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Old 10-30-2012, 09:58 AM   #1
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Sub-Panel wire gauge


Hi All,

I've consider myself to have a solid electrical foundation but after analyzing the electrical setup that I've got in my detached garage which I want to run a heater, I wanted to get some more feedback on my setup.

Here are the facts

Main Panel: 200A w/200 service
Garage: 60A sub panel
Sub panel feed: approximately 50' of 8/3 wire running through 1" conduit.

Main breaker feeding sub panel are 2 40A single space breakers riveted together. As this is a single phase 240 output, I take it this is improperly rated at 80A? As the only 40A square d breakers I've seen are double spaced, I'm not sure if you add the two 40A single space together for the amperage across a single phase?

So, if the 2x40A single space breakers are actually working as 40A, then the 40A rating makes sense for the 8/3 wire. If they are working as 80A, from what I've researched the wire is only good for roughly 45A and I would replace the breaker in the main box accordingly.

I've got some more questions but for now I would like to address this first!

Thanks!

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Old 10-30-2012, 10:44 AM   #2
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Sub-Panel wire gauge


It is 40 amp not 80 amp. You are good on question number one.

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Old 10-30-2012, 11:03 AM   #3
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Originally Posted by fr@nk View Post
Main breaker feeding sub panel are 2 40A single space breakers riveted together. As this is a single phase 240 output, I take it this is improperly rated at 80A?
No, the two joined breakers are the rough equivalent of a double-pole breaker as long as the handles are tied together properly. So the rating is proper at 40A. (The breakers are in series, not in parallel, across the 240V legs.)

If they are indeed two single-pole breakers tied together with a handle-tie, you may want to make sure that they are properly engineered for this use -- you don't want one of the poles disconnected or tripping without pulling the other one, because it could create a backfeed situation for the other leg in the subpanel through a 240V appliance downstream. The way you've described the "double-space" breakers makes me think you're using a QO panel, where handle ties are more common. You may want to replace it with a proper double-pole breaker.
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Old 10-30-2012, 09:57 PM   #4
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Great news.

So my second question goes as follows. There's a 30a 240 outlet and breaker installed for a heater which I've never used. Most of the stronger shop heaters are rated 5kw and their manuals instruct a 30a circuit - so everything seems fine there. A 5kw / 240 = 20.8a continuous draw meaning if I've got 800w (6.7a) of lights on, I should still have roughly 12a of peak demand available before the breaker in my house goes out, meaning smaller tools shouldn't cause too much problems?
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Old 10-30-2012, 10:21 PM   #5
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Originally Posted by flasherz View Post
No, the two joined breakers are the rough equivalent of a double-pole breaker as long as the handles are tied together properly. So the rating is proper at 40A. (The breakers are in series, not in parallel, across the 240V legs.)

If they are indeed two single-pole breakers tied together with a handle-tie, you may want to make sure that they are properly engineered for this use -- you don't want one of the poles disconnected or tripping without pulling the other one, because it could create a backfeed situation for the other leg in the subpanel through a 240V appliance downstream. The way you've described the "double-space" breakers makes me think you're using a QO panel, where handle ties are more common. You may want to replace it with a proper double-pole breaker.
If there are any 240V loads on it, then a true double-pole breaker is required since it must have a common-trip mechanism. Single poles with handle ties only work if there are only 120V loads.
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Old 11-01-2012, 08:31 AM   #6
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Originally Posted by fr@nk View Post
Great news.

So my second question goes as follows. There's a 30a 240 outlet and breaker installed for a heater which I've never used. Most of the stronger shop heaters are rated 5kw and their manuals instruct a 30a circuit - so everything seems fine there. A 5kw / 240 = 20.8a continuous draw meaning if I've got 800w (6.7a) of lights on, I should still have roughly 12a of peak demand available before the breaker in my house goes out, meaning smaller tools shouldn't cause too much problems?
Did this question make sense? I basically want to know if I'm ok to run a 5kw heater and how temperamental the system (mainly breaker in house) will be while its running.

Also, is it safe to run a 45a double pole breaker on the main panel for this wire?

Thanks!

Last edited by fr@nk; 11-01-2012 at 08:57 AM.
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Old 11-01-2012, 10:29 AM   #7
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Keep in mind your circuit (breaker and wire) is only rated to run continuous loaded at 80%.
It will run and yes you have 12a remainder. It would be like having 1 available receptacle in your garage, and if heat is on when you pull the trigger on a circular saw, or small welder etc it could trip the breaker. Then you have battery chargers, radios, trouble light maby. Could end up being a pain constantly thinking about overloading the breaker
What is the sq ft of your garage? Maby you could go with a smaller heater?
Also if your wire is rated for 45a then it is good to be protected by a 45a breaker

Last edited by Marc05; 11-01-2012 at 11:03 AM.
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Old 11-02-2012, 07:42 AM   #8
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Keep in mind your circuit (breaker and wire) is only rated to run continuous loaded at 80%.
It will run and yes you have 12a remainder. It would be like having 1 available receptacle in your garage, and if heat is on when you pull the trigger on a circular saw, or small welder etc it could trip the breaker. Then you have battery chargers, radios, trouble light maby. Could end up being a pain constantly thinking about overloading the breaker
What is the sq ft of your garage? Maby you could go with a smaller heater?
Also if your wire is rated for 45a then it is good to be protected by a 45a breaker
Nicely stated. Garage is around 300 sq feet but my winters are quite cold so I am not yet convinced 5600w is even going to warm the place up all that well when it's 0 degrees out.

40 vs 45, how can I find out what is safe to run on the 8/3?
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Old 11-02-2012, 12:31 PM   #9
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You mentioned that this was run in pipe, so I assume THHN inside the pipe. You would be good to use a 50A breaker.
If this was ran with romex, you would only be allowed to use a 40A breaker.
If this was ran with SE cable, you would be allowed either 40A or 50A, depending on the code cycle in your area.

EDITED: I read your post over again and see you mentioned that it was ran in 1" pipe with "8/3 wire". What do you mean by that? Is it a single cable or individual conductors? If it's a single cable, describe it and see if you can find any identifying labels on the sheath. I have a feeling it is 8/3 romex which means you would be stuck with the 40A breaker.

Last edited by Flaps; 11-02-2012 at 12:33 PM.
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Old 11-02-2012, 05:01 PM   #10
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Originally Posted by fr@nk View Post
Nicely stated. Garage is around 300 sq feet but my winters are quite cold so I am not yet convinced 5600w is even going to warm the place up all that well when it's 0 degrees out.

40 vs 45, how can I find out what is safe to run on the 8/3?
If you have a Iphone i found a decent btu calculator app on there. Typical residential rule of thumb is 8-10w/sq foot in my area (nova scotia). There is a lengthy calculation process including area you live, r-value of insulation, windows/doors, ln ft of exposed wall etc etc to calculate btu's required.. Never seen anyone actually do it that way.
I could ask you all the info but probably easier to look for a btu calc app or online.
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Old 11-04-2012, 08:18 AM   #11
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Originally Posted by Flaps View Post
You mentioned that this was run in pipe, so I assume THHN inside the pipe. You would be good to use a 50A breaker.
If this was ran with romex, you would only be allowed to use a 40A breaker.
If this was ran with SE cable, you would be allowed either 40A or 50A, depending on the code cycle in your area.

EDITED: I read your post over again and see you mentioned that it was ran in 1" pipe with "8/3 wire". What do you mean by that? Is it a single cable or individual conductors? If it's a single cable, describe it and see if you can find any identifying labels on the sheath. I have a feeling it is 8/3 romex which means you would be stuck with the 40A breaker.
Single Cable and here is what's on the sheathing:

General Wire & Cable 8 AWG GENEREX NMWU 300V (-25C)

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