I am researching adding a whole house surge protector to my load center. Specifically I am looking at a Siemens FS140, which requires a 20amp 2 pole breaker.

I do not have any empty spots to add another breaker, but I do have an existing 20amp 2 pole that is for an in-wall electric heater in my garage. The heater is very rarely used, less than 20 hours per year.

The breaker is a Square D HOM220 which supports 2 wires per pole, and the documentation for the SPD indicates that this is an acceptable configuration.

The heater wire is 12ga solid, while the SPD is 10ga stranded.

  1. Are the different gauges a problem?
  2. Is having solid and stranded on the same breaker a problem?

If it is a problem, could I pigtail some 10ga stranded to the heater wires, so that all connections to the breaker are 10ga stranded?

1 Answer 1


I would avoid changing wire types that are already to code. SPD units use stranded wire due to what can be a tight-bend radius in several places.

House/home wiring is mostly regulated by the NEC code book, while the SPD industry follows UL 1449 and ISO regulations.

Make sure to twist the bare strands and try to get them pinched under the solid wire or set-screw. Also, every foot of SPD wire length will add about 100 volts to the SPDs rated clamp voltage so keep it as close to the breaker panel as possible.

Yes, in this case where you add a SPD to existing wires you have no choice but to mix wire gauges and solid with stranded. UL 1449 3rd and 4th edition allow for this else these add-ons would not be possible. At the industrial level dedicated breakers for SPD are common, but at the residential level spare double pole breakers are rare.

  • I'm confused by your answer. Are you saying it's OK to attach the #10 wire of the SPD to the 20a breaker which is designed for #12 wire? If so, please include a code reference (not doubting you, just totally unclear, and code references make good answers great).
    – FreeMan
    Commented Feb 27 at 13:18
  • 2
    The specs for the HOM220 breaker state that AWG#10-14 is acceptable. I'm just concerned about uneven torque if I have #10 & #12 on the same screw. Each screw has 2 "channels" for wire, the wires aren't touching each other in the same channel.
    – DaveinPA
    Commented Feb 27 at 14:08
  • 1
    @DaveinPA Yes, you will need to twist the stranded wire into a tight ball so it will be crushed tight when you tighten the screw. That wire must NOT pull out after you are done.
    – Sparky256
    Commented Feb 27 at 16:27
  • 1
    The screw terminal needs to be torqued to spec, per NEC 110.3(B) and specifically in 110.14(D). Commented Feb 27 at 23:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.