I'm installing overhead raceway for aproximately 50 machines in a machine shop.Most will be on 20 amp three pole breakers.I'm going to be running 2" EMT from the panel to the raceways.I'm thinking about running a #6 ground wire from the panel ground bar to the raceway,and installing(bonding) a terminal ground bar inside the raceway.From the ground bar in the raceway I would run a #12 ground wire to each machine saving me time and wire.I'm thinking this is legal

Any and all comments are appreciated.

I should add that I'll be running 1/2" emt from the raceway to each machine.

Legally you could run a single #12 all the way to where you start splitting them out. The problem is the splice in the raceway. If there is adequate space, you can splice in a conduit body.
This is really a gray area since all grounding conductors only count as one.

You are also going to have some serious derating problems with that many conductors in a single raceway so we are not even talking about #12s. All of the wires, including the ground, have to start getting bigger when you go over three 3 phase circuits in a pipe. (9 current carrying THHN)
In the 10-20 wire range you will need #10.

At a certain point more raceways ends up being cheaper.

Bossanove5:

Heed these words...

"You are also going to have some serious derating problems with that many conductors in a single raceway so we are not even talking about #12s. All of the wires, including the ground, have to start getting bigger when you go over three 3 phase circuits in a pipe. (9 current carrying THHN)
In the 10-20 wire range you will need #10.

At a certain point more raceways ends up being cheaper."

I have seen what Greg is saying happen in the field a few times...results were downright ugly, expensive, and one cost a young guy an unwanted 'vacation.'

As to your question on the EGC...yes it is no issue to install a ground block within a wireway/troff/gutter and run a single EGC to the source panel. Common practice is to size the EGC from the panel to the block to the largest OCP of the circuits, although that is not an NEC requirement.

Really?

How is that accessible? It is OK in a condolet tho.



Each of those "unlesses" refers to a removable cover.

gREG:
i FEEL THAT THE op IS REFERING TO WIREWAY, TROFFS, NOT CONDUIT.

OK my fault, I was confused by the 2" EMT thing.

Not as confused as my pittiful typing job above.!!!!!

Let me clear up the confusion:

The first 14 set of machines will have there own 6x6 square raceway.This raceway will be fed by a 2" conduit so the maximum #12 wires including the ground inside the raceway will equal 43 (Counting all the ground wires as one)

The second set of 19 machines will be fed the same way by (58 #12 wires) their own 6"x6" gutter and 2" pipe.

I know section 310.15(b)(a) calls for derating which permits only 10.5 amps for each #12 wire for the amount of wires I'm using.

The machines pull about 15.3 amps each.So I would have to use #8's which is kinda crazy.

Any more thoughts on this?

Thanks

Just to be clear, grounds count as one for box fill but they are all counted in pipe. Again tho, you really only need one.

If you ran 3 raceways with the wires split up you would avoid the derating problem. Even 2 would be better, even if not legal.

RE-thinking the one conduit, maybe??

2" is legal for 36-#8 (fill) but you need 43. 2x2" drop you to 22 conductors each, but still 45% derate leaves you at #8.
3 conduits, 14 circuits (5-5-4)16 conductors, 50% derate, back to #10. 1/2" or 3/4" for each machine, panel to machine...back to #12 unless there is a VD issue for length.

Other options? Bus Duct? There are 'small amperage' bus ducts available. Cost effective??

Like I said above....this can get really ugly, depending on when that 'number of conductors' rears its ugly head!!

Bossanova;

The Project, as described, is going to require several different Derating Methods, as the Wiring Methods will be a combination of Two different Raceways:
EMT and Metallic Wireways (AKA "Gutters").

Suggest reviewing Article 376 prior to any choice of installation.

With the Wireways, there are Two things for consideration, per Section 376.22 - Number of Conductors and Ampacity:

376.22(A): Cross Sectional Fill.
This limits the Wireway to a Maximum 20% Conductor fill.
Equipment Grounding Conductors _ARE_ included.
Although there may be 541 #12 THHN CU. in a 6"x6" Gutter, without exceeding 20% fill, the next part kills that idea instantly...

376.22(B): Adjustment Factors.
Where the Total Current Carrying Conductors does not exceed 30, no Derating is required.
Equipment Grounding Conductors _ARE NOT_ counted.

Having 31 or more CCC's will require Derating Factors, per Table 310.15(B)(2)(a), as follows:

a. 31 to 40 Conductors: Adjustment Factor = 40% (Imax x 0.4),
b. >40 Conductors: Adjustment Factor = 35% (Imax x 0.35).

As you can see, there is a point where either (A) or (B) will influence the Wireways.

Along with the Wireway issues, feeding them with Raceways, such as EMT, has an altogether new Conductor Fill and Derating concept to figure in.
Using Section 310.15(B)(2)(a), exception #3, if the Segment of EMT between the Panelboard and the Wireway, does not exceed 24", the Conductors may be installed without Derating Factors applied, so there is a little give-way in that situation.

Good luck!

--Scott (EE)

The intent of the derating/conduit fill is to limit heat buildup from so many conductors in a race way. I've dealt with lighting panels that had only two 2" conduits coming out of the panel. One had the feeders and the other had all of the circuits in the panel. The feeder conduit was cool to the touch while the other conduit dang nearly burnt my hand

You should seriously look at cable tray.

With it you can massively cut down on wire make-up.

http://www.legrand.us/cablofil.aspx#.UOCs4vnQTso

It's also going to be quicker and easier.

Expect that your loads will actually need at least #10 THWN-2 conductors -- just because of voltage drop at start-up.

Distant machines may warrant up-sizing to #8.

Cable tray also makes for very smooth changes and additions.

Thanks everyone for your valued input.I have some re-engineering to do.