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Old 07-14-2009, 11:57 PM   #1
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Should use 60amp or 100amp sub-panel?


I have a couple questions, so please bear w/ me!

I'm helping a friend run a sub-panel to a two sided detached garage in his new home...I have some electrical background so of course I have to help him! The garages are about 110-120' away from the main panel. One side of the garage he'll work on his cars and will need power for an air compressor, basic hand tools, lights, and receptacles. The other side he'll be turning into a game room/ bar...so power for refrigerator, lights, fans, heat/window AC, and receptacles.

I think a 60 amp feed will be fine, but he's thinking 100 amp for future additions (he doesn't have much room to add anything). Plus when we went to Depot to price things, all the 60amp sub's only had 2-4 slots. I wasn't sure if I could run 60a to a 100a sub...but after reading some posts I'm pretty confident. First question: do you think 60a is enough for what I described? If so, how exactly do I back feed a panel? Is it necessary?

The main problem I'm running into is getting the feeder from the main out of the house. It is a very old house and I only have a couple options...I think. The first is to drill a hole near the panel through the stone wall (about 2-3' thick) and directly under a 4ft sidewalk (right next to the house), which would start the trench to the garage. This looks like a huge PITA, and I really dont want to have to pipe this!!

My second option is to run either 6/3 UF or 2/2/2/4 SE (depending on 60a or 100a) through the basement to the back of the house (where there is a big concrete patio w/ a roof). My thoughts then are to run the exposed wire up the outside of the house up to the underside of the patio roof and to a post at end of roof where I'd spot a weatherproof box. From the box, I'd run PVC down the post into a trench and to the garage (about 50' from this point). This way I only have to drill through wood and siding instead of stone, and I don't have to dig (or wrestle pipe) under a sidewalk.

My other questions are: are these size wires sufficient considering voltage drop, what size PVC should I use for the wire, can I run the 6/3 or SE cable in PVC pipe, if not... is it legal to splice/bug the 6/3 or SE in the box to stranded single conductors?

Sorry for the length of this post and I'd appreciate any advice.

Thanks!!
Phil

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Old 07-15-2009, 12:39 PM   #2
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Should use 60amp or 100amp sub-panel?


I bought a 100a main breaker panel & fed it with 60a for my pool cabana

You would need to add up the power draw of everything that will be on at once to determine if 60a will suffice
Keep in mind that that is 60a 240v of power

You do need conduit to protect the wire any where it may be subject to physical damage

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Old 07-15-2009, 04:58 PM   #3
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Should use 60amp or 100amp sub-panel?


For your first question, if he is considering future upgrades at all, I would definitely install the 100a panel. Considering what all you say he wants to put in there, I'd actually go ahead and go for the 100a anyhow. I don't recommend running unprotected wire outdoors, but if you decide to go that route, make sure the wire you buy is rated for direct sunlight. Also it should be at least 10 feet above ground level unless it's going over a driveway or fence. I'm not sure about the driveway, but I think it's 18 feet. For a fence, it should be 10 feet above the top of the fence.

It's been a while since I've done voltage drop calculations, but I believe 6 gauge wire is big enough to run 100a on, not sure about the conduit size though.

Remember that, for a sub panel, you need to run 4 wire cable. That's two hot lines, a grounded(or neutral) wire, and a grounding wire. Inside your sub panel, the grounding and neutral buses cannot be bonded like they can inside a main panel. The reason for this is because electricity always tries to find the easiest path to earth ground. If your neutral and grounding buses are bonded, then your grounding wire can become that easiest path and thus electrifying anything that wire is attached to.
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Old 07-15-2009, 05:24 PM   #4
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Should use 60amp or 100amp sub-panel?


I would consider putting a separate service at the garage.
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Old 07-15-2009, 06:14 PM   #5
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Should use 60amp or 100amp sub-panel?


My sub was 60a & #6 is rated at 55a - allowed to upsize to a 60a breaker
So you will need bigger wire for a 100a panel
It is better to run a bigger panel/wire, rather then pay to upgrade later
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Old 07-15-2009, 07:14 PM   #6
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Should use 60amp or 100amp sub-panel?


You're right, I didn't double-check my math. #6 is too small for a 100a sub panel, however, you said it is allowed to use a 60a breaker on a wire rated for 55a? I thought you could only oversize your wire like that was when it was on a motor circuit. If you used type THHN wire, then the #6 size is rated for 75a and everything would be kosher.

Getting back to the OP, according to table 310.16 in the NEC, you would need a #3 THHN wire for a 100a service. That's unless Scuba Dave is right and you can go down a size, that's not something I know about though.
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Old 07-16-2009, 02:05 AM   #7
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Should use 60amp or 100amp sub-panel?


Quote:
Originally Posted by Atroxx View Post
You're right, I didn't double-check my math. #6 is too small for a 100a sub panel, however, you said it is allowed to use a 60a breaker on a wire rated for 55a? I thought you could only oversize your wire like that was when it was on a motor circuit. If you used type THHN wire, then the #6 size is rated for 75a and everything would be kosher.

Getting back to the OP, according to table 310.16 in the NEC, you would need a #3 THHN wire for a 100a service. That's unless Scuba Dave is right and you can go down a size, that's not something I know about though.
There are few items if you are on 2008 NEC code it will really change your plans a bit if not checked in ahead of the time.

I do understand that you will read the chart for 60C and 75C those two are most common rating for the conductors however it will affect depending on which type of conductors you will be using and do not use the 90C rating chart that is only used for derating.

I know it may confuse some peoples when you look at the conductor size and tempture rating.

I will give you quick facts here

The following items are on 60C
Romex ( becarefull with this there are few ratings on this one )
UF cable
SE(R) cable { only for Feeder useage like from Main breaker box to subpanel }

The following items are on 75C

THHN/THWN conductors
SE cable from meter socket to MAIN breaker only
Romex { heed this part carefull the older verison are only rated for 60C and new one are rated at 90C but restricted to 75C rating }


The following items are on 90C
few conductors as listed on 75C but only used for derating so just basically not used here at all { it have very limited use for this rating or higher }

Merci,Marc
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Old 07-16-2009, 08:14 AM   #8
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Should use 60amp or 100amp sub-panel?


Quote:
Originally Posted by Atroxx View Post
Getting back to the OP, according to table 310.16 in the NEC, you would need a #3 THHN wire for a 100a service. That's unless Scuba Dave is right and you can go down a size, that's not something I know about though.
No - not downsizing really that I am talking about
The #6 wire is rated for 55a, since there isn't as 55a breaker you are allowed to install a 60a

If the wire was rated 50a you would need to install a 50a breaker
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Old 07-16-2009, 11:08 AM   #9
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Should use 60amp or 100amp sub-panel?


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Originally Posted by Scuba_Dave View Post
No - not downsizing really that I am talking about
The #6 wire is rated for 55a, since there isn't as 55a breaker you are allowed to install a 60a

If the wire was rated 50a you would need to install a 50a breaker
Yeah, I got that. I knew 55a wasn't a standard size breaker, but I thought your wire had to be rated higher than your OC protection unless it was on a circuit dedicated to a motor and nothing else. You seem to know more about what you're doing than I do, so I'm not trying to argue, I'm just a little confused here.
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Old 07-16-2009, 11:22 AM   #10
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Should use 60amp or 100amp sub-panel?


Quote:
Originally Posted by Atroxx View Post
Yeah, I got that. I knew 55a wasn't a standard size breaker, but I thought your wire had to be rated higher than your OC protection unless it was on a circuit dedicated to a motor and nothing else. You seem to know more about what you're doing than I do, so I'm not trying to argue, I'm just a little confused here.
You are not supposed to use a breaker rated for more than the wire. If you do so you can overheat the wire causing a fire hazard. Upgrade your wire.
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Old 07-16-2009, 11:28 AM   #11
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Should use 60amp or 100amp sub-panel?


I've been trying to figure out the exact NEC code that allows this

Under 230.90 it states that an OC must have a "setting or rating not higher then the allowable ampacity of the conductor"

I'm not sure if this is the code that allows this - but all I have found so far & others seem to agree that a 60a on #6 is allowed. Some codes I know (familiar with), others I have to look up, other info is based on feedback from multiple Inspectors, electricians etc

If anyone has further info on the exact code I'd like to know
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Old 07-16-2009, 11:43 AM   #12
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Should use 60amp or 100amp sub-panel?


Section 240.4 states the same thing
Section (B) refers to the use of the next higher standard OC device (800a or less)
There are conditions to be met - not a multioutlet branch circuit, not greater then 800a
I do know that this is not allowed on 15a & 20a house circuits

So I think I found the correct code reference
And no - my wires do not need to be upgraded from #6 on a 60a breaker
All of my work is inspected - & my inspector is very thorough

Last edited by Scuba_Dave; 07-16-2009 at 05:17 PM. Reason: multioutlet not MWBC
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Old 07-16-2009, 11:49 AM   #13
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Should use 60amp or 100amp sub-panel?


Dave,
Don't forget fire codes come into play as well as nec and irc codes. Unfortunately some codes also contradict each other and it is then often left up to the building official to decide. I am by no means an electrical engineer but what I have found to be enforced through experience is the wire must be sufficient for the load. I also want to point out that the building code is the bare minimum safe requirement. It is always better to build above code then simply meet it, especially when electricity is invloved.
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Old 07-16-2009, 11:50 AM   #14
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Should use 60amp or 100amp sub-panel?


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Originally Posted by Scuba_Dave View Post
I've been trying to figure out the exact NEC code that allows this

240.4(B) is what you need.

Also, #6 is rated at 65 A, it is only NM-B or UF that's limited to 55 A.
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Old 07-16-2009, 12:31 PM   #15
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Should use 60amp or 100amp sub-panel?


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Dave,
Don't forget fire codes come into play as well as nec and irc codes. Unfortunately some codes also contradict each other and it is then often left up to the building official to decide. I am by no means an electrical engineer but what I have found to be enforced through experience is the wire must be sufficient for the load. I also want to point out that the building code is the bare minimum safe requirement. It is always better to build above code then simply meet it, especially when electricity is invloved.
I ran a 60a 240v sub panel for a 10a 240v pool pump & 2 lights
If that isn't above code I don't know what is
Not one Inspector, but several Inspectors AND electricians all stated that the wire & setup met code. I actually upsized the ground wire from what was required

I also know I have enough extra power out there to run any hand tools I need. Maybe even insulate & install an AC/heat if I end up in the dog house w/the wife

Thanks for the confirmation on the code reference Inphase
I do like to know the code reference when possible
I was actually going by the 60 degree rating, but I guess since it was THWN I should have gone by the 75 degree rating which is 65a
I guess I could put a 70a breaker in if ever needed
But I don't really need any more power out there

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