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I'm helping my son build a house and he wanted a whole house standby generator. I did the research and found a 14,000 watt Kohler generator and service rated transfer switch the most highly rated. It will be fueled by propane. The generator will be mounted to a concrete pad near the house. 2 underground conduits will connect the transfer switch to the generator (one for low voltage signal/control, the other for power).

I know how to do the electrical connections. My simple question is what's best practice for type of conduit above ground? The underground conduit will be stubbed off right next to the pad. I'm thinking of extending it via typical PVC and entrance ells into the generator, but wondering if flexible, rain tight conduit might be better. I wish we could have come up underneath the generator, but the Kohler install instructions literally say to drill a hole in the "dog house" cover for the power and control. There isn't room to come up under the generator.

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Well just FYI, "rain tight" is false advertising. Code requires you to assume all exterior conduit is 100% full of water 100% of the time; the defense against wet is in the insulation.

Code requires you fully assemble the conduit first, and then pull the wires through it. I don't like pulling through flexible conduit; it fights too much. So I'd either go Schedule 80 PVC or (if I had a pipe threader on site) IMC or Rigid. The latter two can be used as the grounding path, so one less wire. Sched 40 PVC is too fragile and easily cracked, especially after being sun-bleached. The sun is not kind to PVC pipe; paint it.

Sched 80 has a smaller inside diameter (since the OD is the standard dimension for fittings), so you need to upsize the pipe sometimes. You should upsize anyway if this is a DIY install, as it eases pulling - the last thing you want is get stuck on a pull and have to call an electrician merely for his truck full of pulling tools.

A bit for that reason, I would split it up, and have a conventional main panel straight off the meter, then a 100A transfer switch (the gen is only 70A anyway) to a 40-space subpanel. Put most of the house circuits in the sub, and the non-generator-critical biggies in the main. That way you're muscling around #1 aluminum (or #3 copper if it's short enough for the price not to matter) instead of #4/0 or #2/0. The bigger reason is the main/transfer/sub will play well with emerging technologies like Tesla Powerwalls, EV chargers and solar. That stuff's price is just gonna keep falling.

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  • Thanks for the reply. I agree completely regarding conduit underground. It's always filled with water. The setup we are doing for my sons house is a class 320 meter base (one for the main panel in the house and the other for an existing pole barn, that interestingly had it's own 200 amp service before). The Kohler transfer switch is entrance rated at 200 amps for the house. So the circuit goes from the meter base to the transfer switch (with 200 amp over current protection) to the main panel in the house. The main panel then becomes a sub-panel and must be treated as such. – George Anderson Jan 22 '20 at 3:57
  • Ran out of space in the comment section. The new house is pretty "light weight" electrically: All lighting is LED, super insulated, mini split system that only requires a 20 amp circuit at 240v, gas water heater, gas range. So even though the gen is only 70 A, I"m sure that would be enough and not have to require a separate sub-panel for gen power. If it ended up that we did need that, the transfer switch supports "load shedding" (with additional components) if the demand gets too high. I highly doubt we'd need that. – George Anderson Jan 22 '20 at 4:04
  • @GeorgeAnderson -- the advantage of a separate standby subpanel is that you aren't relying on the relative complexity of load management hardware (with its accompanying current and/or frequency sensing, and possibly the need for central control, as well as separate load shed contactors that can fail) in addition to knowledge of the need for load shedding in order to keep the generator from being overloaded by future developments – ThreePhaseEel Jan 22 '20 at 4:29
  • In general, I agree with you 3Phase, but in his case, the house is very energy efficient. And he is smart enough not to run a clothes dryer when on the generator. He could also shut off a few breakers if needed, but I'm sure it won't be. – George Anderson Jan 22 '20 at 17:45
  • also because the tiered subpanel idea will prepare the house for a variety of emerging tech with a minimum of additional switch gear (some of the interlocking requirements for PowerWall are out of this world in a 1-panel house).., that will not far in the future be cheap enough that everyone will want to do them. But if you already bought all the gear, don't worry about it... – Harper - Reinstate Monica Jan 22 '20 at 17:55
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If it were my generator I would transition to flexible liquid tight, I don't like having solid connections on vibrating equipment.

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  • Thanks, I've seen lots of installations with rigid connections, but that doesn't make it best. I was thinking of the liquid tight but wondering if it was overkill. Thanks again. – George Anderson Jan 22 '20 at 17:47

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