gauge wire needed for subpanel

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  #1  
Old 01-15-14, 05:56 PM
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gauge wire needed for subpanel

I want to add a subpanel in my attached garage, but I'm not sure what gauge wire to run. My main panel is 200A. The subpanel will be about 80-90 feet away. I'm thinking a 100A subpanel should be adequate for some tools, table saw, washer & gas dryer, shop lighting, and a few circuits for outdoor lights. I ran 1 1/4" conduit while the ceilings were open for renovation, so hopefully that will accommodate the wires needed. (If not, what is the largest subpanel that I can install?) And, I know from other posts that the ground and neutral bars should not be tied together in the subpanel.
 
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Old 01-15-14, 06:17 PM
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Welcome to the forums!

I want to add a subpanel in my attached garage, but I'm not sure what gauge wire to run. My main panel is 200A. The subpanel will be about 80-90 feet away. I'm thinking a 100A subpanel should be adequate for some tools, table saw, washer & gas dryer, shop lighting, and a few circuits for outdoor lights.
A 60A subpanel should be able to handle those loads and more. #6AWG copper conductors will handle 60A.

I ran 1 1/4" conduit while the ceilings were open for renovation, so hopefully that will accommodate the wires needed.
It should.

I know from other posts that the ground and neutral bars should not be tied together in the subpanel.
In the subpanel, the neutrals must be isolated from all paths to ground, including the enclosure. The enclosure must be bonded to all branch circuit grounding conductors and the grounding conductor from your main panel. See Sub Panel Diagrams.
 
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Old 01-15-14, 10:04 PM
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Thanks! I'm sure a 60A subpanel and associated wire will be significantly cheaper than a 100A subpanel. I believe the ground can be smaller than the other 3 wires. Would it be #8AWG if the 2 hots and neutral are #6AWG?
 

Last edited by Andy Bourgeois; 01-15-14 at 10:21 PM.
  #4  
Old 01-15-14, 10:11 PM
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From the house to the garage, 60A should be plenty. Have you considered running 2-2-2-4. It's rated at 75A, so you could run it off a 60A service from the main.
 
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Old 01-16-14, 06:28 AM
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Would it be #8AWG if the 2 hots and neutral are #6AWG?
I'd just use a #10 ground. If the conduit is EMT you really don't even need the ground wire, but I prefer pulling the ground wire too.
 
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Old 01-16-14, 08:02 AM
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@ CasualJoe -
After doing a bit more research, your recommendations look good -- according to NEC table 250.122(A) a #10AWG ground protects up to 60A, and #8AWG protects up to 100A.

And for the conductors, NEC table 310.15(b)(6) seems to be the one to reference -- #6AWG can handle up to 65A for types RHW, THHW, THW, THWN, XHHW, USE, ZW.

Were those the correct tables to use?
 
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Last edited by Andy Bourgeois; 01-16-14 at 08:59 AM.
  #7  
Old 01-16-14, 09:41 AM
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A 60A subpanel should be able to handle those loads and more.
Is voltage drop a non-issue? 60A over 200' of #6 copper would be about a 6V drop. Sure, he'll likely never draw full current, but figured I'd ask.
 
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Old 01-16-14, 12:46 PM
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And for the conductors, NEC table 310.15(b)(6) seems to be the one to reference -- #6AWG can handle up to 65A for types RHW, THHW, THW, THWN, XHHW, USE, ZW.
Were those the correct tables to use?
That's the table I would use.

Is voltage drop a non-issue? 60A over 200' of #6 copper would be about a 6V drop. Sure, he'll likely never draw full current, but figured I'd ask.
The OP said the subpanel would be 80 to 90 feet away. I generally don't even worry about voltage drop in residential installations unless the length exceeds 100 feet.
 
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Old 01-16-14, 05:02 PM
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The OP said the subpanel would be 80 to 90 feet away. I generally don't even worry about voltage drop in residential installations unless the length exceeds 100 feet.
Nearly 200 feet round trip is what I meant (180 feet to be exact). Don't you double the one-way length to get the total resistance? Current travels back through the neutral so you have to add its resistance when calculating voltage drop, correct? In the case of #6 copper for conductor and neutral, this would be about 6V of drop.
 
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Old 01-16-14, 05:12 PM
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Nearly 200 feet round trip is what I meant (180 feet to be exact). Don't you double the one-way length to get the total resistance, or does current not return through the other conductor to the main panel?
I have never seen an engineer spec increasing the wire size for a branch circuit or branch feeder under 100 feet in length so I just don't worry about it. If the circuit is over 100 feet, I start thinking about voltage drop and apply the appropriate information into the calculator which only asks for one way circuit length.

electrician2.com voltage drop calculator
 
  #11  
Old 01-16-14, 05:18 PM
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It asks for the one-way circuit length, but the calculator doubles it. In the case of the OP, this would be 6V/5% drop at full current. However, I have no clue what NEC says about the acceptable amount of voltage drop and whether they say to use full load or a percentage of full load, such as 80%. The 6V/5% is the DC drop, so the average AC drop would be less, so it's likely that some lesser percentage is used. I'm sure it will be fine with #6 copper.

Found this on Wikipedia (notice the '2' in the denominator):

In situations where the circuit conductors span large distances, the voltage drop is calculated. If the voltage drop is too great, the circuit conductor must be increased to maintain the current between the points. The calculations for a single-phase circuit and a three-phase circuit differ slightly.

Single-phase voltage drop calculation:

VD = 2 x L x I x R / 1000

VD% = VD / Source voltage

Another reference:

ASK THE EXPERTS: Voltage Drop | Home Power Magazine
 
  #12  
Old 01-16-14, 05:29 PM
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With the loads the OP has listed I would pull three 6AWG THHN and one green 10AWG. I use a different calculator than the one Joe uses: Southwire Voltage Drop Calculator. Both ask for one-way conductor length.

Mine says to use 4AWG copper to keep the drop under 3% if you're feeding a full 60A load. His says the voltage drop with 6AWG copper and a full 60A load will be 2.21%

Go figure.
 
  #13  
Old 01-16-14, 05:33 PM
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Note in the fine print on the bottom of the Southwire page:

The total combined load and length of the circuit must be used
His says the voltage drop with 6AWG copper and a full 60A load will be 2.21%
Check again. I just used the electrician2.com calculator and got 5.9V drop/4.92% using 100 feet as the one-way length. If you work the equations manually, you'll see that you have to double the one-way length to get the same numbers. The current travels down one conductor and back through the neutral then vice versa, once per cycle. Thus, you have to use the total resistance of the circuit (resistance encountered there and resistance encountered on the way back) to get the voltage drop.

Further proof:

I opened a debugger for the electrician2.com calculator and you can see it multiplies by 2:

form.voltdrop.value=round2( 2*materialk*(eval(form.distance.value)*(eval(form.amperes.value))/cmask) *voltsk );
 

Last edited by mossman; 01-16-14 at 05:59 PM.
  #14  
Old 01-17-14, 05:01 PM
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DC (worst case) voltage drop equations for single phase:

VD = 2 x k x I x d / cmil

or

VD = 2 x R x I x d / 1,000

and

VD% = VD / Vsource

Both equations yield the same result

k = resistivity of conductor (Cu = 12.9 ohms)
I = Total current
d = one-way distance ("2 *" doubles distance for round trip)
cmil = (5 x 92[SUP]36-AWG/39[/SUP])[SUP]2[/SUP]
R = resistance of conductor (per 1,000 ft)
 

Last edited by mossman; 01-17-14 at 05:27 PM.
  #15  
Old 01-17-14, 05:12 PM
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I would run # 2 copper for the 2 hots & the neutral with a # 8 earth ground . This will fit in a 1-1/4" conduit . Use individual wires , not a cable .

God bless
Wyr
 
  #16  
Old 01-18-14, 11:15 AM
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I would run # 2 copper for the 2 hots & the neutral with a # 8 earth ground.
See post #6..............................
 
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