Generator transfer sub-panel: is this OK?

[Tman]

Going to install a generator tranfer sub-panel.. I'm getting a permit, but I wonder if there is anything obvious here that I plan on doing that would be amiss before i get that permit....

I have a 8-place Square D subpanel with a 70A switch and a 30A breaker wired with one of those mechanical interlocks that prevent both from being on at the same time.

So I'll run the 30A breaker via 10/4 romex to an outside plug, which is where I'll connect my approx 5500W portable generator.

Then I'll run the 70A switch (2 hots) and a neutral via some 6ga wire in a conduit to a 60A 2-pole breaker in my main panel. Including a 8ga ground in the same conduit, being sure that the ground and neutral are not tied together in the sub-panel.

Then I'll load the remaining 4 spots in the subpanel up with the QOT tandem breakers. One of these is for a 240 circuit (water pump), and i'll tie two adjacent breakers together with a tie pin from Square D to make a 2-pole breaker. This leaves 6 circuits for 120V service.

Two things I want to make sure about:

If I add all the breakers in the subpanel up, I'll have 70A (2x20, 2x15) per leg, which exceeds the 60A subpanel feed breaker and certainly the 30A generator feed breaker, and even more so, the 5500W generator capacity. But I don't think the feed breaker will ever trip, and I'll just have to be careful when on genny. Is there anything not kosher about this setup?

I need to transfer a handful of circuits from the main panel to the sub panel. Can I do this the easy way.. by leaving each circuit run in the main panel, and splicing the hots ONLY with a wire nut to feed to the subpanel via wire of the appropriate guage in a conduit? Anything to be aware of here, especially if it is OK to leave the ground and neutrual for these transferred circuits terminated in the main panel? I could run those to, but not if I don't need to...

Thanks!! Tman.

[RBM]

It would seem to me the easiest way to accomplish what you want is to buy a six or ten circuit transfer panel kit from HD or similar place. This has a ready made panel with multiple switches built in and a cable with a pile of wires you connect right into your main panel, takes just a couple of hours and you're done. The way you're going about it is fine, you don't add up the size of the breakers, but the loads attached to them and I believe you cannot use a panel as a junction box the way you planned and need to remove the cables from your main panel and install them into the generator sub panel.

[Pop]

I think this site might help you out in some of your decisions:

Offhand it sounds OK but it's fuzzy in a couple of areas (to me). The single most important thing is to not allow the generator, under any normal "fault' conditions, to feed the power lines.

A Transfer Switch panel is the easiest way to do it, IMO, and the most reliable. Lots of companies make them. Yours almost, not quite sounds like a transfer switch, so I'm not making too many judgements on the validity of your plans here.

HTH Pop

[RBM]

[RBM]

The thing he has is a UL approved double breaker system with mechanical interlock made by Square D for the purpose he is using it. It's not "odd" or Rube Golberg, just not the easiest way to do the job

[toller]

Why are you screwing around with this, when real transfer switches are available? I would be reluctant to buy a house that contained what you are describing; fearing you had done equally odd things elsewhere.

If you chose to design your own. just be sure that the net current going though any metal hole must be zero. It was not clear if you were doing that.

If you try to draw more than 5500w from you generator it will let you know quickly by stalling. Hopefully you will not burn anything out in the process.

[Tman]

ahh so I need to run both the hot and neutral (and ground also?) from the subpanel, through the conduit, then splice in the main panel. This makes a lot of sense... I suppose a net current in the conduit could induce a current in the conduit itself.

The generator breaker actually pops before the voltage or frequency gets out of bounds enough to fry something...

[Tman]

ahh so I need to run both the hot and neutral (and ground also?) from the subpanel, through the conduit, then splice in the main panel. This makes a lot of sense... I suppose a net current in the conduit could induce a current in the conduit itself.

The generator breaker actually pops before the voltage or frequency gets out of bounds enough to fry something...

[toller]

I don't doubt you are correct, but to anyone who is unfamiliar with this item, it will look odd. So unless there is a compelling reason to do it that way...

[HorneTD]

Try cost. Adapting a panel or sub panel for generator transfer service cost a lot less than stand alone transfer switches or premanufactured transfer panels. You trade your additional labor for lower material cost.

[HorneTD]

There is no need to run the neutral or each ground to the transfer panel. You can run just the ungrounded conductor as long as it goes out and back through the same raceway, cable, and/or knock outs.

[Calvin Henry-Cotnam]

So far, so good. I have nearly the same setup, except the panel I have came with two 30A breakers for the interlock. My connection to the generator inlet is the same, except I used conduit with three 10 gauge wires for the two lines and neutral and a 12 gauge grounding conductor (the ground can be one size smaller, except once you get to 14 ga).

My connection to the main panel used #10 conductors and a #8 ground and runs off a 40A breaker (sized larger in case I want to upgrade it later, but the 30A breaker at the transfer panel is ultimately the overcurrent protector here).

No, it doesn't. Unless Square D has come out with new mini-breakers that provide a proper 2-pole option (I have seen some other manufacturers that provide a 2-space mini breaker configuration, where the two outer breakers are single pole and the two inner are tied together to give a 2-pole breaker), you will have to use a full-size, two position breaker for your

240 volt circuit (a QO215 or QO220, for instance). This will leave you with only two full positions on the panel, where you could install two mini breakers (QOT1515, for instance) to give you four more circuits.

Not really. Have you ever added up all the breakers in your main panel (on each leg)? It is not unusual for them to exceed the main breaker's rating.

A branch circuit protected by a 15 A or 20 A breaker typically does not run constantly with that amount of current (forget, for the moment that breakers are only supposed to be loaded to 80% of their capacity, since

80% of the sum of all breakers compared to 80% of the main breaker's rating is still relatively the same comparison!).

Certain load assumptions are made when figuring out how many devices are on a single circuit, and these assumptions are worst-case scanarios. For instance, when planning a new circuit where general receptacles and light fixtures are concerned (as oppposed to knowing exactly what will be there), I tend to use 100 Watts for lights and 200 Watts for duplex receptacles. Many receptacles in a home are there because code requires you to be no more than six feet along a wall from an outlet, so

1500 Watts worth of receptacles on a 15 A circuit (1500 Watts is a close estimate for 80% of 15 A at 120 volts!) may only end up with 200 Watts on the circuit under normal use.

Some outlets require a dedicated circuit. So a 1200 Watt refrigerator would use up one full circuit, but not be accounting for any more than

10 A when it was actually running.

Hmmm. Something I didn't have to deal with, since mine was new construction so all the transfer-switch circuits had their home runs to the transfer switch. What you are essentially doing is running a "switch loop" for each of these circuits. On the surface, this seems acceptable, but I would run it past your inspector before getting started.

The cost of your permit actually buys you a bit of a consultant. Inspectors are generally available for an hour in the morning to answer questions. Make use of this service (just don't abuse it!).

[Calvin Henry-Cotnam]

Let me correct what I said here, as I was thinking only about the non-generator operation.

You will have to run a full cable for each branch circuit that was in your main panel to the transfer panel. Both its hot and neutral must be terminated in the transfer panel.

There is part of me saying that the heavy neutral between the main panel and the transfer panel would handle the neutral current in generator mode, since it only exists when mains current is cut off, but there's something nagging me that this is not quite right.

[Calvin Henry-Cotnam]

The panel in question made by Square D is simply a small panel with eight breaker positions (4 on each leg), a neutral bus (isolated from the chassis) and a ground bus.

Power coming into one of these panels is "back-fed" through a 2-pole breaker that acts as the "main breaker" for the panel.

When the same panel is adapted as a transfer switch, it comes with two

2-pole breakers (mine was a pair of 30A breakers, but the OP mentioned a 30A and a 70A) along with a mechanical interlock that not only prevents the two from being turned on at the same time, but also ensure that there is a break-before-make action (meaning that trying to turn one on will force the other to be shut off before the first one even comes close to being on). My inspector did a close check to see that this was the case.

Since 4 positions in the 8 position panel are used up, you only have four circiuts with full-size single-pole breakers, but can double that with the use of the QOT mini breakers from Square D.