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I purchased a Reliance Q510-C transfer switch to use with a 50 amp generator at my house. The transfer switch is rated at 50 amps. I have wired it into my main electrical box.

I also wired a 50-amp power inlet box into the transfer switch so I can plug the generator into that (rather than run the cord into the house). I used 6 gauge wire to connect the inlet box to the transfer switch.

The transfer switch and the inlet box are both rated at 50 amps. Everything is installed and works fine.

There's just one thing that bothers me. The transfer switch has a terminal block to connect the inlet wires. So the four wires coming into it (from the inlet box) are 6 awg wire. Two hot wires, one neutral and one ground. The four wires coming out of the terminal block are 10 awg wire. So it goes from 6 awg to 10 awg at the terminal block. Red 6 goes to red 10, black 6 goes to black 10, etc.

Here is my question...

Why would the manufacture use 10 awg at the terminal block? I expected the wires to be 6 awg (the same size coming from the generator power inlet). The 10 awg wires are only about 8 inches long and they run into the main bar where all the breakers connect. I assume the manufacture knows what they are doing and that 10 awg will carry the load, but I have never seen 10 awg used in 50 amp applications.

Is this OK, should I call and ask the manufacture if they made a mistake (they sell 30 amp boxes exactly like this... maybe this is a 30 amp box mislabeled as a 50 amp box) or should I not worry about it and just use the box as is?

The neutral wire they use at the terminal block is 8 awg. The others are 10 awg. I'm really only concerned about the two hot wires (red and black) that go from 6 awg to 10 awg at the terminal bolck.

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Definitely call. I think you spotted a 30A design mis-manufactured as 50A. –  HerrBag Aug 25 '13 at 14:40
    
@HerrBag - Thanks. I plan to call and talk to them about it this week. I just wanted to post here to get other opinions. Maybe it's OK in this application, I just want to make sure before I use it a lot. –  user1356386 Aug 25 '13 at 20:55
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If the device is UL listed, there shouldn't be a problem. The internal workings of devices are outside the scope of building codes, and would fall to the standards organizations. Most building codes cite using listed and labeled devices, because these devices have been designed and tested to be safe when installed and used properly. –  Tester101 Aug 26 '13 at 11:55
    
@Tester101 - Yes, the transfer switch is UL listed and has a rating for 50 amps. I don't know much about that (I'm more of a building code guy). I do appreciate your comment. –  user1356386 Aug 26 '13 at 11:58
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The fact is, building codes err on the side of safety. there is no telling exactly how (or through what) wires are going to be run, so they generalize and maybe rate things a bit on the low side. Whereas internal device wiring has a known environment, so you can push things a bit further. The device manufacture may have proven that they can dissipate any additional heat, so they're able to use #10 for 50A. –  Tester101 Aug 26 '13 at 12:03
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2 Answers 2

up vote 3 down vote accepted

That does indeed describe a properly engineered 50 amp device.

It is normal for appliances to use slightly lighter gauge wire than would be used in building wiring. You can observe that inside electric ranges, dryers, and water heaters, where you will see the 10 or 8 gauge incoming supply wires to a terminal and then 12 or 14 gauge wire going from the terminal block off to the components.

Because the wire is inside an enclosed box, and is not tightly bundled or surrounded by thermal insulating materials, its elevated temperature will not cause any danger. It will run a little warm at peak current, but be far below "hot". The same is not acceptable inside dwelling walls were it will be in contact with wood, fiberglass, cellulose insulation, etc.

As for standards, I have determined this is beyond the scope of the NEC and falls into the domain of the National Fire Protection Association (NFPA) standards. That seems to cost $$ to view.

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Thanks. Is there any standard or documentation about this practice in the NEC or somewhere else that you can cite? –  user1356386 Aug 25 '13 at 16:49
    
@BradTilley: I am sure there is. I vaguely remember reading it in a NEC decades ago. But it wasn't what I was looking for. –  wallyk Aug 25 '13 at 16:51
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The heat resistance of the insulation is also a factor. A thinner wire can have the same ampacity as a thicker wire if the insulation can withstand higher heat for any given installation. –  bcworkz Aug 25 '13 at 20:10
    
@bcworkz - I read that too, but the highest rating I could find for 10 awg copper was 40 amps with 90 degrees Celsius insulation. en.wikipedia.org/wiki/American_wire_gauge –  user1356386 Aug 25 '13 at 20:51
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I received a good answer from the manufacturer. I wanted to post it as an additional answer. Thanks to all for the mention of UL and appliances being separate from general NEC building codes. I've learned a lot from the comments and the other answer (which I've accepted).

UL 1008 is the standard these devices meet.

Here is the email response. They have been very professional and timely in their response and the product is well-built. I recommend them:

Hello, thank you for the inquiry and for choosing our product!

The wiring we use inside the Q-series cabinet, specified as factory wiring, is sized in relation to the testing for heat-rise on wires. In a factory wired electrical product, the wiring within can be potentially rated at any size that passes the heat-rise tests set forth by the testing lab, in this case Underwriters Laboratories Inc. 6 AWG is rated for 55 amps and is required for most 50 amp applications when dealing with general wiring, but the wiring in the transfer switch does not fall under this category as it's enclosed and presumed to be unchanged after the factory build. If number 12 wire were to be sufficient in passing all of the stringent UL testing, we would use that instead of number 10 wire.

I understand the concern and it's true that you will see 6 AWG wire for most 50A applications, the number 8 and 10 wires in the Q510C are permitted through the listing of the product by UL as they passed all of the testing that is required by the national standard for transfer switches (manual), UL 1008. There are actually a lot of potential applications that could use smaller wiring than what is considered normal, but the NEC isn't able to differentiate every application in the codebook so they have very standardized wiring regulations to enhance safety and avoid undersized wiring. In this scenario, we are using a smaller gauge wire but due to its short length and enclosure in the cabinet it can handle a 50A load with no problems. Please let me know if you have any questions, thanks!

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Thanks for the follow up! –  wallyk Aug 27 '13 at 16:55
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It's awesome that the manufacturer took the time to respond. Thanks for posting it. –  Tester101 Aug 27 '13 at 17:49
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