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I need to add both a 120v (probably 20a) circuit as well as generator hookup (120v 30a) to my backyard. The 120v circuit will be used for landscape lighting, that sort of thing. The generator hookup will be wired to a weatherproof outlet at the back of house, and a manual transfer switch (adjacent to the main 200a load center) in the garage to run stuff like a fridge, lights, select outlets, etc..

House is block construction on stem wall, so I will be surface mounting sch80 conduit on the exterior, run is approx 60-70 feet. With conduit derating I'm looking at probably 6AWG for the generator hookup. While I only have a 120v genset now, I was going to future proof and run 3 conductors for when I go 240v. Of course any ampacity for 120v will be plenty for 240v. The other circuit is probably going to be 10 or 12AWG copper.

My question is can I share the ground between these or will that be unsafe from a line/generator isolation standpoint? Im assuming sharing neutrals is big no-no. Is running in the same conduit problematic? I assume if sharing ground is fine, then same conduit is okay.

Any other wisdom also appreciated.

NB - I am well aware of the dangers of generators to linespeople, and it will be appropriately permitted/inspected.

Edit: if I go 240, it would be to run AC. I have a 2 ton unit.

Generator only would be for emergencies like hurricanes (I live in FL), so don't need an automatic unit.

I'm down with better quality components, what do you suggest I look at?

Thanks.

  • Do you plan to use your generator solely for standby power, or do you want to use it as a standalone portable generator as well? – ThreePhaseEel Nov 12 '18 at 5:39
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    Don't get one of those horrible 6, 8 or 10 circuit transfer switches. They are expensive, cheaply built by low tier vendors not qualified to make service equipment, and are built that way to allow generator installers to CYA legally. A DIYer can do a much better job with service grade equipment and get a better performing solution too, for half the cost. Of course you can't put that in a box and sell it, so nobody will tell you that,s the better way to go. – Harper Nov 12 '18 at 9:22
  • When you upgrade to 240V, exactly what size generator are you planning to install and exactly what do you plan on running on it once it is installed? The reason I am asking this is it affects the way we answer your questions. This is in conjunction to @ThreePhaseEel's comment. – Retired Master Electrician Nov 12 '18 at 15:41
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Sharing the EGC is fine

No matter how many circuits you run in a single raceway, you only need one equipment grounding conductor for the whole lot (unless you require redundant or isolated grounding conductors, of course). This is implied by 250.122(C):

(C) Multiple Circuits. Where a single equipment grounding conductor is run with multiple circuits in the same raceway, cable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the raceway, cable, or cable tray. Equipment grounding conductors installed in cable trays shall meet the minimum requirements of 392.10(B)(1)(c).

For your use case, a single 8AWG EGC (adequate for all circuits up to 100A) is sufficient here. Note that this conductor must be a wire in the setup described below as it serves as the separately derived system's grounding electrode conductor as per 250.30(A)(4) and the Exception to 250.121 in addition to being the equipment grounding conductor for both circuits, and the ground wire from the transfer switch to the main panel must likewise be an 8AWG wire.

Derates aren't too big a deal here either

Given that we can guarantee that there are only 4 current-carrying conductors (generator L/L1, generator N/L2, landscape lights L, landscape lights N), we can operate at an 80% derate from the 90°C column in Table 310.15(B)(16), allowing 6AWG THHN to carry 60A, more than you will need for just about any generator you want to use.

Conduit sizing

Given all this, and your (not unreasonable) desire to use Schedule 80 PVC for this wiring run, our 6 conductors (3 6AWG THHNs to make sure we have enough room for a 240V generator hookup, an 8AWG bare ground for the lot, and 2 12AWG THHNs for the landscape lights) add up to 127mm2 of fill. A 1" Schedule 80 provides 178mm2 of fill, but I would go up to 1.25" here which gives us a nice, generous 320mm2 of fill in order to avoid having to call in a pro to rescue a botched pull job.

As to transfer setups

You are on the right track with breaking your standby loads out into a separate panel -- this allows the use of inexpensive interlocked-breaker type manual transfer switches instead of loadcenter interlock kits (which limit flexibility) or select circuit manual transfer switches (which are awkward and hokey). It also reduces the size of the equipment required compared to service entrance transfer setups (which are a far too common occurrence with ATSes).

Given all this, I would recommend either an Eaton CH10GEN5050(R)SN if you want a prepackaged solution, or the combination of a suitable main lug Eaton CH loadcenter with a matching "Type A" Eaton CH generator interlock cover, two Eaton CH350SW breakers for the interlocked mains, and Eaton CH branch breakers if you wish to build something up yourself and can get the CH350SW (its production status is a bit questionable at the moment). If neither of these are an option, a Reliance Controls XRR1006D(R) (or XRR1006C if you want wattmeters for the generator) with suitable 1" wide branch breakers (HOM, QP, or BR, but not THQL or CL) can be substituted instead. Of course, you'll need a suitable inlet box for plugging your generator in outside as well -- either a L5-20 inlet for 120V only, a L14-30 inlet for 240V 30A, or a CS6365/CS6375 inlet for 240V 50A.

What makes these three solutions special is that they provide a switched neutral between the utility and generator sides of the system, allowing the generator N-G bond to stay intact. Practically, this means that just about any generator out there can be used plug-and-play with a suitable generator cord instead of having to fiddle around with the generator to pull the neutral-ground bond out of it, as most generators ship with their neutral-ground bond installed for portable standalone use, such as on jobsites.

Other residential transfer setups do not switch the neutral, which requires pulling the neutral-ground bond from the generator to avoid a paralleled bond that can falsely trip GFCI or many AFCI breakers and pose a safety hazard due to current on grounding conductors.

Other notes

You'll need a preprinted warning placard at your inlet in accordance with NEC 702.7(C) that specifies whether your system uses a bonded-neutral or a floating-neutral generator, as follows:

                          WARNING:
            FOR CONNECTION OF A SEPARATELY DERIVED
                 (BONDED NEUTRAL) SYSTEM ONLY

or

                          WARNING:
           FOR CONNECTION OF A NONSEPARATELY DERIVED
                (FLOATING NEUTRAL) SYSTEM ONLY

depending on what you have. You will also need a label on your main panel specifying where the generator inlet is, as per 702.10(A) -- this could be incorporated into the panel directory, though. More importantly though, your main panel will need a preprinted warning label or placard, conforming to NEC 702.10(B), warning that disconnecting the EGC to the generator will cause the generator to lose its grounding electrode connection as well, as follows:

                         WARNING
              SHOCK HAZARD EXISTS IF GROUNDING
          ELECTRODE CONDUCTOR OR BONDING JUMPER
          CONNECTION IN THIS EQUIPMENT IS REMOVED
           WHILE ALTERNATE SOURCE(S) IS ENERGIZED.

Last but not least, you will need to ensure that all connections are torqued to manufacturer specifications with an inch-pound torque wrench or torque screwdriver as per NEC 110.14(D).

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On the ground question, no problem sharing. How do I know that? Change your conduit type to metal conduit (you were talking quality): if it wasn't legal to share ground, it wouldn't be legal to use metal conduit! Because it's almost impossible to keep one metal conduit isolated from another, nor would you want to try, because if a voltage diffential did develop, it would shock passersby.

As far as up to 3 circuits in the same pipe, you don't need to derate as long as you are using THHN or THWN-2 or other 90 degree C rated wire. Why? Because grounds, and neutrals in 240V circuits, don't count; so any circuit in residential use only has 2 wires that count... 3 circuits is 6 wires and you only derate to 80% of the 90C column, so your max is #14=20A, #12=24A, #10=32A, #8=44A, #6=60A, etc. You see where those derate limits are above the normal wire limits you still must obey. So not an issue.

Ah, I see ThreePhaseEel posted. My work is done :)

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