I've needed external relays several times in the last few years, controlled via an Elk M1, for sprinkler pumps and an HVAC fresh air duct fan. I chose the G7L series from Omron with DIN rail mount, covers and a test button. 2 are DIN mounted in a NEMA box by the pumps.
http://www.component...le/G7L_0609.pdf
G7L-2A-BJ-CB-AC24 DPST NO, Screw terminals, 25A, Test button and bracket mount
P7LF-D DIN Rail mounting adapter
P7LF-C Cover for screw terminals
The PDF also has the spacers and end plates for DIN rails if you need them along with options for DIN-mounted socket base.
These are not cheap, so your 100A contactor may work out cheaper. The were about $17/ea when I bought them in 2008 and about $22 each now. Watch that some of the sites play games with shipping prices.
Jay
After reading BSR's comment I re-read what you are trying to accomplish - don't do it.
When I answered with suggested relays I was contemplating a different scenario and not "hot" switching between power sources.
If there is any load on the circuit at all, you run big risks unless the sources are "in sync" for frequency, phase and voltage:
- Resistive loads (aka, lights) will probably do fine with a flicker, though you may be shortening the life of the bulb
- Motor loads (fridge, a/c) will cause the motor to be "slammed" back into phase to match incoming power which on the motor will damage it (bearings, windings, output gearing) with a huge inductive kick back through the relay contacts well in excess of the 20A/25A rating
- I don't know how electronic switches (ALC, UPB, ...) would react, but it can't be good
Tying neutrals also has some interesting questions because:
1) while they ultimately both tie back to ground, you have different paths so there likely will be a ground current loop;
2) because neither neutral can take the "full load" return for both power sources it is probably a code issue around current load (Yes, if everything is balanced the neutral current should be zero but design is worst-case);
3) if one source is disconnected you may have a lineman safety issue because they wouldn't expect a neutral return current if that source is disconnected.
The correct contactor for switching has 3 key features you need: 1) Current load for live switching, 2) Guaranteed break-before-make and 3) and "disconnected" position to allow everything to power down before re-applying power. I realize #3 is not what you want but if your sources are not exactly matched then it is required.
Note that my experience is from working with data centers that have multiple power sources, generators and UPS's (3 phase) all playing nicely together (usually) so it may not translate 100% to residential but the fundamental issues still apply.
My 2 cents for Saturday morning.
Jay
