According to the Manufacturers Installation Guide (PDF)...
Physical - install the UTS within one foot of the building circuit breaker panel
Page 4
Caution: The UTS must be installed within one foot of the building circuit breaker panel. If the UTS must be located further than one foot from the circuit breaker panel, a licensed electrician must extend the wiring using standard electrical wiring conduit and junction boxes.
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According to National Electrical Code (NEC)...
National Electrical Code 2011
ARTICLE 110 Requirements for Electrical Installations
110.3 Examination, Identification, Installation, and Use of Equipment.
(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
I'm guessing the unit has permanently attached wires in a raceway, which is of a limited length. So mounting the unit further from the panel would require you to extend these wires and raceway, which requires knowledge of wiring methods to do properly. It's not likely you'll have to upsize the wires, but you will have to understand basic wiring methods (Chapter 1-4 of NEC).
I will join longneck, you need to give to give this one to a pro.
If you do not understand about back-feeding a circuit, already, it is too easy to kill someone. I say this without exaggeration.
The biggest problem with running a generator with a transfer switch is that you have to wire it into your main panel.
For basic protection of life, (as well as the law,) it must NOT be possible to have your generator and the power to your house on the same wire, at the same time.
This is not a case of, just for a few seconds, or I would not do that, or I will triple check a written list.
It must not be possible under mechanically, electrically, on with quantum physics.
The reason is that if you put power on the 'dead' wires to your house, it is possible to kill someone a mile away, or more.
Emergency workers expect down lines to be dead. Yes, they do check first, but that does not mean that the situation can not change while they are working on them.
Transformers work backwards, low voltage at your generator (240) quickly becomes high voltage (thousands).
And sometimes people, and pets, or cars, just do not even see a fallen power line.
This is a case of some money to a pro vs life.
Best Answer
Nope. This question comes up a lot on here, and you can't do it because it creates a paralleling problem with the neutrals.
These Reliance switches are designed for one thing: to provide a generator salesman a switch to offer to anyone without having to understand or investigate their panel. But they aren't well made, and they have a lot of limitations -- GFCI/AFCI support is a serious problem. And because of how they mishandle neutrals, supporting 2 separate panels is simply out of the question.
Your only Reliance-side option is to move the circuit in question so it's in the same panel with all the other circuits.
The better way to do it is to do what the generator salesman was trying to avoid: Evaluate your main panel for the feasibility of fitting a Generator Interlock. That lets you feed the entire main service panel (and subpanel too!) from the generator; on the proviso that you limit appliance use (or turn some off) to keep current draw within the generator's range. That will neatly moot your entire problem.