Let's hope there is a proper switch in there.Most folks who use a portable generator to feed a home:
1. Engage the main disconnects to isolate the generator from the utility should the utility service come back
2.Connect the generator to the dryer 4-wire receptacle circuit and feed the whole house
3. Manually trip off less important loads
This way you have both 240 & 120V. If you have a well pump - odds are it is 240V and you would want that as a priority load. But wiring for 240 gives you both 240 & 120V
What I did was wire in a 4 wire generator receptacle rated for wet space/outdoors in parallel with dryer receptacle but located the generator recep outdoors in the vicinity of the generator. My port gen is 7.5 kW and use 10 awg flex service to feed the dryer's 30A circuit breaker. This arrangement is my "Temporary Emergency Backup" in case my fixed 50 kVA diesel gen is down.
The size of the cable you need between the gen and the receptacle will be driven by the diesel gen breaker rating. If its 40A, then you should be using 8 AWG. Technically, if the dryer circuit is fed from a 30A breaker, you could argue to use the 30A cable. You may find it difficult to get flex service 8 AWG cable that will fit into the standard gen receptacles.
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