# How many amps can 6 THW Copper handle



## dw1256 (Apr 7, 2010)

Hi,

I have wire that feeds into my garage load center from the house. The wire jacket says "6 THW 600 volt AMMCO". I am wondering how many amps this can handle. 

However, the wire enters into the house into a service box. For whatever reason, the wire that it is connected to that runs back to the main panel is 10 guage aluminum wire. It is my understanding that the 10 guage aluminum can only handle up to 25 amps. Currently there is a 50 amp double pole breaker in the main panel.

So basically what I am wondering is if I replace the 10 guage aluminum wire back to the main panel with more 6 THW, what is the maximum size of the breaker I can use in the main panel?

Also, if I bought new wire would it still be called THW, or is that something out of date?

Hope that made sense.

Thanks in advance.


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## Code05 (May 24, 2009)

#6 copper THW is good for 65 amps. But your cable may be only 55.

You can use Romex and keep the 50 amp breaker.

It may possible to increase to a 60. Do you need 60?


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## dw1256 (Apr 7, 2010)

I think I can get away with 50. I really just want to be able to take full advantage of the wire that is already going out to the garage. However, I know I can't do that with the aluminum wire that is there right now.

What I really want to do is replace the box in the garage that only supports 4 breakers. If I am only running 50 amps to the garage, is there a limitation on the number of circuits I can run? Someone told me that if I had 3 20 amps circuits it would be too much. I don't plan to run much load on any of the circuits, but in the event of tripping a breaker I don't want too much going out, if that makes sense.


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## Code05 (May 24, 2009)

dw1256 said:


> I think I can get away with 50. I really just want to be able to take full advantage of the wire that is already going out to the garage. However, I know I can't do that with the aluminum wire that is there right now.
> 
> What I really want to do is replace the box in the garage that only supports 4 breakers. If I am only running 50 amps to the garage, is there a limitation on the number of circuits I can run? Someone told me that if I had 3 20 amps circuits it would be too much. I don't plan to run much load on any of the circuits, but in the event of tripping a breaker I don't want too much going out, if that makes sense.


I assume that this a 4 wire cable.

The person was wrong. 

As long as the circuits are "lightly loaded", you can put in a bigger panel. A 8 space is common on a 50 amp breaker.


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## dw1256 (Apr 7, 2010)

Yes, it is 4 wire. Two hot, neutral, and ground.

My plan was to swap it out for a main lug panel with 8 spots.

I think this is my last question. There are also some other wires that run out from the basement to the garage that power lights on side of the garage and the roof. They do not get any power from the load center, but they do run through the panel as if it were a junction box. Any idea if that is not allowed?

Thanks again. You have been very helpful.


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## brric (Mar 5, 2010)

Is the garage attached or a seperate structure?


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## vsheetz (Sep 28, 2008)

60a to a workshop is common and usually ample power. I have 60a on #6 to a nearly full 12 slot panel in my workshop/garage. There I have a MIG welder, air compressor, lights, table saw, misc. power tools, heater, etc. Also powers my motorhome when parked at home. All works great.


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## Code05 (May 24, 2009)

dw1256 said:


> Yes, it is 4 wire. Two hot, neutral, and ground.
> *
> Good*
> 
> ...


You are welcome.


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## frenchelectrican (Apr 12, 2006)

Code05 said:


> #6 copper THW is good for 65 amps. But your cable may be only 55.
> 
> You can use Romex and keep the 50 amp breaker.
> 
> It may possible to increase to a 60. Do you need 60?


I just don't want to step on your toes but I hope you did not read the wrong tempture chart due the OP mention THW not the THHW IIRC the THW is on 75°C rating not on the 90°C 

Merci,
Marc


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## dw1256 (Apr 7, 2010)

brric said:


> Is the garage attached or a seperate structure?


Separate structure.


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## dw1256 (Apr 7, 2010)

frenchelectrican said:


> I just don't want to step on your toes but I hope you did not read the wrong tempture chart due the OP mention THW not the THHW IIRC the THW is on 75°C rating not on the 90°C
> 
> Merci,
> Marc


What does it mean if it is on the 75 rating?


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## Code05 (May 24, 2009)

frenchelectrican said:


> I just don't want to step on your toes but I hope you did not read the wrong tempture chart due the OP mention THW not the THHW IIRC the THW is on 75°C rating not on the 90°C
> 
> Merci,
> Marc


65 amps is the 75C ampacity for THW. His cable is probably limited to 55 amps at 60C.


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## Code05 (May 24, 2009)

dw1256 said:


> Separate structure.


You need 2 ground rods if they are not there already. Separate grounds and neutrals.


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## Code05 (May 24, 2009)

dw1256 said:


> What does it mean if it is on the 75 rating?



You have a code book, it would be easier to explain.

I will see if I have a link.


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## brric (Mar 5, 2010)

dw1256 said:


> Separate structure.


Panel will require a main if there are more than six breakers.


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## Code05 (May 24, 2009)

brric said:


> Panel will require a main if there are more than six breakers.


Good catch, I thought garage was attached when I said MLO.


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## Code05 (May 24, 2009)

dw1256 said:


> I think this is my last question. There are also some other wires that run out from the basement to the garage that power lights on side of the garage and the roof. They do not get any power from the load center, but they do run through the panel as if it were a junction box. Any idea if that is not allowed?
> 
> Thanks again. You have been very helpful.


If this garage is detached, now you got a problem here. More than 1 supply to a building.


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## dw1256 (Apr 7, 2010)

Code05 said:


> You have a code book, it would be easier to explain.
> 
> I will see if I have a link.


I have access to the 2008 National Electric Code book.


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## Code05 (May 24, 2009)

dw1256 said:


> I have access to the 2008 National Electric Code book.


Go look at table 310.16.


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## dw1256 (Apr 7, 2010)

Code05 said:


> If this garage is detached, now you got a problem here. More than 1 supply to a building.


The reason for this is because there are 3 way switches between the house and garage for certain lights. Is that not allowed?


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## Code05 (May 24, 2009)

dw1256 said:


> The reason for this is because there are 3 way switches between the house and garage for certain lights. Is that not allowed?


Only if the switches are fed from the garage or the wire feeding the lights does not enter the garage.


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## dw1256 (Apr 7, 2010)

Code05 said:


> Only if the switches are fed from the garage or the wire feeding the lights does not enter the garage.


Let's say I rewired them so that they were using power from the garage. They can still be controlled by switches in the house, correct? Also, now that I have to use a panel with a main breaker due to the amount of breakers used, can I still make connections inside of this panel to wire the 3-ways?


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## frenchelectrican (Apr 12, 2006)

dw1256 said:


> Let's say I rewired them so that they were using power from the garage. They can still be controlled by switches in the house, correct? Also, now that I have to use a panel with a main breaker due to the amount of breakers used, can I still make connections inside of this panel to wire the 3-ways?


Oui you can with three way switch however there will be a sticky item may come up is you will have no netural connection at the house side due some case you may want to change to timer but with convental three way switch you are fine but just make sure you do NOT steal house's netural conductor at all if you have electrionic timer.

{ note : my SOP is have seperated switch box or have two gang swtich box with divder in there so you know you can not get it mixed up at all }

Merci,
Marc


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## Code05 (May 24, 2009)

Hmmm. An argument could be made that the wire feed for the lights could be okay in addition to the feeder.

225.30(D) Different Characteristics. Additional feeders or
branch circuits shall be permitted for different voltages,
frequencies, or phases or for different uses, *such as control
of outside lighting from multiple locations.*

225.31 Disconnecting Means. Means shall be provided
for disconnecting all ungrounded conductors that supply or
pass through the building or structure.

225.33 Maximum Number of Disconnects.
(A) General. The disconnecting means for each supply
permitted by 225.30 shall consist of not more than six
switches or six circuit breakers mounted in a single enclosure,
in a group of separate enclosures, or in or on a switch-board.
There shall be no more than six disconnects per
supply grouped in any one location.

225.37 Identification. Where a building or structure has
any combination of feeders, branch circuits, or services
passing through it or supplying it, a permanent plaque or
directory shall be installed at each feeder and branch-circuit
disconnect location denoting all other services, feeders, or
branch circuits supplying that building or structure or passing
through that building or structure and the area served by
each.

225.38 Disconnect Construction. Disconnecting means
shall meet the requirements of 225.38(A) through (D).
Exception: For garages and outbuildings on residential
property, snap switches or 3-way or 4-way snap switches shall
be permitted as the disconnecting means.


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## bob22 (May 28, 2008)

I didn't think a breaker box could be used as a junction box for splicing wires that pass through and are not being protected for overload?


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## Code05 (May 24, 2009)

bob22 said:


> I didn't think a breaker box could be used as a junction box for splicing wires that pass through and are not being protected for overload?



312.8 Switch and Overcurrent Device Enclosures with
Splices, Taps, and Feed-Through Conductors. The wiring
space of enclosures for switches or overcurrent devices
shall be permitted for conductors feeding through, spliced,
or tapping off to other enclosures, switches, or overcurrent
devices where all of the following conditions are met:
(1) The total of all conductors installed at any cross section
of the wiring space does not exceed 40 percent of the
cross-sectional area of that space.
(2) The total area of all conductors, splices, and taps installed
at any cross section of the wiring space does not
exceed 75 percent of the cross-sectional area of that
space.
(3) A warning label is applied to the enclosure that identifies
the closest disconnecting means for any feedthrough
conductors


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