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I have a Square D QO 200 amp main panel. I want to add a Square D Homeline 150 amp panel as a sub panel (it has a 150amp main breaker).

Does Square D make an expansion lug kit for the QO panel that would allow me to tie my sub panel's service wire into the main panel after the 200 amp breaker?

I see that Siemens offers an extension kit, but I can't seem to find an equivalent for Square D. I called my local electrical supply store and they didn't know what I was talking about.

So is the only solution to add a $200 feed breaker to the main panel for Square D?

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    Spend a couple of bucks and make the new panel ALSO QO so your breakers are all compatible, rather than having incompatible Homeline breakers in the new panel.
    – Ecnerwal
    Apr 18 at 14:01
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    Lug kits at Schneider if the link works: se.com/us/en/product-range/7249-qo-load-centers/… List prices of ones that will do more than 125A are not favorable, though. Street prices might be less?
    – Ecnerwal
    Apr 18 at 14:07
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    Important related question: How much capacity do you have in your existing 200A panel? You may well have plenty of excess capacity, but you might not, depending on gas vs. electric stove/oven, water heater, dryer, etc. as well as other factors. You need a load calculation when making changes this big. It might well be that you need to go to a larger service (400A, aka 320, split to two 200A panels) to make this actually work. Apr 18 at 15:08
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    I have to support @manassehkatz-Moving2Codidact comment about a class 320 service. It means swapping out the meter base and class 320 meter and they aren't cheap, probably around $400-$500. But it means you don't have to get into your current main panel and you'll have plenty of power in the new garage (you'll be installed a 200 amp panel there). EVs are probably the future and they take a lot of power to charge, esp. if you are doing 2 at once. I don't like very high power breakers on a bus bar, the connections just aren't that good. Yeah, personal opinion, but that's me more cmts follow. Apr 18 at 15:27
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    Ran out of space. like @manassehkatz-Moving2Codidact said, you haven't provided what your current loads are, so our (and my) advice is necessarily limited. So here are some questions: Heat source for existing home? IE: resistance, heat pump, solar, mini-split? Clothes dryer, water heater, Range...? Gas or electric? Solar array? Type of heat source planned for new garage? Any A/C loads? Hot-tub? Oh....and one more thing related to previous comment: You'll need to work with the power company to make sure they can supply a class 320 system as well as to coordinate the swap. Apr 18 at 15:33

2 Answers 2

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A subfeed lug kit and the wires behind it must be protected by a breaker.

Electrical equipment and wiring must be protected by a circuit breaker.

A subfeed lug kit is Square D's name for the thing. It fits where breakers go, but has no breaker. Since your main breaker is 200A, and a subfeed lug kit has no breaker, it means a 125A subfeed lug kit is simply not usable in your panel as nothing protects it. The only subfeed lug kit you can use is 200A.

The wires beyond the lug kit are in the same situation. They too need to be behind breaker protection. You can't protect 125A wire with a 200A breaker. So not only must you use a 200A subfeed lug kit, you must use 200A wire.

  • 200A wire is 250 kcmil aluminum or 3/0 copper.
  • If your entire service is 200A, rule 310.15(B)(7) kicks in, allowing 4/0 aluminum or 2/0 copper.

So to use a subfeed lug kit, a) the lug kit must be >=200A, b) the wires must be >=200A and c) the subpanel must be >=200A.

That's perfectly allowed if you want to do it.

You would need a QO2225SL subfeed lug kit, with 200A wire to a 200A subpanel.

A Load Calculation is seriously warranted here.

That is the proper and NEC-specified procedure for determining the load on a panel or service. The Load Calculation will tell you whether the house's service can support all this stuff, and how big subpanels need to be.

Electric Vehicle charging is adjustable. So if your Load Calculation won't support 80A, you can simply change the commissioning settings on the EVSE to set the current you have available. EV charging is actually pretty sophisticated, and they've thought of everything.

There is even a way for multiple EVs to share a single current allocation, called "Share2". It dynamically allocates current according to the EVs' needs and abilities. You implement this feature by choosing EVSE's designed to work with each other via Share2.

What I would do

I'm OK with bring 200A to the sub, but the cost of the QO2225SL subfeed lug will be prohibitive.

I think I would run 2/0 aluminum wire (135A) which is the largest wire that will fit on a "QO" 80A through 125A breaker. I'd use the smallest breaker that will do the job for now, since cost is a concern. The larger wire will allow upgrading the capacity later simply by enlarging the breaker. The largest readily available QO breaker is 125A.

If cost was no object or if distance was very short, I'd run 2/0 copper, which is 175A and could actually carry an entire 200A service due to 310.15(B)(7).

I already have the HL panel mounted to the wall.

Don't get swept into the "fallacy of sunk costs", trying to force a wrong thing to work merely because you already own it. Forgive yourself the error of "buying before researching", and get the item that's right for you.

The #1 priority when selecting a subpanel is breaker spaces. Of course, the "thrifty gene" motivates us to "think cheap" and try to chintz out as much as possible, e.g. using the most el-cheapo panel on the market, HOMeline. That's a mistake. Spaces are actually cheap, the cost diff to a "plenty of spaces" panel is pretty minimal. And running out of spaces is very costly.

HOM and QO are 100% incompatible and share nothing except enclosures. So "staying with Square D brand" does you absolutely nothing unless you stay with QO, which is a costly (but quite good) industrial-tier panel.

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    When I first read your answer, I read that you said the breakers were 100% compatible, I about had a heart attack! Did some research and found they are NOT compatible. Then returned to your answer and after re-reading it, realized you said the same thing....WHEW! You almost always get it right and I seldom disagree, so I thought, "how could he have said that????" Anyway, I'll only install SD QO or CH panels these days. Apr 19 at 0:17
  • So what makes homeline Inferior to qo? I asked an electrician and sales guy and both shrugged and said they sell/install homeline and that there’s nothing wrong with it. The cost difference is so large that having compatible breakers is meaningless. And why would the sub panel need to be 200amp? I know the wire has to be sized according to it’s breaker, but from what I know running a 150amp sub panel (which has a 150 amp main breaker) on a 200 amp service line is absolutely fine. Apr 19 at 1:01
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    @PaulWieland HOM panels use tinned aluminum busbars, which is OK but not great in corrosive environs, and the contact jaw design is kinda "meh" (it's similar to what you find in other 1" breakers) QO uses tinned copper busbars with a very different jaw design that's considered more robust/reliable -- the breaker guts are basically the same Apr 19 at 3:53
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    @PaulWieland That works if the 150A panel has a main breaker. I was assuming it did not, for some reason I thought it was in the same building so it wouldn't require one. (but it's welcome to have one). HOM is also the shoemaker's son (not well supported) - took the brunt of availability problems during COVID, they don't make 60A GFCI breakers for some reason, and no 3-phase model has ever been made. Ironically, they do do a good job supporting 225A subfeed lug kits and 150 and 200A plug-on breakers. Apr 19 at 6:18
  • I initially has the same misread you did, @GeorgeAnderson, so I emphasized the text a bit to get everyone else to slow down and read it right.
    – FreeMan
    Apr 19 at 13:10
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If the subpanel is protected by a "main" circuit breaker in the sub panel the conductors are protected by overload only by that breaker. The "feeder" conductors must be sized for that panel breaker with no round up, and using the 75 degree C column for the conductors as they are terminating in a 75C device. The short circuit protection is from the "main" breaker in the main panel and as long as the feeder is bigger than the required EGC that will show protection from short circuit. Look at the code book under feeders. Can be done if requirement are met. Best and easiest to go with a 200-225A capapble panel at the end ( and correct sized conductors ) as the AHJ may not like it. All protection must be "upstream" -not true but the inspector is always right, if not see first statement.

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