Posted By: watersparkfalls
conductor fill and non linear - 10/12/10 07:19 AM
I am at a quandary, do I pull a #10 grounding conductor in my 1/2" EMT and exceed my wire fill by .0084 in2 or use my raceway as the grounding means?
Here is the scenario I have roughed in a raceway to my utility room with 1/2" EMT to feed a 240V 30A dryer(4 qty #10's w/grounding)120V 20A washer(2 qty #12's ) and a 120V 15A (2 qty #14"s) gen. use ckt. However after doing calculations per table 5 and table 1 I see that I have exceeded conduit fill by .0084
Calc thhn Tbl 5:
TBL 1: 40% .122 in2
My code question:
Is it code compliant to use raceway as grounding conductor per 250.118 from a jbox which is bonded with a #10(romex) grounding conductor to my dryer/washer/gen use jbox? If it is legal should I consider pulling #10 grounding and exceed fill by .0084 in2(I am only asking for best advice--not approval to violate code)Also am I forgetting that washer and dry may be considered non linear(are they)? Leaving interpretation weather gen use could also be a non linear load(IE. computer plugged in)which would throw the calcs out because I would have 7 current carrying conductors.
Whew...I know this is a long post, appreciate the input.
Posted By: renosteinke
Re: conductor fill and non linear - 10/12/10 03:06 PM
First, I advise folks to, in general, avoid getting even close to maximum wire fill. Naturally, a lot depends on the specific of the run - but I find it doesn't take much to make for a hard pull - especially if there are LB's in the run.
Now, for your real questions: If you're not required to run a wire at all, can you run a small wire? Is a small wire better than no wire? Is a wire any real improvement over using the pipe as your ground?
Pipe makes an amazingly good conductor- even when the connections are loose or otherwise flawed. Still, it has limits; over a long enough run, even a perfect install will not clear faults. While the NEC does not consider this fact directly, it's pretty glear if you look at the GEMS calculations.
In some areas, some 'professional circles,' the use of the pipe as the ground is simply assumed to be poor, unreliable, and inadequate; wire alone is considered 'good.' As best I can tell, this attitude is not supported by either the NEC, nor the actual test data.
But isn't 'more' also 'better?' Not always - especially when that additional wire starts cluttering up already busy switch boxes.
When would I want, as a design choice, an additional ground (the wire)? For long runs, for complex runs, and for runs where the pipe is (let's be realistic here) going to get damaged.
Otherwise, I'd do without the wire. Pipe fill aside, even an undersize wire still adds to the clutter in every box.
That's my take.
Posted By: watersparkfalls
Re: conductor fill and non linear - 10/12/10 04:49 PM
I have decided to redo the general use circuit and run a new ckt back to the panel mostly because of non linear and a little because of pipe fill.
Is a motor a non linear?
It is code compliant to under size a grounding conductor? I so can you give code ref?
Posted By: Niko
Re: conductor fill and non linear - 10/12/10 04:54 PM
You can use the raceway as a EGC, but how many times have we seen broken/disconnected couplings, connectors. so i recommend to keep the #10 EGC.
however, how about if you drop the #14 and install one additional #12 and make it a MWBC. I did the quick math on the following:
.0211 x 4 = .0844
.0133 x 3 = .0399
total = .1243
This way you are still over but by only .0023"
Posted By: Tesla
Re: conductor fill and non linear - 10/12/10 06:35 PM
By the time you are getting past 65% NEC maximum branch circuit wire fill you have certainly crossed over into poor economics.
Unless it's no more than a drop to a switch, or some isolated emergency circuit run I just don't ever run 1/2" EMT. The labor cost for 3/4" EMT is virtually the same, and the extra cost of the upsized EMT is a joke.
Moving to 3/4" for all overhead rack work means that you need only one of what used to be two items: one bender, one-size of strapping, uniform bends, one size of couplings and connectors; easier inventory control, etc.
All of this work is typically done up in the air, too.
And then there's the ease of passing your inspections and throwing in an extra wire or two when the pressure is on -- with last minute change orders.
In all my days, my jobs never ran over budget on minor materials. It's the wastage of labor that sinks a project. Tight branch circuit pulls will bury you every time. I once had the displeasure of pulling nine #12 solid THHN into a twenty-five foot run with two-nineties. It took two man-hours just on the pull. Another time I hurt my back -- I could hang by my weight and not budge nine-into-one-half EMT. (That particular run involved three-ways with A,B switching. All over that job the EMT was undersized because the original foreman failed to understand the explosion of travelers required in three-way A,B switching.)
Your loads sound like residential...
Which would make me think that Romex is the way to go. Are you in Chicago?
Posted By: renosteinke
Re: conductor fill and non linear - 10/12/10 07:33 PM
An undersized ground in addition to the code-compliant pipe as a ground?
The NEC doesn't directly address this; while you can probably make the case to allow it, chances are that any AHJ would not allow it. Here's why:
When you're doing parallel runs, the ground wire is required to be able to handle ALL of the fault current - except that the NEC modifies this by stating that the ground need not ever be larger than the power conductors in the pipe. If we apply that standard to your situation, I can see a requirement for the ground to be able to handle ALL of the fault current. Maybe, just maybe, the AHJ would allow you to size the ground by the 90 degree column - but that would be his allowing you to vary from the code mandate that smaller circuits use full-size grounds.
Tesla has a point: the NECA "trade practice" standard for installing EMT advises that you avoid the use of 1/2" in industrial settings.
Motors, of themselves, are linear loads. There is only a 'non-linear' issue when there is a frequency drive AND three-phase power. Other non-linear loads can be electronic ballasts, power supplies, and induction equipment. In any event, you deal with these issues by having a larger neutral- the ground wire isn't involved.