I found this curious item in my main load center (circuit breaker box), marked "SQUARE D COMPANY / SECONDARY SURGE ARRESTER / J9200-10". It has been there for at least 20 years, attached to the ground/neutral bar and the load side of the main breakers, and dangling free inside the load center. At Schneider's site (apparently bought Square D), "the J9200-10 was a 175Vac to neutral/ground maximum, single-phase arrestor". So I assume it was installed in my panel with the intention of offering whole-house surge protection.

enter image description here


  1. Should I keep it installed ?

  2. Should I replace it, since it's so old, that is, do surge arrestors "wear out" ?

  3. Not visible in this image, but it has a round threaded protrusion and locknut, obviously meant to be installed in a 1/2" knockout. Seems like the body of the thing is meant to be on the outside of an enclosure (as opposed to the inside), since loose wires come out the other side. Perhaps it was simply left dangling inside the box due to laziness. Should I install it in a knockout (of the load center) ? I imagine this is a metal-oxide varistor (MOV) and these things dissipate surges as heat, which gives me pause in having it outside any enclosure.

  4. If I install it in a knockout (with the body outside), the wires will not reach. Is it ok to extend them with soldered THWN and heat-shrink insulation ? I prefer to avoid the clutter of 3 add'l wire nuts. I believe solder is considered satisfactory electrically, but not mechanically; but since the solder joint and both ends of wire are within the same enclosure, I don't think the mechanical thing is of concern.

Here is the backside of it.

enter image description here

  • What is the conductivity of solder as compared to copper? Jul 8, 2020 at 21:16
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    The resistivity of copper is 1.68e−8 ohm-meters. For various solders it's approximately 1.3e-7 ohm-meters (alasir.com/reference/solder_alloys) so approximately 10X that of copper. Given the size of the joint, I'm not very concerned though. Let's model it as an annular cylinder 5mm long, 5mm circumference, and 1mm wall thickness (very conservative assumptions); so the area and thickness are 2.5e-5 and 1e-3 meters, respectively, and the resistance of the joint is somewhere in the neighborhood of 10^-5 ohms (if I got my math right). Jul 8, 2020 at 21:46
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    I did not see the 2nd photo when I answered. You can test with a megger if it shows open with a standard ohm meter that is normal from each leg to ground with no power of course. Using a megger above the breakdown voltage it should conduct if it doesn’t it is blown.
    – Ed Beal
    Jul 8, 2020 at 23:06
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    The SOVs I use have a knockout connection, but instructions suggest installing them inside a box to contain any shrapnel / flambé from too much of a strike.
    – Ecnerwal
    Jul 9, 2020 at 0:08
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    @Harper-ReinstateMonica IIRC, the reason solder is not allowed has nothing to do with its conductivity and everything to do with its mechanical properties.
    – user253751
    Jul 9, 2020 at 23:23

4 Answers 4


They are intended to be mounted outside the box...

You are correct that they are intended to be mounted to a KO (usually on the breaker box), instead of being left inside, all flop-a-dop, as yours was. You can use this unit still, although you will probably want to leave some space around it, as its further lifespan is unknown. (All MOV-based suppressors have a finite life; a MOV that's seen too much will break down under normal mains voltage, which on good suppressors will cause a thermal fuse to blow and take it out of the picture.)

...and not double-tapped either

While you're in there, you'll want to clean up the "double tap" on your main breaker's lugs and move the surge suppressor wiring to its own two-pole breaker of an appropriate ampere rating, or use wirenuts and appropriate-gauge pigtails to add it to an existing 240V circuit of a suitable ampere rating. Note that Siemens breakers (unlike Square-D QO and HOM and Eaton CH) are not rated for two wires under a branch breaker lug, by the way.

  • Per my earlier question about possibly adding a subpanel, or replacing my panel with a bigger one, I simply do not have space to add a two-pole breaker for the suppressor. So it's either delete it, or leave it connected to the load-side lugs on the main breaker. Jul 9, 2020 at 18:50
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    @RustyShackleford, there's a third option, assuming your panel currently has a two pole of the proper rating: add it into one of your current circuits. Just wirenut it with the wire currently going to a breaker, with a pigtail to the breaker.
    – Nate S.
    Jul 9, 2020 at 19:59
  • So even as @ThreePhaseEel points out that Siemens doesn't allow 2 wires on a branch-circuit breaker's lugs, it's always ok to wirenut wires inside the box before the lugs. Ok, that's a good solution. I have double-pole breakers at 30, 40, and 50 amps. Probably best to use one of the 50s, but I can't find any guidance online about this particular unit. And I guess whichever circuit's breaker I piggyback onto is going to get a little extra protection from surges, if the surge pops that breaker, even though at that point, the surge arrestor is no longer helping the rest of the panel. Jul 9, 2020 at 21:42
  • Googling, I came across forum.nachi.org/t/secondary-surge-protector-for-square-d-box/… and the last post quotes the install instructions as saying it should be connected to the "service cables" and that it can be suspended by its leads (yikes !). Jul 10, 2020 at 4:40
  • @RustyShackleford -- yeah, the install instructions for older Type I or II SPDs are a bit scary Jul 10, 2020 at 11:46

Attached to the load side of the main breakers

How? A special mini-lug just for the purpose? It sounds like it was double tapped on lugs not made for that. That is a common problem with how people install these things.

Instructions usually say to put them on a breaker.

That makes physical installation make more sense.

You can double tap some branch circuit breakers.

  • 1
    There are brackets that connect the busbars to main breaker - I can't quite see how they attach to the busbars (top breakers are in the way) but it's a standard Siemens main-breaker load center, so nothing weird there. The brackets attach to the breaker with screws. They loosened the screws and slipped male spade connectors under there. So yeah, half-assed. Jul 8, 2020 at 22:03

That needs to be enclosed, modern ones have LED’s to show when they blow. If there are any dark areas on the case I would replace it they don’t really wear out but when a large spike hits they dump the spike to ground when they take two large of a hit they overheat and many times burn up, So if there are dark areas replace. If you live in an area with clean power it may be ok.

  • No dark areas visible. Jul 8, 2020 at 20:56
  • I don't understand how they intended for it to be enclosed. It is clearly meant to be installed in a 1/2" knockout. If it's installed with the body on the inside side of the k/o, then there are loose wires (not romex and no provision for transitioning to conduit) on the outside. Jul 8, 2020 at 20:58
  • @rusty you’d have to read the instructions. Maybe your found install method is legit. Jul 8, 2020 at 21:29
  • Looking at the instructions for the Eaton one linked in the answer below (which has a comparable threaded protrusion clearly meant for a k/o), they appear to intend it to be mounted on the outside of the panel - scroll down to p.3 of images.homedepot-static.com/catalog/pdfImages/ec/…. They also mean for it to be attached to a breaker, and not directly the load side of main breaker (i.e. to the busbars) as mine is; that doesn't seem so important if wires never exit the load center. Jul 8, 2020 at 21:59
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    Rusty the original photo did not show the threaded shaft, some early ones were required to be enclosed that one can be mounted on the box. you can test that with a megger. meggers are very low current high voltage ohm meters some with a crank will show that as a short cranks are usually 1kv , I have a digital that I can set the voltage , 100, 250,500,1000 I would test that at 100 it should show high resistance each leg to ground, then test it at 200 it should break down and show low resistance to ground possibly bump to 500 if 200 doesn’t break down if it shows low it is still functional.
    – Ed Beal
    Jul 8, 2020 at 23:01

This is a modern one. I imagine the threaded connector is similar? https://smile.amazon.com/gp/product/B00VWGY3YK/ref=pe_2602560_258877390_em_1p_0_ti

I'd probably replace the one you have with the CHSPT2MAX so that you have the lifetime warranty on anything that blows. Some competitors only had the warranty for a number of years - you could look into the square D device warranty but since they were acquired I wonder even if the warranty would still be valid.

The Eaton Cutler Hammer CHSPT2MAX Surge Trap warranties $50,000 for lifetime limited (on both the product and anything connected to the protected panel) which handles 72k Amps.

I have one installed where the wires from the panel enter a square junction box that is located above the device and the threaded connector uses a knockout in the junction box. I'd probably opt for a similar setup so you can replace it in the event it fails without having to mess with the panel.

  • 1
    Yep, in the 4th image over (in the Amazon link), you can see a threaded protrusion, with locknut, and the wires coming out of it (I just added a photo of mine to OP). Sorry, but I'm confused about how you installed yours. You added an add'l junction box to enclose the suppressor ? If it's inside that box, how do the wires get from there to the main panel, without being un-enclosed at some point ? Jul 8, 2020 at 21:28
  • The junction box is on the wall with a cover plate. The device sits below it such that the face of the device is flush with drywall that surrounds the device. Jul 8, 2020 at 21:29
  • Ok, so the device itself in un-enclosed, as I surmise mine is meant to be. I'm ok with it on the outside of my main panel, because the wall around the panel is not enclosed. Of course, when AHJ comes to sign off on my PV installation, he may object to this. Didn't object when house was first inspected though, and it is in a utility room. Jul 8, 2020 at 21:50

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