Number Of Wire In Conduit

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  #1  
Old 10-27-04, 11:41 AM
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Number Of Wire In Conduit

Running 8 gauge (two hots plus ground, three wires) and 10 gauge (two hots plus groundthree wires) 220 v. Total of six wires.

Can I run all six wire thru pvc 40 ""3/4"" electical conduit?

I have the chart for number of same size wires in a conduit but no info on mixing?? Anyone know of such a chart or how to compute.

And if understand "deratig " correctly, I have to take into account the load must be 20% less, correct??


One othe question, all this is for my shop which has it's own meter. When must I use GFCI recepticles.

Wire are THHN I believe.

Thanks
John1
 
  #2  
Old 10-27-04, 11:52 AM
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The general rule is that you are allowed to fill to '40%', with a bunch of details in how the calculation is done.

All of the tables for how many wires you can use are based upon this.

For your application, you can share the ground for both circuits, just use the #8 ground for both. This gives you 5 wires, and since 5 #8 wires are allowed, a mix of 3 #8 wires and 2 #10 wires is clearly also allowed.

Next thing to check: derating. With 4 conductors in a conduit, you have to reduce the current rating. What are you carrying on the #8?

-Jon
 
  #3  
Old 10-27-04, 05:26 PM
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Winnie:
Thanks,

The #8 is an air compressor 23 amps max.

Since 220v require separate circuits, can I have both share the #10 ground as you said??

Just want to make sure I understand.
John1
 
  #4  
Old 10-27-04, 09:17 PM
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Depending upon local codes, all you need is one ground in the conduit going back to the panel... however, the ground should be sized accordingly so that it provides adequate protection for the largest wire. A #8 wire usually can be derated to a #10 ground, but I would recommend keeping it the same size as your other wires. Derating doesn't become much of an issue or money saver untill you get into larger wires. Anyways, one ground in the pipe is legal, providing it is large enough for the largest current carrying conductor AND, all the wires are going back to the same panel. If you have a subpanel of some sort and some wires are going to the main panel and some are going to the subpanel, each panel must have its on ground with the circuits that go to each designated panel. Oh yeah, this would be in the case you have a junction box (I don't see how you wouldn't) that will split both circuits in each different direction. Than just run a ground to each piece of equipment and tie them together in the junction box to the one ground going back to the panel.

Basically, you are far from using up all of the conduit. One ground will be fine if your situation agrees with what i said before. Also, you should not just worry about the heat the wires will generate in the conduit, but also take into account the distance of the run you have. If you are over 100', post back, as you may need to take voltage drop into account.

Best of Luck,
Paul
 
  #5  
Old 10-28-04, 08:18 AM
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John1,

I didn't say that you could use a #10 ground for both; I said that you could use the #8 ground for both.

morpheusoptic is correct that circuits which commonly require #8 for the current carrying conductors can use #10 for the ground conductor. However this is not the case in your application.

When you use an oversized wire for a circuit, you have to oversize the ground conductor by the same amount. Since you could put 23A on #10 wires with a #10 ground, the fact that you are putting the 23A on #8 wires means that you need to use a #8 ground.

If you use 2 #8 hot conductors for one of your 220V circuits, and 2#10 hot conductors for the other, and a single #8 ground conductor for both, then you will be fine.

Just as a side note: when the ground (or neutral) is reduced in size relative to the hot (or phase) conductors, this is _not_ termed 'derating'. Derating is reserved for situations in which a conductor's current handling capacity must be reduced to compensate for ambient conditions.

-Jon
 
  #6  
Old 10-28-04, 06:05 PM
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SO IF i KEEP IT TO 2#8 HOT THEN I HAVE TO USE A #8 GROUND.
I THOUGHT I COULD USE A #10 FOR 2 #8'S??

jOHN1
 
  #7  
Old 10-28-04, 06:44 PM
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Seems to me like a #10 EGC woud be okay.
 
  #8  
Old 10-28-04, 08:21 PM
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John Nelson: See 250.122(B), which is new or changed in the 2002 NEC.

John1:

I was wrong in the instructions that I gave you. You need to read NEC article 250.122 and look at table 250.122. Basically if you have a circuit protected by a 40A breaker, you can _in general_ use a #10 conductor for the ground. In fact, #10 is good for use on circuits protected up to 60A. So the real question for you is 'What size breaker is being used to protect the circuit made with the #8 conductors?' This breaker has _two_ functions, protecting the #8 conductors, and also protecting the compressor. The nameplate or the instructions for the compressor will tell you a recommended breaker size, and you should carefully follow this recommendation.

Now, there is an exception to the rule about wire sizes for the grounds. If, for any reason, you increase the size of the circuit conductors, then you are required to increase the size of the ground conductor. Say, for example, that you have a 30A circuit. _Normally_ this would require #10 wire for the conductors, and a #10 ground wire. But if you increase the size of the conductors to #8, say for reasons of voltage drop, then you are required to similarly increase the size of the ground wire to a #8.

So if these #8 wires are for a 30A circuit, then your ground needs to be a #8, and if they are for a 40A circuit, then the ground is only required to be a #10. This does seem counter-intuitive (why in hell should the higher amperage circuit get away with a smaller ground) until you consider _why_ conductors get increased in size: voltage drop and derating. If the circuit is so long that you need to make the wires larger, then you should make the ground wires larger in proportion. Imagine a 20A circuit that is _very_ long, so that you need to use #6 conductors for reasons of voltage drop. In order to make certain that enough fault current will flow to trip a breaker, the ground would also need to be a #6.

-Jon
 
  #9  
Old 10-30-04, 06:17 PM
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So is it #10 or #8 ground??

Thanks
John1
 
  #10  
Old 10-30-04, 06:27 PM
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So, is it a 30A circuit or a 40A circuit? The answer to your question depends upon the rating of the #8 circuit.

-Jon
 
  #11  
Old 10-31-04, 04:32 AM
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?

30 amp circuit
 
  #12  
Old 10-31-04, 04:44 AM
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Then it is a #8 ground.

-Jon
 
  #13  
Old 10-31-04, 01:00 PM
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Just curious John, but why do you have a 30-amp circuit wired with 8-gauge wire?
 
  #14  
Old 10-31-04, 05:25 PM
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My 30 amp water heater has THHN #8s in the conduit because Lowe's had a remnant of #8 cheaper than #10 would cost.

I find it interesting that #10 is now approved (table 250.122) as grounding for any circuit up to 60 amps, unless it conflicts with 250.4(a)(5) or 250.4(b)(4)
 
  #15  
Old 10-31-04, 05:43 PM
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Originally Posted by SteveBausch
I find it interesting that #10 is now approved (table 250.122) as grounding for any circuit up to 60 amps
Now approved? Has been for as long as I've had a code book. At least back to '87 when 250.122 wasn't even the table.


I agree with Mr. Nelson. 30 amps on #8cu wants a #10cu ground.
Even if you upsize a 30 amp circuit to #8 the ground called for is still #10.
 
  #16  
Old 10-31-04, 07:25 PM
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250.122 Size of Equipment Grounding Conductors
[...]
(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.
[...]
By table 250.122 a circuit with a 30A overcurrent device requires a #10 copper or #8 aluminium equipment grounding conductor.

By section 240.4(D) a #10 conductor can be protected by no larger than a 30A overcurrent device. #10 is the conductor size commonly used for 30A circuits.

Since a #8 is being used to carry what could be carried on a #10, I stand by my assertion that the NEC requires #8 ground as well.

-Jon
 
  #17  
Old 10-31-04, 09:40 PM
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I agree with Jon that a strict interpretation of the NEC will require #8. However, I also agree that #10 will be safe in this case, and the #8 is only required to meet the letter of the law. However, I would certainly bend this tiny bit to meet the code and install the #8. Why violate code when you can just as easily follow it?
 
  #18  
Old 11-01-04, 10:48 AM
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Ok, Just Finished Pulling 3 #8' For
Two Hots And A Ground.

Also In The Same Conduit I Have A 220v 20 Amp Receptacl With Two Hot #10 And Using The Same Ground Conductor As Above.

3/4" Conduit, Winnie Pls Don't Tell Me This Is More Than 40%

John Nelson, Do You See Any Problems??

John1

Ps, Disregard Any Of My Other Post.

Winnie, Thanks For Your Help But I Think You Made It More Confusing Than It Is!

Remember We Come Here For Help!!
 
  #19  
Old 11-01-04, 12:22 PM
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I don't see any problems, but I didn't do the conduit fill calculation. Is that what you're asking for confirmation on?
 
  #20  
Old 11-01-04, 02:21 PM
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Assuming 3/4" sch 40 and THHN insulation:

You are good for 5 #8s.

So your three #8s and 2 # 10s are compliant.
 
 

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