I've ordered a Siemens FS140 Whole House Surge Protector that I will install myself.

The instructions say to keep the leads as short as possible and straight as possible. I have an open square d slot farther away from where I want the FS140, but have a couple of 20 amp breakers already installed near where I'd want it. They feed the kitchen counters.

Is there a disadvantage to connecting the FS140 to the existing breakers (with pigtails) or does it really need its own breakers?


Response to moving it -- my two free slots are at the very top of the panel. At the bottom are 2 x 30amp solar disconnects and then 2 x 20amp tied kitchen breakers. I'm realizing now my bigger problem is the conduit underneath and beside which won't let me fasten the device to the wall. Maybe I should just put it the top, which makes it harder to see, but you can't win all the time!

Panel Open Panel Closed


Thank you for the advice. I ended up shifting breakers up to make room for the SPD breaker near the middle (where the power comes in). I couldn't get it quite next to the middle because the bonding bar was in the way, so I ended up a slot or two higher, but this allowed the wires to be straight.

I was also worried about the 1.25" OD conduit on the right being in the way, but had forgotten there is already a 1" offset built into the SPD.

Finished Project

  • Can you rearrange the panel to move the open space adjacent to the SPD, or has Mr. Snippy been through and cut all the wires to exact length to prohibit you from doing this? Also, is this panel flush mounted, surface mounted, or outdoors? Feb 18, 2020 at 3:50
  • So that gets into my other complication - mounting. The panel is surface mounted outdoors. I haven't looked inside recently but I know an electrician did some cleanup at one point of excessively long wires and tangles. I dont know yet if I could relocate the two twenties. I also realize now that conduit is going to make it not possible to wall mount. I'll add a picture.
    – rrauenza
    Feb 18, 2020 at 3:53
  • Yes, pictures of the situation would be quite helpful indeed, especially considering that the "obvious" way to mount the unit may require some extra parts compared to the norm Feb 18, 2020 at 4:02
  • @ThreePhaseEel Pictured added. I've been assuming that it wouldn't really be great to just mount it horizontally just hanging cantilevered out on the conduit nipple. Maybe vertical would be ok hanging on the bottom or sticking up at the top. Or maybe an elbow on the right side suspending it down?
    – rrauenza
    Feb 18, 2020 at 4:18
  • I have installed dozens of whole house surge protectors. The closer the protector is to the main breaker the better the performance especially if you have ground fault or arc fault breakers, if you cannot put the breakers in the 1-2 slot with short wires make sure the total length of wiring from the main to the device is shorter than the distance to the electronic breakers. The reason this is important is you want the spike dumped to ground before it can affect your electronic breakers (standard breakers are thermal magnetic) AFCI and GFCI are loaded with electronics.
    – Ed Beal
    Feb 18, 2020 at 6:08

2 Answers 2


If the options

  • A) choosing the shortest wire connections and option
  • B) connecting the SPD to its own breaker

are mutually exclusive, option A) is more important for the surge suppression, since possible capacitive or inductive couplings are minimzed.

The breaker is needed if the SPD is damaged or has been loaded with too much surge energy resulting in a long term low resistance - e.g. after a near lightning hit. The normal circuit breakers (e.g. 20A) are much too slow to cut the surge current, which may be in the range of hundreds or thousands of Ampere, but will only flow for microseconds.

A SPD should be connected as close as possible to the incoming lines (power provider) of the breaker panel or main panel with as short connections as possible. At the same time, these wires should be "lonely" wires, i.e. all other wires should have a big distance to those SPD connection wires again to minimize any couplings.

  • Not following with the lonely wires. Or are you just saying that's a reason not to share a breaker? Because if I share one, I don't see how I can isolate those other wires at all since they will be pigtailed. But at the same time, the breaker bus doesn't really seem to isolate any more than a pigtail? Or does the lower resistance on the bus make the difference?
    – rrauenza
    Feb 18, 2020 at 3:43
  • It should be a breaker that is close to the incoming lines. If a breaker is shared, the connections/pigtails should be done as close as possible to the breaker, the other lines (e.g. to the kitchen) should have a big distance. Of course, at the breaker they must be in even galvanic contact, but should not be bundled (i.e. running parallel in close contact is bad). Not the resistance of the bus is a concern, but the deflection of the wave energy as early as possible. A tsunami protection /wave breaker should also be installed close to the water front and not in the market place.
    – xeeka
    Feb 18, 2020 at 4:32

Those are Square D "HOMeline" breakers, or are supposed to be. That means they support double taps on the breakers, if the wires are copper.

The surge suppressor recommends but does not require a 20A breaker. A 30A or 40A is fine.

The surge suppressor has some indicator lights, but they're not very important. The alarm light is doubled with an alarm buzzer. The 2 green lights merely indicate correct connection. So the surge suppressor doesn't have to mount outside the panel, though it's able to to save space.

The suppressor is meant to be near the main breaker.

All that adds up to, double-tap the breaker in 11/12 (A/C breaker?) which is right next to the main breaker.

And it's time to start planning to upgrade that panel. A 20-space panel is wholly inadequate for a modern home, and double-stuff breakers are no longer allowed (because circuits need AFCI protection, though that is less important in all-metal-conduit installations). The right approach is to leave this panel here (why go to the expense of replacing a meter-main when it's unnecessary), but move circuits to one or more subpanels inside the house. So think about that for the long term.

  • Wouldn't the leads of the SPD be inadequate gauge for the AC breaker? (I think they will be 10...)
    – rrauenza
    Feb 18, 2020 at 15:47
  • @rrauenza Factory pigtails (leads) built into a device are in the jurisdiction of UL. If UL required a 20A breaker, then UL would require the instructions to say that. UL approves labeling and instructions as part and parcel of the device approval. That's why so many instructions are confusing, dated and generally awful: they can't amend the instructions without doing another UL listing. Feb 18, 2020 at 15:51
  • I used a 30A breaker since the leads are 10 gauge. Thank you!
    – rrauenza
    Feb 21, 2020 at 0:17

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