I am running a series of (6) 20 amp outlets, via conduit, in (3) double gang boxes, each outlet on a separate circuit…. a couple of questions.
1. Breakers… single or double? Advantages?
2. The neutral…can one be used or are multiple runs required? (the surface mount outlet run is approx 60 ft with an additional 75ft pull thru 2″ cond to panel…if it matters)
Any advice, suggestions or scathing criticism (“Hire an electrician, idiot!!”), appreciated.
Thanks,
TN
Replies
As I understand this, you want 6 receptacles, each on it's own breaker, run through the same pipe.
I prefer that 'full size' breakers be used. The panel is crowded enough as it is. If you don't have room, I suggest that you consider installing a 'sub' panel, and feed your receptacles from that.
I suggest that you consider the actual loads you intend to power. Having more than two circuits in a pipe, you have to "de-rate' the wires ... a fancy way of using bigger wires. (I'll get back to this point).
You can 'share' a neutral between but two circuits. The circuits need to be on different 'legs;' that is, there needs to be 240 volts between the 'hots.' If there is ever a loose connection on this shared neutral, you will wind up supplying 240 volts to those receptacles. Not good!
Since keeping all the neutrals straight can be confusing, I often find it easier to to just run separate neutrals. I take pains to identify them by circuit number in the panel; this can be a big help later.
6 circuits means 12 current-carrying wires if you run separate neutrals; 9 if you do not. Now, for derating:
12 wires means a 50% capacity ... you need to run #12 for 15 amp circuits, or #10 for 20 amp circuits.
9 wires means you can use #14 for 15 amp circuits, and #12 for 20 amp circuits.
Let's look at your pipe sizes:
For 12- #10's, you need to run 1" pipe.
12 -#12's will fit in a 3/4" pipe.
9 -#12's ought to be run in a 3/4, but can be squeezed into a 1/2" pipe if you don't run a ground wire as well.
9 -#14's fit in a 1/2" pipe without a problem, even if there is also a ground wire.
Now ... a look at your box fill. There simply is no standard box that has room for all those wires, as well as the receptacles. So, you will need to have some junction boxes.
I will assume here that you are mounting these boxes ON the wall (not IN it), running EMT (steel pipe), and that the receptacles will be mounted on "industrial style" covers. For this arrangement, you do not need a ground wire.
While there are several possible ways to do this job, the one I suggest is to run your pipe from the panel to a junction box ... then exit this box with two pipes ... each of which goes on to serve three receptacles.
For simplicity sake, I suggest that you use all 4x4x2 1/8" deep boxes.
So ... looking over the entire job .... it looks like your best arrangement is to have 15 amp breakers, feeding #12 wire. Run 3/4 pipe to the junction box, as well as the first receptacle. After that, you can use 1/2" pipe.
Now does everyone see why an electrician needs four years to learn the basics of his trade?
6 circuits means 12 current-carrying wires if you run separate neutrals; 9 if you do not. Now, for derating:12 wires means a 50% capacity ... you need to run #12 for 15 amp circuits, or #10 for 20 amp circuits.9 wires means you can use #14 for 15 amp circuits, and #12 for 20 amp circuits.
I'm no electrician here, but how can you use #12 for 20A circuits after derating? My understanding was that 20A circuits had to be #12 to begin with, without taking any derating into account.
Bob
The NEC has a variety of factors that are used in 'derating.' Apart from the various tables and formulas, the material has to be applied in a systematic way. Just as important, there are various things that go into arriving at the 'load' that you need to cover. But, to answer your question: In your derating calculations, you start off with a value that is determined by the type of wire. I assumed a very common type (THHN) that has an 'ampacity value' of 30 amps. This is the starting point. When I look at the rules on multiple wires in a pipe, I see that 9 wires have a multiplier of 70% ... 30 amps x 70% = 21 amps. This compares to a value of 50% for 12 wires (30x 50% = 15). Yet another part of the NEC goes a step further: In most common situations, I am given a maximum breaker size for wire sizes #10, 12, and 14. That is, the code says in effect "no matter how the math works out, don't put a breaker larger than 20 amps on a #12 wire." There are a lot of other factors that I did not discuss, just to keep things simple. For example, I assumed that he didn't really need 20 amps available for an extended period. I assumed that he was not using a load (like an air conditioner or welder) that has specific rules. I often tell folks that an electrician is hired as much for his knowledge and judgement, as he is for his physical skills. This thread is a good example of that. You have certain practices that you actually have to learn ... and that learning has to be done in a systematic way. This also is a good example of what the introduction to the NEC means when it says it's not a design, or instruction, manual. To apply the NEC, you simply have to know the trade first - using the Code as a 'final check.'
First, give us the big picture. You're talking about buying half a dozen rather expensive breakers. So the first question is, do you really need to run that many 20 amp loads, or random combinations of them, simultaneously? This kinda sounds like it might be a home workshop, in which case you're not likely to really need to run more than one machine and a dust collector at once.
If this really requires six circuits in one place, then definitely the sub panel idea is the best way to go.
-- J.S.
Renosteinke - thanks for the great overview and advice .... of course good advice leads to more questions!
JS- yes, the big picture...I should have included more detail in my original post!
The 120V, 20amp dedicated circuits are required for "shore power" to inverters on custom hi-way coaches (converted MCI buses used to transport kids/families to Shrine Hospitals). The buses are housed in a commercial, concrete tilt-up warehouse. They originally wanted (10) 20 amp cir., but today we reduced that to (6). The building is new and has a large, 3ph panel, so brkr space is not an issue....cost of course is!
The lay-out is as follows: Panel located on east wall - (2) 2" pvc cond stubs below panel - 65ft under slab stubbed up at 2 locations on west wall. My six outlet "run" is 12 ga.wire from the panel, thru one of the 2"pvc to large JB on west wall, then through 3/4" EMT to each device box (I had planned on 4X4X21/8) each housing two 20 amp duplex outlets. (the other circuits are on east wall on separate, single runs)
A ground from the panel thru PVC to main pull box will be required...EMT from there on will provide the gnd. OK so far??
My question on the neutral Renosteinke...I think I get what your saying re: loose conn.=240V to one recep...so to clarify... the neutrals, individual or shared must all be home run to the neutral bus bar in the panel, not pig-tailed, correct?
Breakers....do not require tie-bars for the shared neutral circ. right?....should these be individual sngl pole 20A or double 20A or quad type. Does it matter and why?
Thanks again for your input. I can appreciate the "4 years to learn the basics"... I have done lots of res. electrical modification/alteration and often wish I had taken it up as a trade... I really enjoy the trouble-shooting/service end of it! My commercial/conduit experience is limited, hence this post! Just want to do it right!
TN
Since each "hot" wire will be going to a separate device (duplex receptacle), you are not required to use handle ties on the breakers. Yes, the neutral must be a 'home run', and serve only two circuits. In a three phase panel, you can (in theory) share each neutral with three circuits; but, with the layout you've described, you won't get any help for this arrangement. Here's the rub: Let's assume you are using #12 wire. That wire ought not ever be expected to carry more than 20 amps. If you combine two circuits on the same phase on that neutral, a circumstance can develop that would have nearly 40 amps on that neutral. Not good! Some might argue for other approaches, and these others might be allowed under a strict reading of the NEC. I have found, however, that it makes things a LOT simpler, and prevents many other issues from arising, if your neutrals are limited to only one "round robin" (one hot from each phase, no more), and run all the way back to the panel before being joined with other neutrals.
Reno, thanks again...now its making sense....the potential "40 amp on the neutral" is what I was concerned about. Now that the original 6 circuit run has been reduced to 4, I will take your advice on the independent neutral runs...just didn't want to pull and overfill pipe with 400 ft of un-necessary wire!
Also, (again due to my limited comm. experience) the 3ph is what confused me. Looking at the existing circuits in the panel, I saw (3)15amp circuits, 2 outlets and 1 HWT, all on 1 neutral...hence the 3ph. didn't think of that. They must all be on different legs. Correct?
TN
They should be. How can you tell? The voltage between any two should be somewhere near 208 volts, and between any one of them and ground 120 volts. IF you get different readings, PM me and we can discuss this in greater detail.