having too small of a load?

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  • Where in the world are you located? That will impact the answer – Machavity Jan 11 at 17:14
  • FYI, 110v and 220v nominal ratings are very, very obsolete (half a century, actually). We're really talking about 240v appliances and circuits here. – isherwood Jan 11 at 17:19
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    The reason for that is power started at 100V and was 95% for lighting. As they needed more capacity, they conspired with light bulb makers to make their bulbs tolerant of a 5% bump, then in some months (a light bulb's life) they bumped to 105. Then to 110. Then, they switched to AC and started marketing electricity to the public with great fanfare. That stuck in people's minds, like Xerox. "Get your 110 volts!" After that was over, they did two more bumps, to 115 and 120, now we are at WWII. They teed up a 125V bump but never did it. – Harper Jan 11 at 19:21
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Wire can always (or almost always, I'll bet there is an exception somewhere) be oversized. Using a larger wire than required by code will either make no difference at all or only improve things - e.g., by reducing voltage drop over long distances.

Breakers must NEVER be oversized when used as a protective device. Generally this is pretty clear from the particular device - e.g., a device that requires a 20 Amp circuit must have at least matching size wire (typically that would be 12 AWG) and must have a breaker that is exactly 20 Amp. There are certain exceptions - e.g., a 20 Amp breaker (with at least 12 AWG wire) may be used for 15 Amp receptacles, provided there are at least 2 (e.g., a typical duplex 15 Amp receptacle). But you can't put a 30 Amp breaker on a circuit that is rated for 20 Amps (either because of the device specifications, the wire size or both).

Breakers CAN be oversized when used merely as an on/off switch. This is the case, for example, in some subpanel installations. For example, if you have a 60 Amp breaker in your main panel feeding appropriately sized wire to a subpanel, then the subpanel can have any size breaker (though obviously it should be 60 Amp or larger in this example) as a main breaker because it is not providing protection, it is only used as a switch (e.g., to be able to safely work inside the subpanel without going back to the main panel). If you search around a bit on DIY, you will find many examples of using a large panel with a 100 Amp (or even larger) breaker for a subpanel that is actually fed by a smaller breaker in the main panel.

Now on to your situation:

220 V baseboard heater with a 10 amp draw, wall mount thermostat. Can I run 12-2 wire with a 15 amp breaker?

The first determining factor is the specifications of the heater. It should have some information about what type of circuit it works with. Barring any specifics, 15 Amp is the typical bottom meter size in the US, and is obviously big enough (and should be safe enough) for a 5A or 10A heater. However, more typically with 200 V loads you will see larger breakers as one of the reasons to use 220 V is that you can get a lot more power than with 110 V. In particular, if you will be connecting multiple baseboard heaters on one circuit then you need to make sure that they are designed so that they can be on the same circuit and protected appropriately. Your circuit (including the breaker) needs to be larger than the continuous load (heaters may be on for long periods of time, so they definitely count for continuous load), I believe by 20%. So a 15A heater, for example, would actually need a 20A circuit (12 AWG wire and 20A breaker).

As far as wire, as already noted in another answer:

  • You need to see if a neutral is needed (you can't share ground & neutral). If so, then you actually need 12/3, not 12/2.
  • Even if you are using a 15A circuit now, going up to 12 AWG doesn't cost much for 40 feet and gives you more flexibility (i.e., up to 20A) for the future.
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  • I don't get it. Shouldn't the device be fused? – Joshua Jan 11 at 21:59
  • I believe the breaker is the fuse, its the only means of safety in this situation. – Nobody Inparticular Jan 12 at 11:53
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Code says "Follow the instructions or labeling" (NEC 110.3).

Why? Because the heater presumably has a UL or CSA listing. That means it went through a testing lab to confirm its safety. The lab didn't test in every possible use, only the uses described in the instructions. The UL listing is contingent on use according to the instructions.

Remember both the wire (215.2) and the breaker (215.3) must be sized for at least 125% of the heater's rating, because heaters are a continuous load (424.3b). So you must provision 12.5A of wire and breaker for your 10A load, which fits comfortably on a 15A breaker. This 125% derate is the same thing as the 80% derate discussed in the instructions, so don't apply it twice... this method is official because it allows you to calculate correctly when mixing it with other loads that don't need the derate.

The instructions should answer your question about how large a breaker to match this heater up with. Generally I would focus on the wire and aim to not exceed wire capacity (over-wiring is always ok).

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Is this baseboard 240 only? Some also need a neutral when the controller is digital. I would run #12 wire better to go big with heaters, as far as the breaker a 15 amp would work, check the info on the heater as it may state max breaker size.

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  • These heaters are fixed loads. It is acceptable (in Canada) to max out the breaker and the conductor to 100%. No need to derate because this is a fixed load. No other loads are wired to this dedicated circuit. – Chris Taylor Feb 5 at 21:08
  • Where did I say anything about derating? – Ed Beal Feb 5 at 21:26
  • I was responding to other's comments, not your initial post. But you do mention something about 80%, which is the common derating factor, in the fifth paragraph, the sentence beginning with the words "The maximum amperage load you can put ..." – Chris Taylor Feb 5 at 21:29
  • In the NEC the wire size needs to be 125% of the load for continuous loads and a heater is a continuous load, but I said nothing about that. If you want to provide a comment on an answer place after that answer or no one understands or it gets totally lost in the shuffle. – Ed Beal Feb 5 at 21:43

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