wire amp rating & temp

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  #1  
Old 02-08-11, 07:29 PM
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wire amp rating & temp

I'm trying to understand the temperature part of amperage rating of a wire.

Seems that it depends on terminals rated for 60 or 90 deg C.

My Reliance panel has a 50A generator circuit in the 1/3 slot, the 50A wattmeter box states to use no larger than #8 wire.

I had it originally setup for a 30A gen so it has a 7' run of 8-3 wire from the inlet to the breaker going through the 30A wattmeter box.

I'm now going to replace the 30A inlet and wattmeter box to a 50A but I thought #8 only was rated at 40 or 45A.

Sone places I read #8 is safe for 50A so what is the real truth and please put in laymans terms...
 
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Old 02-09-11, 07:24 AM
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The problem is that the NEC has multiple ratings on wire and cables. Single conductor THHN, which is used in conduit has a 90C rated jacket and is rated for 40 amp ay 60C, 50 amps at 75C and 60 amps at 90C. Depending on usage you could fuse 8 gauge thhn at 50 amps. However ROMEX cables while the inner conductors are 90C rated can't be fused the same way due to NEC requirements. Romex 8 guage cable is rated 40 amps, period. NEC requires the lower temp ratings even though the wire itself could mechanically handle more. Not sure if this explains it well enough, but the type of wire and NEC dictates amperage allowances.
 
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Old 02-09-11, 10:13 AM
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Originally Posted by i6pwr View Post
I'm now going to replace the 30A inlet and wattmeter box to a 50A but I thought #8 only was rated at 40 or 45A.
A cable assembly of #8 copper (Romex, NM-B, SER) is limited to 60C temp rating which means no more than 40A. Individual THHN #8 copper wires in a conduit system most of the time have a temp rating of 75C which allows up to 50A.
 
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Old 02-09-11, 08:17 PM
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You've already got the right idea. The limiting factor is the temp. rating of the connection points/terminals. The common wire insulation THHN can handle 90C before failing, but terminals are normally rated at 60C or sometimes 75C. (it's usually in the fine print on the side of a circuit breaker). So yes, 8ga w/THHN can handle 50 amps without the wire failing, but it would cook the terminals at the ends of the wire. Electric stoves used to be fed with 10ga wire, on 40A breakers... the wire held up just fine, but open the back of the stove and you'd see the terminals were toasted.

One of the reasons THHN is popular is that the extra heat resistance it gives makes it possible to sidestep all manner of ugly complexities of installation. Basically, it's quite easy for parts of the wire run to be hotter than inside the boxes at the ends, and the Code requires that these be taken into account. But if your wire is limited to its 60C amperage, and has 90C insulation, the fact that it's run under insulation in a hot attic (for example) won't push it past its 90C heat rating. That 30C overcapacity makes for great insurance against warm environments.

The trouble in your case is that your 50A box said to not run more than 8ga. I don't know why it said that. My suggestion would be to get a short piece of #6 and see if it'll install cleanly into the Reliant box terminals. If that won't work nicely, you should put a 40A breaker on the line so you don't end up frying something.

One final thought: Some parts of electrical systems are designed expecting no more than 80% continuous load. Is your generator actually rated for 50A continuous load? If 50A is a peak rating, and it can only put out 40A continuous, then keeping the 8ga wire looks more reasonable. I'd still use a 40amp breaker with 8ga wire to be safe, the breaker won't throw with 40A continuous load.
 
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Old 02-12-11, 11:59 AM
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I checked with Reliance and they investigated the situation and came to the conclusion that the manual for the wattmeter box was written when it had actual terminals in the enclosure, it now consists of two donuts that the wire passes through and using 6 GA is perfectly fine.

I heard 8 GA will handle 50A, but it was all dependent on the insulation etc. Now if something were to happen, heaven forbid, that it would be an insurance issue if I used the wrong wire even if it wasn't the root cause....I don't want that burden for sure.

The generator is rated for 11KW continuous and 13.75KW peak.
 
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Old 02-14-11, 11:43 AM
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I would definitely go with #6 in this case. The genny continuous rating is 11kW = 45A, and the peak is 13.8kW = 58A. These are out of range for #8 in the best circumstances.
 
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