I bought an electric heater and I am going to install it on 240V. So I will put a 30A double pole breaker in the box and run 10-3 romex to the heater.
Unfortunately, I am not sure how to read the directions for this commercial model of heater. There are three lugs on the heater, L1, L2, and L3. I also don’t see a ground screw anywhere.
The instructions including a wiring diagram is here. I took pictures of the heater here and here.
I figure Dopers know everything, so I will ask you guys where the hots and neutral go. I can post more pictures and answer any questions you have for me.
It looks like it’s designed to be used on either single- or three-phase power, so it can probably run on anything from 208 to 240 V. Looking at the wiring diagrams on pp. 5, figure 12, you’ll want to connect one leg of your 240 to L1 and the other to L2. There appears to be no neutral connection. I don’t see a ground terminal listed, but it’s almost certainly present–usually they’re green. You may need to remove one of the red jumpers as indicated if your 240 V circuit can’t handle the full load.
Missed the edit window: Yes, I see a green ground screw mentioned on page 4, item #14. Do not connect the neutral here; this is only for the Earth ground connection. You will not need a neutral at all.
I see what’s going on now. The ground screw is on the enclosure, which I have not unpacked out of the box yet… no wonder I didn’t see it. And the two hots go to L1 and L2 but there is no neutral because the two hot legs in opposite phase from each other so they end up cancelling each other out, right? Well that means that I spent extra money on 10-3 wire when I could have gotten away with 10-2 and putting tape on the white to mark it as hot.
FWIW, this isn’t quite right. If you were using a hot and a neutral, the neutral would be a current-carrying wire, it is only designated as a neutral because the neutral bus is connected to earth ground somewhere, a ground rod for instance. When you connect two “hots” they are opposing in phase, meaning essentially that for one-half the AC cycle the current flows in one wire(L1) and out the other(L2), the next half-cycle, it flows in (L2) and out (L1). Your heater is still going to work just fine whether you aware of this or not, but with electricity it’s much safer if you understand some of the basic principles.
OK, I see what you are saying. I guess I was thinking of an instance where you would have a 20A circuit and a 15A circuit coming off of a subpanel on different legs. When both are drawing full current, the neutral back to the main panel is only carrying 5A, and the most it would ever carry is 20A if the other circuit is drawing zero. Do I have this right at least?
I don’t know how much safer any of this makes me though… I just follow code and make absolutely sure the breaker is off before I do anything.
I don’t think so. Each circuit would have it’s own neutral in that case, assuming 120V circuits. The voltage from leg to leg is 240V, and from leg to neutral is 120V.
Each circuit has it’s own neutral which gets joined together in the neutral bar. Then there is only one neutral that runs back to the neutral bar on the main panel. That one neutral serves for both legs.
The line I ran from the main to the sub was 6-3. The red was 6awg, the black was 6awg and the neutral was also 6awg - not 4 or 2 or whatever would be needed to carry twice the load of one hot.
On a 240V circuit the neutral isn’t an issue. The only reason I nit-picked on the issue earlier is I’ve seen people working on the neutral live without being aware that it becomes hot if they disconnect it at the wrong end while everything is live. Also, if the neutral is loose you get serious voltage drop issues, or you create a situation where it carries too much and overheats. You seem to have a decent understanding of it but didn’t want to assume that based on the OP, better safe than sorry.
You can use the same neutral if the circuits are on different phases.
That neutral would carry the unbalanced load as Stinky Burrito has mentioned. He was also right in wiring his sub-panel with a #6 neutral in that it only carries the unbalanced load. In certain cases you can actually downsize the neutral.
When you share a neutral in this manner it is not necessary to use a two pole breaker unless both circuits are feeding the same outlet or appliance.
If you run two circuits to a location and they are both on the same phase then you must run a separate neutral for each.
The more I thought about that statement, the more it makes sense WHY 240V is 240V. (And why I don’t need a neutral at all). I think it helps to picture a graph in my mind. The distance from a peak to the ground/neutral is 120V while the distance from a peak to the opposite peak (if you imagine the opposite sine wave overlayed on the linked graph) is twice that, or 240V.
This is all very interesting - I am glad I am able to learn more about this topic.
I hope I’m not belaboring the point too much but when I’m training a new technician I never use the term “neutral”. I’ve found that calling it a “return” helps them get it faster, in that it provides a path back to the source. Many people find it easier to understand by comparing it to a belt running around two pulleys; the “hot” leg represents the belt on one side of the pulleys, the “return” is the part of the belt on the other side of the pulleys. Together they make one complete belt just as the hot and neutral make up one complete circuit.
I was wrong. I started wondering about it as I was driving to work.
Regarding the two pole breaker, the 2008 NEC has a new section on disconnecting means for multiwire branch circuits (210.4(B)). It basically says that you need a means for simultaneously disconecting all ungrounded conductors where the circuit originates. Seems like the two pole breaker would be the simplest way to do that. Of course, that is a very recent addition to the code.
