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1 GROUND OR MORE? For multiple circuits

toolbear's picture

1 GROUND OR MORE? For multiple circuits in one conduit


Folks,


Situation:  Setting out to upgrade the garage electrical for workshop.  Meter main is on outside of wall, with space, so that is not an issue.  Plan surface mounted EMT with several circuits in it and 4" J boxes on the walls:


2 - 20A, 12/3 WG, 120v  balanced circuits for the outlets on both sides


1 - 30A, 10/2 WG, 240v compressor/welder/whatever circuit for later (Got no compressor, got no welder - and I am not sure what amps they pull. Might have to increase wire size.


Question is:  with several circuits in one run, can they share a ground wire or should each circuit have it's very own ground wire.


Please! Don't make me read my obso 1996 copy of the code.  I though I spoke and wrote fluent English - until I started reading that.  It only seems to be written in English. 


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #1 of 25)

If you are running EMT, don't use 12/3 wg or any  type NM cable. Around here that would be a major code violation. Type NM cable is rated for heat by boyh the wire insulation and the outer sheating, both for how much it can witjstand, and how much heat it can shed. Enclosing it in conduite changes that rating to a lower value,


If you are running EMT, pull in single conductors, in appropriate gauges for the fuse or breaker size. Pull a seperate ground for each circuit, not each device.


My last shop garage/shop was wired as you described. I used  #12 THHN stranded wire in 1/2 EMT for the lighting runs and the receptical.  If you are not using spec. grade devices, stranded wire is a pain to land on terminals. It is also difficult to  push through anything other than short straight conduite runs. It is easier, however to pull with a fish tape. Using solid wire may be easier for you to both land on devices, and feed thru conduit by youself.


 


Dave

(post #96912, reply #2 of 25)

"If you are running EMT, pull in single conductors, in appropriate gauges for the fuse or breaker size. Pull a seperate ground for each circuit, not each device."

Why a separate EGC for each circuit?

In fact the EMT is qualified to be the EGC, but as been mentioned here many times lots of people thing that running a separate EGC is better practice.

But only ONE should be enough, but sized for the largest circuit. And even if you ran multiple ones you would need to tie all of them together at each j-box. Sounds like making more work for nothing.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #96912, reply #3 of 25)

Dave,


Glad you caught that so I can clarify.  I do tend to think in romex terms, but I was not planning on running the stuff in EMT.  


All seperate conductors.  I am thinking of 3/4 EMT for the main run as I can do ten 10g THHN or 16 of the 12g - which means there is room for later additions.  Drop 1/2 legs  as needed.


I use only commercial spec backwire outlets and put them in J boxes.  No 18CI handiboxes.  I like room.


I am partial to flex after having stripped 6" of insulation off some solid as it ran across a liquidtight 90 fitting.  Is it naughty to use crimp down terminals on these?  Trying to get stranded under a screw is not on. 


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #4 of 25)

"All seperate conductors.  I am thinking of 3/4 EMT for the main run as I can do ten 10g THHN or 16 of the 12g - which means there is room for later additions.  Drop 1/2 legs  as needed."


Remember that if you run multiple conductors in a single conduit you may need to de-rate their current carrying capacity...the code allows up to three current carrying conductors without derating, 4 to 6 conductors you derate to 80%, 7 to 9 conductors derate to 70%, 10 to 20 conductors derate to 50%.  This applies to only the hot conductors.


Edited to clarify: to match how this is described in the Code, "hot conductors" should be changed to "current carrying conductors."


Edited 12/19/2004 7:39 pm ET by Stuart

(post #96912, reply #5 of 25)

>>the code allows up to three current carrying conductors without derating, 4 to 6 conductors you derate to 80%, 7 to 9 conductors derate to 70%, 10 to 20 conductors derate to 50%.  This applies to only the hot conductors.


Why only the hot? It seems to me that since the returns carry the same current and have the seam resistance, they would generate the same heat as the "hots".


Ah wait, I see, since the current is limited by what the derated hot can carry, it effectively derates the returns.


Doh!


Of course, you wouldn't want to say "I gots 2 hots at 50%, so's I kin use a shared return at 100%."


SamT


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #96912, reply #6 of 25)

Using EMT is the way to go.If you're just needing the two general use and one 240v dedicated circuit,3/4" pipe will be OK.If you're going to come anywhere close to 16 #12's you ought to bump it up to a 1" w/ 4 11/16" boxes.A lot of what the NEC says you can do is a serious PITA in real life.

(post #96912, reply #7 of 25)

I want to put in two 20A balanced circuits for the outlets and a 30a/240 for compressor.

My ElectriCalc 96 says the limit in 12g THHN in 3/4 is 16, so I think I have room to spare, or do I.

Can they all share one ground or does each circuit get a personal ground?

The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #8 of 25)

Each circuit does not need an individual ground.You would be packing the pipe with extra ground wires needlessly.For what you have said you need,pull a #10 ground and make joints at each point that you need to split off,remembering to add a ground tail to the box.I run conduit and pull wire every day, and I'm just trying to be helpful in saying that you don't want to pull the maximum number of conductors in a pipe that the NEC says you can.It's a lot of pi$$ing around if you have any number of boxes to pull through,and if you start splicing in them you'll end up with overloaded junction boxes.In laying out homeruns at work the most I ever want to go is 3 sets of three phase( 4 wires each) per 3/4 pipe.It's not hard for the wire count to creep up when you decide to put the garage door openers on their own circuit,and then there's three way switching for the lighting,etc....

(post #96912, reply #12 of 25)

Barry,


BTDT trying to deal with well-stuffed conduit and overstuffed boxes.  I like to oversize so there is room to pull, room to work and no question of a box fill violation. 


So, I can put one 10g ground to share among a pair of 12g balanced circuits and one 10g 240 circuit. 


Tap off the 10g. with the appro. gauge - 12g to the 120 stuff, 10g to the 240 stuff.


Include each 4x4 J box with a pigtail.


Should be fun.


 


 


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #9 of 25)

For all us plain folk, would that be 1 black & 1 white for each circuit, but a single continuous green tapped into at each termination?

(post #96912, reply #10 of 25)

This whole thread is slowly converging on truth. To begin with, the Code is poorly written because it is written by committees who pretend to be lawyers.


For the grounding issue: EMT can serve as a ground but a real ground wire [green] is preferred. It is sized to the largest conductor; in your case it would be #10.


You can run Romex [secret code name = non-metalic cable] in conduit but this is usually only done for protection.


The derating applies to all the current carrying conductors which includes the neutrals. [Exception: balanced three phase circuit]


You can terminate stranded wire with those crimp on terminals. You might want to use a larger conduit for future expansion and leave a pull string.


~Peter


Martha Stewart is solicitating.
Page two: Well, she is solicitating a donation of a wheelbarrow full of gold coins. The wheelbarrow must include a store receipt.

(post #96912, reply #13 of 25)

Please tell me more about the derating and how to avoid it. 


I will have two sets of 12g R,B, W for the balanced circuits.  One set of 10g R,B for the 240 and one 10g G for the common ground for all circuits. 


I'll run the numbers again, but I think 3/4 EMT should handle that with room to spare.  Hope so, since my biggest bender is 3/4.


Yes!  I should add a pull cord for Later. 


 


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #15 of 25)

Spent the Sat. night and all day Sun. at the hospital with my Dad.


Looks like you got all the suggestions and answers you needed in this thread.


One other caution I'll add from experience.


Attach all conduit runs and boxes as if you are going to hang stuff  on it....cause you will......eventually!


 


 


Dave

(post #96912, reply #17 of 25)

Dave,


You're right.  That conduit is great for hanging stuff. 


I secure it with straps and deck screws to the studs.  You can chin yourself on it - at least once.  Twice...we'll see.


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #20 of 25)

Yeah, I was helping wire a TV studio where I worked, and I demanded that the conduit be attached well enough to support my weight, given that, while up on a ladder playing with lights, I might suddenly "desire" an additional handhold.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #96912, reply #19 of 25)

"Please tell me more about the derating and how to avoid it. "

First the requirement is based on "net" current carrying conductors.

So a 240/120 circuit (2 hots and neutral) only count as 2 current carrying conductors. Because, at max load you either have full current on the 2 hots or one hot and a neutral.

Now 2 "separate" 120v circuits would be 4 current carrying conductors. But if you run it as a multi wire circuit with a shared neutral then it is only 2 current carrying conductors. But in this case it does not matter as you will still be in the 4-6 count.

But many people do not understand the actual current limits in the wiring charts.

Using the 75 degree column and 90 degree column the acutal ampacity rating of wires are

#14 20 25
#12 25 30
#10 35 40
#8. 50 55

Now if you use THHN/THHW (which is the common individual wire) then it ratd for upto 90 as is NM-B. But few terminals are rated for 90, most are rated for 75.

But at 75 rates about you derate to 80% you have

#14 16 amps
#12 20 amps
#10 28 amps
#8 40 amps

Which for the #14, 12, and #8 they are large enough for the nominal circuit ratings of 15, 20, and 40 amps.

Now for the #10 I would stretch go ahead and use it for things like wood working equipment because of the low duty cycle. Also I know that I would have limited loads on the 120 at the sametime. This is ok for a home workshop, but not for a commerial establishment.

In fact whenever I redo my shop I am going to run dual 20 amp 120 and a 20 240 circuit in a loop around the shop.

But for air compressor and dust collector I am going to run separeate lines.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #96912, reply #23 of 25)

Thanks, that helps make sense of 316. 


Looks like for what I plan I will still have a working pair of 20a balanced circuits. 


The 240 will get it's own conduit.


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #21 of 25)

"Please tell me more about the derating and how to avoid it."


The Code is a confabulation of experience, distorted theory and backroom politics. Derating is a perverted attempt to keep wires from overheating, melting the insulation off and causing ensuing problems.


Derating is based on the theory that each current carrying conductor releases heat in proportion to its resistance and the amount of amps it is conducting. The relevanent sections are Table 310-15(b)(2)(a) and good old 310-16.


Here is a summary of 310-15:


Number of conductors   % darating
4-6                    80%
7-9                    70%
10-20                  50%


Table 310-16 is what separates the professionals from the hacks. There are three columns: 60º C, 75Cº and 90º Centigrade. You never actually use the 90º column except for derating. Furthermore, according to 114c, you must use the 60º column for every thing less than 100 amps [exceptions exist but not here]. Here is a sumary of 316-16:


AWG  60º  75º  90º
14*  20   20   25 amps
12*  25   25   30
10*  30   35   40
 8   40   50   55
*240-3 says these sizes are to be protected at 15, 20 and 30 amps repectively. [This is for copper]


At the bottom of the table are more derating tables for ambient temperature. Except for an attic in Phoenix, Arizona, these are a non-issue. 


Assming that you are using THHN in your conduit, it is rated for 90º.  "two sets of 12g R,B, W for the balanced circuits.  One set of 10g R,B for the 240 " I get a total of SIX current carrying conductors.* This qualifies for 80% darating. Since this is THHN, you start with the 90º column. Your #12 could carry 30 amps X .80 = 24 amps and your #10 could carry 40 amps X .80 = 32 amps. But 114c says you must use the 60º column because the terminals of the breakers under 100 amps are only rated for 60º. So now your down to 25 and 30 amps -- or to be precise, 24 and 30 amps. But 240-3 limits these to 20 amps and 30 amps so this whole exercise is a lesson in futility.
*310-15(4)(a).


Hope this totally clarifies things and I didn't make any mistakes.


~Peter

(post #96912, reply #11 of 25)

Those portions of the circuits run in conduit can operate with a single ground sized for the largest required ground. Watch the derates both for fill and ambient temperature.

In other words if the largest required ground is a #8 all other circuits can be tapped into this ground conductor. The taps should be sizes for the circuit feeding the outlet. In your case matching the wire size of the hot feeding the outlet. IE: A receptacle fed with a #12 conductor has a #12 ground connected to the #8, as per the example above only.

Those circuits or portions of circuits run in cable, Romex, have to each have a separate ground. The requirement is that all circuit conductors, including the ground, must be run together. Either within the cable or within the conduit.

This has to do with the effects of impedance on circuit balancing, the timely functioning of breakers and how magnetic forces cancel each other out, or fail to, within circuits and surrounding materials.

Circuits run in cables, multiconductor assemblies, Romex and sleeved in EMT for the purposes of localized mechanical protection also need separate grounds as there is no practical way to do it otherwise as splices have to be in an accessible and properly sized junction box or condulet. Here again you have to watch the pipe fill and derates.

Note that the conductors within the cable, once removed from the jacket come under the rules for separate conductors, under their own ratings, within conduits. Sometimes this can be turned to your advantage but you have to know what the rules are to steer through the rocks.

(post #96912, reply #14 of 25)

Why not just run two or more sets of conduit? I suspect you'd be much happier with having more room to work in both the EMT and the boxes. And since this is a garge, won't those 120 circuits require a GFI receptacle in the first box for each 120 branch? Plus, you have the option of changing circuits down the road. If you pull 11 wires through a 3/4" EMT, I'd bet you are neither removing nor adding to that conduit in the future w/o removing all the wires. With the 120 and 240 circuits in separate runs, you have the option of changing the wiring in the future without interfering with other wires in the EMT. For me, the negligible higher cost of multiple conduit runs far outweighs the hassles of making connections and folding wires in tight spaces.

(post #96912, reply #16 of 25)

I can certainly see the advantages of a double run.  I hate working in stuffed spaces.


I use 4" J boxes for wiggle room.


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #18 of 25)

The Lincon AC/DC stick welder at HD, $375, has a three conductor 50A plug on it. 


 


Big Macs - 99 cents
Big Macs - 99 cents

(post #96912, reply #22 of 25)

Thanks for the data.


I was wondering about what those draw.  Better wire it like a stove.  Means it will have it's very own conduit.


What does a tower compressor draw?


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert

(post #96912, reply #24 of 25)

The 80 gallon 175 PSI jobs come in under thirty amps @ 240 V.  The do things right part of me thinks #8 or #6 to keep the volatge drop low.  I heard that makes motors last longer.


Big Macs - 99 cents
Big Macs - 99 cents

(post #96912, reply #25 of 25)

Overwiring a size does help reduce voltage drop - depending on distance, load.  When in doubt, bump it up.  I think that there is a rule of thumb that after 75' of run, jump up one size.


I use 10g cords for my saws.  HD was offering 100' of 10g with 3 outlet head for $62.  Bought one, cut in half, put a 20a GFCI in RT box on the other half.  50' of 10g then 50' of direct connect 12g cord to the saw. 


 


The ToolBear


"Never met a man who couldn't teach me something." Anon.

The ToolBear

"You can't save the Earth unless you are willing to make other people sacrifice." Dogbert