Bought a house built in the 1970’s in a 1970’s neighborhood. It has a 200AMP main panel. A serviceman replacing 1 of the 2 mainboards in the refrigerator suggested that I install an SPD inline in each circuit that has electronics because he’s had very many calls to replace electronics in this locale due to many transient spikes and power surges. He considers our power supply “dirty”.

It would be easy to add an SPD to the service panel. It’s possible with some rearranging to add (depending on model) the 220V 20, 30 or 50AMP breaker required to install a Type 2 SPD that will cover the whole house. I’m curious why the device requires this overcurrent protection. Thanks.

  • Thanks. Really the the Types are defined in the NEC 2014 and its recommendations apply for all users of these Type devices - that makes it a more general EE design question, I thought, which applies to the installation design for any user, including industrial, commercial or residential. To migrate, if necessary, is there a method using stackexchange, or is it a copy & paste affair? Would it be more appropriate to keep it here and remove my context? Jul 19, 2019 at 13:15
  • I thought the breaker was just an approved way of attaching it to the electric panel. I didn't think it was needed to perform over-current protection. This model connects to the breaker with 12g stranded wire, but calls for a 50A breaker. A 50A breaker isn't going to protect those wires.
    – JPhi1618
    Jul 19, 2019 at 17:46

2 Answers 2


The overcurrent protection is for when the SPD itself evenutally fails — failing shorted is a fairly common occurance.

  • Failing shorted when the MOV’s essentially wear out or are consumed? And that’s the design intention for SPD’s...maybe to indicate end-of-useful-life, if there is no other signal (some have LED’s+/or audible alarms)? Jul 19, 2019 at 13:53
  • Citation on this would be nice. The models that I have seen do have onboard LEDs so you know its working and I don't think they would be designed in a way to produce a hard short when they fail.
    – JPhi1618
    Jul 19, 2019 at 17:48
  • 1
    @JPhi1618 the underlying MOV wearout failure mode is a short. Normally, well-designed units have internal thermal fuses that blow when the MOVs start to conduct continuously from wearout; the breaker is backup protection in case the thermal fuses don't do their job for whatever reason. Jul 19, 2019 at 23:36
  • Good idea to have the circuit breaker for protection, I like belt & suspenders approach for hardwired, unattended devices. Jul 25, 2019 at 17:08

First, hooking up something with no over-current protection at all is right out. It has to go on a breaker.

That said, it is not critical that it be a dedicated breaker. They also tell you it needs to be in the top left (nearest the main) breaker position, but again, that's only ideal, it's not critical.

I simply install it on existing branch circuit breakers, and I double-tap them because Pushmatic breakers allow that. You can wire-nut to a pigtail. Also I don't bother handle-tying if the SPD is going in the main panel, because the cross-connection is obvious to anyone servicing the device.

It is important to NOT place an SPD on a GFCI or AFCI breaker. It is also important to not put a refrigerator on GFCI protection.

No need to install at the main panel — It may be marketed as a whole house surge protector, but that does not need to be installed in the main panel. Wire length has a small spike-attenuating effect, which is why they want it in the main panel. However if you are using multiple SPDs to protect (or suppress) various appliances, the closer you are to the appliance, the better.

Keep in mind your appliance may be a creator of noise and spikes; they don't all come from outside your house. In fact, transformers are fairly good at attenuating spikes, so unless there's industry on your transformer, I would expect most spikes to come from yours and other residences.

  • Bought a Siemans FS140. Going to install very close to the main panel. I’ll be able to use the necessary circuit breakers Siemans specifies - a two pole 30A breake. There’s another question: I read that the best installation was with a separate breaker for each leg so if one breaker trips there is protection remaining on the second leg...that is different than Siemas’ spec. though. Jul 25, 2019 at 17:22

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.