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Old 03-12-2010, 11:56 AM   #1
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correcting existing subpanel in detached garage


Hey folks, I have an existing subpanel in my detached garage that does not seem to be wired correctly and I want to make it safe.
There is a 40 amp 240v breaker in my main panel. There is 8/2 with ground going into buried 1/2 inch metal conduit about 60 feet to the garage. (so no neutral!) The 8/2 goes into the subpanel through a 40 amp breaker. There are 3 120 circuits and 1 240 circuit. There are no ground rods for the garage. Neutral and ground are bonded in the subpanel. There is also a water pipe between the two buildings. The subpanel is not grounded to the water pipe.

I use the 120 circuits all of the time. Occasionaly, my compressor on its own circuit trips the 15 amp circuit breaker.

I would like to use 240 for my welder. (The welder manual says to use a 25a breaker).

I would really like to keep at least 40 amps out there, more would be better, but burying new conduit would be very very difficult. The conduit would have to go under 6' of concrete and 12' of stone patio.

1. is this safe to use at all right now?
2. would it be safe if I just added ground rods?
3. should the subpanel be grounded to the water pipe in the garage?

I looked at this fill calculator http://www.electrician2.com/calculators/rf_calculator.html
4. the conduit appears to be a straight shot. it looks like it technically would overfill, but would 3 x 8 gauge thwn and the suitable size ground (10?) be able to be stuffed in there? would that be safe?
5. if that would not work, it looks like the calc says that 8-8-10-10 will be OK in 1/2. can the neutral be bumped down to a 10?
6. or if that would not work, can I use 3 x 8 and use the conduit itself as ground?

thanks very much.

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Old 03-12-2010, 12:27 PM   #2
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correcting existing subpanel in detached garage


First question is how did someone get an 8/2 cable into a half inch pipe?
I must assume you will remove the existing cable from the pipe.
If the pipe is electrical conduit and not plumbing pipe and if it is continuous from panel to panel then it could serve as the equipment grounding conductor. Isolation ofneutrals and equipment ground and ground rod(s) will be needed.

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Old 03-12-2010, 12:52 PM   #3
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correcting existing subpanel in detached garage


Conduit fill.....

{sorry, info appears to be incorrect for cable conduit fill}

So what type of wire ?

Detached structure needs 2 ground rods

Todays code calls for 2 hots, neutral & ground
I prefer not to undersize my neutral
so....(3) #8's & (1) #10 need a 3/4" conduit
http://www.onlineconveyorparts.com/enghelp/conduit.htm
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Old 03-12-2010, 01:19 PM   #4
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correcting existing subpanel in detached garage


Quote:
Originally Posted by Scuba_Dave View Post
Conduit fill.....
For 8-2 NMB the diameter for conduit fill is .2942 1/2" is OK
(3) #8 THWN would also meet code

For 8-2 UF (required for burial) its .3610 you would need 3/4"

https://edeskv2.belden.com/conduitcalculator/

So what type of wire ?

Detached structure needs 2 ground rods

Todays code calls for 2 hots, neutral & ground
I prefer not to undersize my neutral
so....(3) #8's & (1) #10 need a 3/4" conduit
http://www.onlineconveyorparts.com/enghelp/conduit.htm
Dave, please tell me where you got that .2942 number.
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Old 03-12-2010, 02:11 PM   #5
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correcting existing subpanel in detached garage


The conduit is actually metal conduit, not water pipe.

The wire in the conduit right now is nmb. I know it does not belong underground in that conduit. I want to yank it out and as I said, put thwn in there.

Burying new cable or new conduit is not feasible, so I am trying to pull the correct wire through the existing 1/2 inch conduit.

Even though it's not preferable, would doing 8-8-10-10 still be safe?
Am I reading the calculator correctly that that will go in the existing 1/2?
If I had to choose between running 8-8-10-10 or 8-8-8 and using the conduit as ground, which would be better?
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Old 03-12-2010, 02:36 PM   #6
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correcting existing subpanel in detached garage


I would go 8-8-8-10 THWN

1/2" RMC has a .125 sq. in. allowance for a 40% fill with allowance for 2 or more conductors.
#8 THWN has an area of .017 sq inches.
#10 THWN has an area of .011 sq inches

3 #8 equals 0.051 sq inches plus one #10 @ .011 square inches totals .062 square inches which is less than the allowed .125 square inches.
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Old 03-12-2010, 03:00 PM   #7
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correcting existing subpanel in detached garage


Quote:
Originally Posted by Scuba_Dave View Post
Conduit fill.....
For 8-2 NMB the diameter for conduit fill is .2942 1/2" is OK
(3) #8 THWN would also meet code

For 8-2 UF (required for burial) its .3610 you would need 3/4"

https://edeskv2.belden.com/conduitcalculator/

So what type of wire ?

Detached structure needs 2 ground rods

Todays code calls for 2 hots, neutral & ground
I prefer not to undersize my neutral
so....(3) #8's & (1) #10 need a 3/4" conduit
http://www.onlineconveyorparts.com/enghelp/conduit.htm
From your table .2942 is the cross sectional area not the diameter for 8-2 NMB. NM in a conduit is calculated as a single conductor. 53% conduit fill is allowed for a single conductor. 53% fill for 1/2" RMC is 0.166 square inches. 8-2 NM in 1/2" RMC or EMT would not be code compliant.
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Old 03-12-2010, 03:14 PM   #8
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correcting existing subpanel in detached garage


Quote:
Originally Posted by brric View Post
I would go 8-8-8-10 THWN

1/2" RMC has a .125 sq. in. allowance for a 40% fill with allowance for 2 or more conductors.
#8 THWN has an area of .017 sq inches.
#10 THWN has an area of .011 sq inches

3 #8 equals 0.051 sq inches plus one #10 @ .011 square inches totals .062 square inches which is less than the allowed .125 square inches.
Thanks for your help.
But I am confused.
The areas you are giving for the wires are about half of what the areas the calculators are giving.
for example this one http://www.onlineconveyorparts.com/enghelp/conduit.htm says #8 is .0366, and #10 is 0.0211 square inches, which gives a total of 0.1309 square inches, or just over the .125
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Old 03-12-2010, 03:28 PM   #9
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correcting existing subpanel in detached garage


Quote:
Originally Posted by mtk View Post
Thanks for your help.
But I am confused.
The areas you are giving for the wires are about half of what the areas the calculators are giving.
for example this one http://www.onlineconveyorparts.com/enghelp/conduit.htm says #8 is .0366, and #10 is 0.0211 square inches, which gives a total of 0.1309 square inches, or just over the .125
You are correct. I was looking at the uninsulated conductor table.
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Old 03-12-2010, 03:30 PM   #10
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correcting existing subpanel in detached garage


I would still consider this pull even though it is a little over fill limits. I believe it is better to have the additional equipment grounding conductor just in case the integrity of the conduit is compromised.
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Old 03-12-2010, 03:40 PM   #11
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correcting existing subpanel in detached garage


Quote:
Originally Posted by brric View Post
From your table .2942 is the cross sectional area not the diameter for 8-2 NMB. NM in a conduit is calculated as a single conductor. 53% conduit fill is allowed for a single conductor. 53% fill for 1/2" RMC is 0.166 square inches. 8-2 NM in 1/2" RMC or EMT would not be code compliant.
I'm going by the Online Calculator ? NOTE - the calculation I ran is wrong !!

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Old 03-12-2010, 03:46 PM   #12
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correcting existing subpanel in detached garage


Quote:
Originally Posted by Scuba_Dave View Post
I'm going by the Online Calculator ?

That is not the cable diameter it is the cross sectional are in square inches. The diameter of 8-2 NM is larger than .2942.
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Old 03-12-2010, 04:11 PM   #13
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correcting existing subpanel in detached garage


Quote:
Originally Posted by brric View Post
That is not the cable diameter it is the cross sectional are in square inches. The diameter of 8-2 NM is larger than .2942.
So what is the diameter then ?
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Old 03-12-2010, 04:38 PM   #14
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correcting existing subpanel in detached garage


Quote:
Originally Posted by Scuba_Dave View Post
So what is the diameter then ?
Most 8-2 is elliptical not round. If it is round and you assume the area is .2942 sq inches then te diameter would be around .6 of an inch over 1/2".
pi x r x r = .2942

r x r = .09369

square root of .09369 =.3061
r=.3061 r = radius
2r = diameter
d = .621 inches
That is why I find it hard to believe it was installed in 1/2" pipe.

Last edited by brric; 03-12-2010 at 04:40 PM.
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Old 03-12-2010, 04:58 PM   #15
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correcting existing subpanel in detached garage


Yup - Thanks, I found one Mfg that listed 8-2 as .580
I also measured some 8-2 & it is over 1/2" (measured without my glasses on )
That would be a tight squeeze into 1/2"

I missed the fact the info I had was cross section & calculator wants diameter

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