Generator Hook-up to Main Breaker Panel

Question

I would like to add 240V/50A infrastructure to support a portable generator for power outages. I’m planning to buy a Westinghouse WGen1200DF, 15000/13500 Peak Watts and 12000/10800 Rated Watts (Gas/Propane) due to its fuel flexibility and low THD. A permanent generator was overkill and outside my budget.

The garage contains the sub-panel powering 75% of circuits to my single-story house, although several key (at times) 120/240 branches are also fed from within the MBP. I considered installing an interlock at the sub-panel to facilitate back-feeding there, but can’t because there isn’t a main breaker inside the sub-panel (the 100A breaker for the sub-panel is in the Main Panel). I considered a Generac or Reliance manual transfer switch (with breakers) at the sub-panel, but I wasn’t happy with the limited number of circuits and also felt it wouldn’t be cost effective after buying appropriate GFCI/AFCI breakers for the transfer switch in order to meet NEC. In the end, I felt providing interlocked 240V/50A generator service directly to the MBP was my best course of action. It would cover the entire house and use existing breakers, although I would have to actively monitor the power demand.

My main concern is distance- the main breaker panel (MBP) is on the opposite side of the house from the driveway and garage (hindsight 20/20). I would like to keep the generator on the garage/driveway side of the house for security, ease-of-movement to/from the garage, and close access to my buried 500 gallon propane tank for fueling the dual-fuel generator. I considered using a very long 6/4 cable, but it would require about 120 feet to make it from where the generator would rest, not to mention lay on the ground in front of the house.

Given all that, I believe my best course of action may be to add a 50A Power inlet by the garage, and run the line through the attic (I have easy access, and it would essentially follow the same path of the existing sub-panel cable), out the soffit (in conduit) and down into the MBP mounted on the outside wall of the house. The overall distance from the power inlet through the attic to the MBP would be about 90 feet. Add a 20 ft 6/4 cable from the power inlet to the generator should make the maximum distance about 110 ft.

With all that said, I wanted to ask:

1. Does my thought process and approach make sense and seem reasonable?
2. Is a 110 ft distance from generator outlet to MBP ok? I want to want make sure I have maximum voltage/current available at the MBP.
3. For the 90 ft run through the attic, will 6/3 suffice, or would I be any better off using 4/3 to overcome any increased impedance? My understanding is that 6/4 or 4/4 would not be required for the 90 ft run through the attic.

Answer 1

Your problem: your generator isn't compatible with your current plan

The first and foremost problem with your current plan is that your generator is technically not compatible with ordinary breaker panel interlock kits. Why? Well, in the vast majority of cases, there is only one place where neutral and ground (grounding conductors) are connected together, and that's at your main breaker panel. However, your generator is designed to be legally and safely used as a portable or jobsite power source, not just for powering up a building's electrical system, so it brings its own neutral-to-ground bond to the party. This is a problem because ordinary breaker interlock kits only force the hot wires to be switched, not neutral, leading to the results of Two Bonds Bickering, namely neutral current getting onto grounding wires and making exposed parts "live".

Some people fix this by doing surgery on their generator to remove its neutral-to-ground bond, but since you need it for portable/jobsite type usage, that's not an option for you. Instead, we'll need to build a plan around using a switching neutral transfer panel that breaks up the fight between the neutral/ground bonds by ensuring only one bond is selected at any given time, along with a subpanel off that transfer panel, and some wiring to connect everything together.

For the switching neutral transfer panel, you have two options. If you want to stay with Eaton CH breakers, you can use an Eaton CH10GEN5050SN, but that limits your standby loads to 50A at any one time no matter if you're on utility or generator power, and is also rather costly by itself. If you want more power, you can use a Reliance Controls XRK1005C, but that has the downside of using 1" (Siemens QP, Eaton BR, Square-D HOM) breakers instead of the Eaton CH breakers you currently have, and also requires a bit more work to play nicely with our plan.

Either way, for this plan, we'll need:

• matching two-pole breakers to transplant up to 5 of the 120/240V circuits from the main panel to the new transfer switch
• 90-100' of 1½" ENT conduit (smurf tube)
• About 270-300' of 1AWG (or 4AWG, if you're using a 50A transfer panel) and 360-400' of 4AWG Al XHHW-2 wire
• a 125A, main lug, 24- or 30-space loadcenter to use as a subpanel off the transfer panel, with a grounding bar fitted, and any replacement breakers installed if this subpanel's not Eaton CH
• an Eaton CH250 or CH2100 to serve as our utility feeder breaker for this all
• two 1½" PVC male adapters
• A short length of 1¼" or 1½" conduit of any type with associated adapters (this can be some of the ENT from earlier)
• A length of 6/3 W/G NM-B or UF-B for connecting the inlet box to this arrangement, with matching clamps
• A 50A inlet box (the Reliance Controls PB50 is the easiest/cheapest to find in most cases)
• A Mersen MPDB63103 + mounting screws (or 3 4-14AWG two-port insulated mechanical splices) to splice the 6/3 from the inlet box to the 4AWG wires carrying it the rest of the way
• And some lengths of ½" or ¾" conduit and fittings with THHN wires of suitable size, or NM cables of suitable sizes and types for that matter, along with wirenuts for moving branch circuits between boxes

With this setup, the transfer panel is mounted beside the main breaker panel, connected to it with a conduit for the feeder from the main and another conduit or a set of cables for the circuits that are moved over. Note that hot and neutral need to be moved over for each circuit, individually. The long length of conduit then is connected to the transfer panel (you may have to enlarge the knockout in the top center to do this) and brought up through the attic and over into the top of our new subpanel (next to the existing subpanel). This subpanel also has the power distribution block fitted to it (unless you prefer insulated mechanical splices, that is) so that there's enough room to run wires into both ends of the block.

Once the new subpanel's in, then we can start moving circuits over from the existing subpanel to the new subpanel, using the same strategy as before. With that out of the way, we can then install the inlet box and fit the 6/3 cable from it to the distribution block in the subpanel, landing its grounding wire on the panel's grounding bar. Working backwards now, we run the 7-wire double feeder between the subpanel and main panel:

• 3 of the wires (the 3 1AWG wires, if a 100A transfer panel is used) run from the main lugs on the transfer panel to the main lugs on the subpanel
• Another set of 3 4AWG wires runs from the power distribution block in the subpanel to the generator terminals in the transfer panel
• Finally, the remaining 4AWG wire runs between the two panels' grounding bars

Last but not least, we can use some more 4AWG (or 1AWG, if you're using a 100A transfer panel) to connect the transfer panel's utility terminals to the feeder breaker in the main panel. If a grounding wire is needed here, you can use more of the 4AWG Al for this.

The end result of this all is a switching neutral transfer panel set up so that its utility connection is fed from the main panel and its generator connection is fed using a back-run feeder from the inlet box, while its main lugs are connected to the main lugs of a remote subpanel that effectively extends the transfer panel to provide more circuits.