sourcing and running large utility wires through the attic

In my situation the Nat gas and fies box are far apart. I'm installing the generator near the nat gas and running wire to the fuese box where the transfer switch will be.
Its a 15kW unit so large (#2 minimum) wire will be needed to carry the single phase power to the transfer switch.
I'm having trouble finding this in a bundle. Will I have to run individual wires in a conduit? The run is ~140 ft.
Where's a good source for electrical wire of this size?
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Mook Johnson wrote:

Well 15K Watts = 62.5 Amps.
You should be able to use 4 guage NM/B Wire IMO. That will have your 2 hots, neutral, and ground wire in it. From my understanding, this wire can handle 70 AMPS up to about 130 feet for 2% voltage drop or less. It can handle 60 AMPS up to about 150 feet for 2% voltage drop or less. That would tell me that it should be able to handle up to 65 AMPS(15.6kW) for 140 feet.
When I ran a subpanel in my garage, I used 4 guage NM/B wire. I could not find this at your standard Home Depot / Lowes places. However, a quick look on Yahoo Yellow Pages or in your phone book for electrical suppliers, you can ask around and find it easy. It did hit my pocket book good for about $2.20 / foot.
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My default response would be running it in conduit, it keeps the rodents off of it, and the individual wires are usually cheaper than the bundle, off-setting at least part of the cost of the conduit.
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Eric in North TX wrote:

Depending on how accurate the 140' measurement is, you're also close to 500' for the three conductors (reduced size ground) so you may get a better price buying a full spool. Still need to buy just 140' or so of the smaller wire for the ground.
Pete C.
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I know this sound radical, but I'd try an electrical supply house.
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