View Full Version : #12 AWG copper wire for baseboard heaters

Nov 16, 2009, 02:29 PM
I want to run a circuit for baseboard heaters and have been told to use #12 AWG copper. I have a total of three heaters in separate rooms each controlled by its own thermostat. I would like to use #14 AWG copper and have calculated the voltage drop with #14 to be less than 1% with all three heaters on together. Is there a reason why I shouldn't use #14? The code recommends no more than a 3% voltage drop, lists the ampacity of #14 as 20 amps, and allows a circuit breaker of 15 amps. The heater manufacturer recommends an 80% derating. It seems that #14 complies with all of these. Am I missing something?

Nov 16, 2009, 02:57 PM
Am I missing something?

Yes. Maximum amperage of #14 wire is 15 amps. Maximum breaker size is 15 amps.

Nov 16, 2009, 05:59 PM
A slight nitpick but the original poster is correct on the ampacity of 14 AWG copper wire. The fact that the code limits the overcurrent protection to 15 amps does not change the ampacity listed in the table.

As for the question, we would need to know the wattage of the heaters and what voltage you intend to use. I will say that even at 240V, 12 amps wouldn't buy you much heat. 12 amps being the most you can put on a 15A electric space heating circuit. Did you consider that 12A would be the max or did you think it would be 16 because the wire could handle 20? The breaker must still be 15A and 80% of that is 12A.

Nov 17, 2009, 07:08 AM
I am using 240 volts through a double pole breaker. I was using 12A as the max. There is a 1,000 watt heater in the first room, another 1,000 watt heater in the second room, and a 500 watt heater in the third room. The voltage drop in the third room at the end of the run calculates to be 0.79%. The heater sizes were chosen per the manufacturer's recommendation of 6 watts per square foot of room area. When I pointed out the load and voltage drop to the fellow who recommended #12 wire, he said "I always use #12 to baseboard heaters.". I said "Even with a single 500 watt heater?". He said "Yes."

Nov 17, 2009, 07:16 AM
Why are you de-rating the amperage?

For heaters, you must multiply the amperage required by 1.25 (Continuous ON rate) That equals 15 X 1.25 = 19 amps.

This means that you will actually need a 20 amp line to feed these heaters.

Question, are the heaters daisy chained together?

Nov 17, 2009, 10:11 AM
To answer Don's questions, the thermostats controlling the heaters are daisy chained and I am derating their current because they represent a conitinuous load.

Nov 17, 2009, 10:28 AM
Okay, first of all, you only gave a 15 amp load in your problem description. How am I to know information that you did not pass on to me.

Your plan to parallel connect the three thermostats together and ignore the total amperage required is bothersome to me.

According to what you have given, each heater requires an amperage of 10.4. Just by simple math, one 15 amp circuit can not safely drive three 10.4 loads.

If all of the heaters are on at the same time, the circuit will trip. Right now, that's a opinion only. Let me dig through the NEC to see if I can back my comments with code sites.

I just don't have a code book with me at the present time.

Nov 17, 2009, 10:46 AM
My problem description in #4 talked about a 2,500 watt load. I am not ignoring the total amperage required - it is 10.42A at 240 volts That is for two 1,000 watt heaters plus a 500 watt heater for a total of 2,500 watts.

I wasn't being critical of your answer. Iwas simply trying to let you know 15A isn't the figure to use

Nov 17, 2009, 12:01 PM

Please consider the following code cite from the 2005 NEC.

"424.3 –(A) Branch-Circuits Requirements: Individual Branch-Circuits shall be permitted to supply any sized space-heating equipment.

(B) Fixed electric space-heating equipment shall be considered as continuous load.

The 424.3 (B) tells you that you have no choice but to multiply the amperage listed on the space-heaters plate by 1.25 to get the actual required amperage.

What is the amperage required by each individual baseboard-heater?

Also, I believe that article 424.3 (A) tells you to use an independent circuit for each room. You need 3 independent branch-circuits, one for each room.

Would it trouble you to post the exact information from the plate to this item or at the very least provide the make and model for the 1000 Watt and 500 Watt baseboard heaters? I want to e certain that your plan meets the NEC Code.

With respect to the voltage drop, that should not even be a consideration unless the distance from the main service panel to each room is greater than 100 feet. Voltage drop on a 240/120 circuit (is calculated by using the line to neutral voltage) The maximum RECOMMENDED drop is limited to 3.6 volts.

On a straight 240 (no neutral - line to line) the maximum RECOMMENDED drop is 7.2 volts.

These are recommendations, not code requirements.

Nov 17, 2009, 03:30 PM
There is nothing in the NEC that I know of preventing you from using 14AWG copper wire and a two pole 15A breaker for this circuit.

It's 2500W/240V = 10.42A

You would be fine with a 15A circuit. I'd still run #12 in case I wanted to add more heat at a later date.

I'm sure the guy that gave you the advice has just had to rerun a circuit one to many times when the homeowner changes the heater because his wife and kids are still to cold.

Nov 17, 2009, 09:31 PM
This thread is all messed up. I'm seeing apples and prunes.

Basic math Amps @240 V = 2500W/240 V = 10.41 A

We can use that to determine voltage drop,

One way 110' of 14 AWG dual cable for 3% drop

Next problem; space heating multiply by 1.25 or 13 Amps. Space heating loads are continuous; Can be on for more that 3 hours at a time and there space heating, so by definition, they are considered cotinuous.

Next problem, an buy a 13 A breaker. Breaker must be 15 A

So, 14 AWG, 15 amp breaker, no more than 110' of cable for a 10.41 amp load at 240 V resistive which will have no more than a 3% drop.

Nov 18, 2009, 05:57 AM
Tev, I agree with you. There is no prohibition from using a 12/2 AWG cable within a 15 Amp circuit.

However, as I understand the description supplied by RG, he wants to tie the three rooms together on one circuit.

Since I still don't know how the 10.4 amp demand is to be derived (the total amperage of the two 1K +500 ohm baseboard-heaters or is the 10.4 amp the required amperage for a single circuit)

Still to the best of my knowledge, you do not run three baseboard-heaters each in a separate room off one 240 VAC - 10.4 Amp circuit. Each room has to be on its own branch-circuit.

Nov 18, 2009, 01:04 PM

This thread is all messed up.

So, true, glad you cleared it up.

Don, please show me exactly where this is code:

Each room has to be on its own branch-circuit.

If true, then I wired about 1000 homes wrong, and all passed inspection.

Nov 18, 2009, 02:48 PM

As usual I will defer to you in all electrical postings.

I am referring to 424.3 (A) & (B).

In my very limited experience, I have never come across one 240 X amp branch-circuit feeding three different room base-board heaters as well as each room's thermostat. All by itself that a major statement. Given that I have only done maybe 3 different homes in three different states, but they to also passed each inspection. Also they only involved two rooms.

If I may, is 424 (A) to be interpreted as saying that if you want to use an independent branch-circuit, you may or that a multi-room drop is also permissible?

Nov 18, 2009, 03:01 PM
424.3(A) says two things about electric space heating in dwellings

1. If you have only one heater outlet on the circuit the circuit size can be whatever size you need.

2. if you have multiple heater outlets on the circuit you are limited to a maximum circuit size of 30 amps.

Nowhere does it limit the number of heaters you may have on a circuit or specify they be located in the same room.

Nov 19, 2009, 07:54 AM

Nov 19, 2009, 09:02 AM
Don, Tev is correct.

You need to read the code very carefully. Words such as shall, may, permissible, are very specific.

I easily can have 3 - 1000 watt heaters and a 500 watt unit, each alone in 4 rooms, and all supplied by a 20 amp 240 volt circuit.

Nov 19, 2009, 09:21 AM
T.K. I understand, that's why I thanked him.

As I now understand 424.3 (A), you can choose to use a dedicated branch-circuit for any size heating branch circuit.

If, however, you are serving multiple heater outlets from the branch-circuit, then the circuit is limited to a minimum of 15 amps and a maximum of 30 amps.


Nov 19, 2009, 09:30 AM
If, however, you are serving multiple heater outlets from the branch-circuit, then the circuit is limited to a minimum of 15 amps and a maximum of 30 amps.


For residential, Correct.