View Full Version : electric heater wire size
leelectricco
08-10-2007, 11:11 AM
Installing a 4000 watt, 240 volt garage unit heater and it pulls 16.7Amps.
In calculating 125% for continuous load I come up with 20.8Amp circuit conductor required. Can I use #12 wire which is rated in 310.16 for 25 Amps?
It would still be on a 20 Amp breaker.
The reason I am asking is that the heater manual said to use minimum supply wire of #10. If I use Table 310.16 it looks like #12 is large enough for this heater.
Thanks.
iwire
08-10-2007, 11:32 AM
12/2 with a 20 amp breaker will be a code violation.
At a minimum you may use 10/2 with a 25 amp breaker.
charlie b
08-10-2007, 11:54 AM
12/2 with a 20 amp breaker will be a code violation.
I'm not so sure about that, Bob (spoken as the gauntlet is tossed onto the floor at Bob's feet). ;) What article(s) would you cite as being violated?
(While youse guys are contemplating the audacity of my challenge, I will by typing up a statement in defense of my position.)
iwire
08-10-2007, 11:57 AM
I'm not so sure about that,
Truthfully neither am I. :grin:
I will get back to you. :)
growler
08-10-2007, 12:05 PM
What about 240.4(D) ? I don't see a water heater as an exception.
I'm an idiot I'll have to fall back on good old 110.3(B)
iwire
08-10-2007, 12:08 PM
Installing a 4000 watt, 240 volt garage unit heater and it pulls 16.7Amps.
In calculating 125% for continuous load I come up with 20.8 Amp circuit conductor required.
I agree with that per 424.3(B)
Can I use #12 wire which is rated in 310.16 for 25 Amps?
It is rated 25 amps but it has a "*" beside it.
At the bottom of Table 310.16 you see a reference to 240.4(D) which requires that we use no larger than a 20 amp breaker with 12 AWG in most cases.
Maybe it could be argued that 20.8 can be rounded down to 20 but I do not believe so.
charlie b
08-10-2007, 12:16 PM
OK, here I go:
Start with 424.3(B). It says that this equipment is to be treated as a continuous load.
Next go to 210.19(A)(1). It says that the minimum conductor size must have an allowable ampacity not less 125% of this continuous load. That brings us to the value of 20.8 amps, as correctly calculated by leelectricco.
Now we proceed to Table 310.16. It says that the allowable ampacity of a #12 THHN is, as leectricco correctly observed, 25 amps. We are all so familiar with the combination of #12 wire and 20 amp breakers that we can’t think outside that box. Look at the top of the table. Its title begins with, “Allowable Ampacities.”
We note the asterisk at the far left of the table, next to the “#12.” That takes us to 240.4(D). There we learn that we must use a breaker not higher than 20 amps, despite the fact that a #12 has an allowable ampacity of 25 amps. So that is the breaker size we select, and that fact causes the branch circuit as a whole to be rated at 20 amps.
Looking just a bit higher up the page, at the beginning of 240.4, we see that conductors must be protected against overcurrent in accordance with their ampacities. We reflect that a 20 amp breaker can successfully protect a conductor that has an ampacity of 25 amps, and conclude that we are still in compliance with the code.
Finally, we note that this design choice might be unwise, in that there is a risk that the heater might just trip the breaker from time to time. But we can rest assured that this fact does not place us in non-compliance, because of what is written in 90.1(B).
The bottom line is that that "*" to the left side of Table 310.16 does not alter the values in the table itself. It puts a restriction on the overcurrent protection, but it does not change the allowable ampacities.
All that being said, I agree that using #10 wire and a 25 or 30 amp breaker is the right thing to do. But the original question was not all about the right thing to do.
charlie b
08-10-2007, 12:18 PM
What about 240.4(D) ? I don't see a water heater as an exception.
I don’t either. But then, I don’t need to. 240.4(D) has nothing to do with allowable ampacity, as I explain above.
Maybe it could be argued that 20.8 can be rounded down to 20 but I do not believe so.
Neither do I. But I don’t need that either.
Michael15956
08-10-2007, 12:40 PM
Very good Charlie,
I agree with your post, but Iwire was still right also.
As Browler posted, #12 wire on a 20amp CB is still a violation of 110.3(B).
charlie b
08-10-2007, 01:09 PM
Yes, of course, since the manual said to use #10, it would be a violation of 110.3(B) to use #12. But my little academic discussion was aimed at the universally accepted untruth that the ampacity of #12 THHN is limited to 20 amps. If the manual did not give a minimum wire size (or if the OP did not tell us what the manual said), there would not have been enough information to use as a basis for calling this a violation.
So here is a quick summary of my position. You can use a #12 wire and a 20 amp circuit breaker to feed a continuous load of 16.7 amps, despite the fact that 125% of 16.7 is above 20.
SmithBuilt
08-10-2007, 01:54 PM
You can use a #12 wire and a 20 amp circuit breaker to feed a continuous load of 16.7 amps, despite the fact that 125% of 16.7 is above 20.
Since it would be 20.8 wouldn't that make it a violation. Can't put a continuous load on a breaker over 125%. Sorry I don't have my code book with me to site the rule, plus a poor memory.
charlie b
08-10-2007, 02:19 PM
Can't put a continuous load on a breaker over 125%. Sorry I don't have my code book with me to site the rule, plus a poor memory.
210.20(A), unless that very rarely used exception can be brought into play.
I concede. Oh well. It was fun while it lasted. :rolleyes: ;) :smile:
leelectricco
08-10-2007, 04:40 PM
With so many different opinions, I am not sure if I want to put the heater on the 20 amp circuit. Why do they put a 20 amp cord and plug on the heater and then say it must be 30 amp rated wire? If someone purchased this heater to use on an existing 20 amp circuit, most likely that receptacle would be wired with #12 wire when it was installed.
I think I will search for a 3000 watt heater so as not to worry about it. Thanks to everyone for your opinions.
With so many different opinions, I am not sure if I want to put the heater on the 20 amp circuit. Why do they put a 20 amp cord and plug on the heater and then say it must be 30 amp rated wire? If someone purchased this heater to use on an existing 20 amp circuit, most likely that receptacle would be wired with #12 wire when it was installed.
I think I will search for a 3000 watt heater so as not to worry about it. Thanks to everyone for your opinions.
Is this a Cord and Plug heater? In my opinion, if it is, then you are only required to have a 20 amp #12 circuit.
Dennis Alwon
08-10-2007, 05:08 PM
[quote=leelectricco]With so many different opinions, I am not sure if I want to put the heater on the 20 amp circuit. /quote]
Don't put the heater on a 20 amp breaker. Run #10 and you can either use a 25 or a 30 amp breaker.
I believe Charlie conceded in his last post stating that art. 210.20A will not allow a 20 amp on the circuit for the heater.
Dennis Alwon
08-10-2007, 05:09 PM
Is this a Cord and Plug heater? In my opinion, if it is, then you are only required to have a 20 amp #12 circuit.
Norb there is a 4000 watt heating element--- It would be a violation.
Norb there is a 4500 watt heating element--- It would be a violation.
But if the unit comes with a cord then aren't we able to use it?
Is this garage heater a water heater or a Heater heater?
Dennis Alwon
08-10-2007, 05:15 PM
But if the unit comes with a cord then aren't we able to use it?
Is this garage heater a water heater or a Heater heater?
My mistake it is a unit heater but that doesn't matter. If the unit calls for 30 amps then it will come with a 30 amp plug. Nothing was said about a plug anyway.
My mistake it is a unit heater but that doesn't matter. If the unit calls for 30 amps then it will come with a 30 amp plug. Nothing was said about a plug anyway.
Dennis: Read post #13 from the Original Poster(?)
Dennis Alwon
08-10-2007, 05:25 PM
Dennis: Read post #13 from the Original Poster(?)
I am wrong again. I had a similar experience with a range cord. The specs called for a 20 amp cir but the cord was 15 amp. They said the only time it would be a problem is when both convection motors came on at once. I said that's right and I had the manufacturer send a service man and change the plug.
Now what do you do-- I would confront the mfg who is telling you 2 things. You can't legally use a 20 amp recep. on a single cir. with a 30 amp breaker and that is what is necessary. Actually they only call for #10 wiring not a 30 amp OCP so--- I give up.
I am wrong again. I had a similar experience with a range cord. The specs called for a 20 amp cir but the cord was 15 amp. They said the only time it would be a problem is when both convection motors came on at once. I said that's right and I had the manufacturer send a service man and change the plug.
Now what do you do-- I would confront the mfg who is telling you 2 things. You can't legally use a 20 amp recep. on a single cir. with a 30 amp breaker and that is what is necessary. Actually they only call for #10 wiring not a 30 amp OCP so--- I give up.
I've been following this as well and I think I am going to give up as well. ;)
Have a great Weekend.
growler
08-10-2007, 05:34 PM
Now what do you do-- I would confront the mfg who is telling you 2 things. You can't legally use a 20 amp recep. on a single cir. with a 30 amp breaker and that is what is necessary. Actually they only call for #10 wiring not a 30 amp OCP so--- I give up.
You speak Chinese ? Little humor there.
…and all this has what to do with Electrical Contracting and Estimating/Management? I am sorry maybe I am speaking out of turn but do newbies that join to ask a simple codebook 101 question in the wrong forum usually get this kind of attention?
Not trying to be a jerk here but all this time and energy spent on trying to educate a homeowner on how to hook up his water heater has got me perplexed.
leelectricco
08-10-2007, 05:52 PM
Ito does seem to be a jerk. If he followed this closer he would know that the question was on a garage unit heater. Also, as many different opinions as I had, it must not be as simple minded as he thinks it is.
Sorry I posted this in the wrong area, but, all the other people who posted interesting opinions didn't seem to mind. I am an electrician in search of an answer to my problem, not a smart-ass opinion like Ito has.
So is this unit heater Cord and Plug?
leelectricco
08-10-2007, 05:57 PM
Yes, it is cord and plug connected. It has a 20 amp plug on it, but recommends to run #10 supply wire for the circuit. I had already ran #12 for the circuit ahead of time, not thinking I would have a problem.
480sparky
08-10-2007, 05:58 PM
I'll throw this in, too:
Is this a single 4,000 watt element, or two 2,000 watt elements and there is a mechanism that prevents both from being energized at the same time?
Yes, it is cord and plug connected. It has a 20 amp plug on it, but recommends to run #10 supply wire for the circuit. I had already ran #12 for the circuit ahead of time, not thinking I would have a problem.
With what I know, I would plug it in and walk away.
leelectricco
08-10-2007, 06:00 PM
It is a single 4000 watt element heater.
JohnJ0906
08-10-2007, 06:02 PM
leelectricco, welcome to the forum!
I wouldn't worry about the placement of this question, if the first 2 responders didn't have a problem, then no one else really should either. (both are moderators)
This IS an interesting situation, and I think maybe a call to the manufacturer is in order. Do they want #10 wire on a 20 amp breaker and a 20 amp receptacle, or what?
They created the situation, ask them what they want.
480sparky
08-10-2007, 06:03 PM
Instead of getting a 3,000 watt unit, get a unit with two 2,000 watt elements. It will be more effecient than a single 4,000 watt unit and solve your wire-size dilemma.
leelectricco, welcome to the forum!
I wouldn't worry about the placement of this question, if the first 2 responders didn't have a problem, then no one else really should either. (both are moderators)
This IS an interesting situation, and I think maybe a call to the manufacturer is in order. Do they want #10 wire on a 20 amp breaker and a 20 amp receptacle, or what?
They created the situation, ask them what they want.
Very well stated.
leelectricco
08-10-2007, 06:10 PM
Thanks to everyone for your opinions. It will help me decide which way to go.
I think I will go to a smaller size heater just because I worry if things are not as I think they should be.
Thanks
growler
08-10-2007, 06:11 PM
Yes, it is cord and plug connected. It has a 20 amp plug on it, but recommends to run #10 supply wire for the circuit. I had already ran #12 for the circuit ahead of time, not thinking I would have a problem.
Recommends is not the same as required. The code book is full of things that are recommended but are not enforceable. If you already ran the circuit, slap a 20 Amp. receptacle in and call it a day. Let the homeowner buy the heater.
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