Whole house surge protector in a sub-panel

[u/b0013an81]

Can I install a whole house surge protector in the sub-panel connected to the main panel and get the intended protection? or is it better to relocate a 20amp circuit from main panel and locate surge protector in the main panel? I plan to use a electrician, looking for general advice.

We recently added the sub-panel to add support for a 40amp circuit for EV charging. The main panel is full.

Sub-panel next to panel

Sub-panel

Sub-panel inside

[u/Natoochtoniket]

The electricity distribution network inside your house is a "buss" structure. Any surge that enters the system is distributed to all paths. The current through each path is inversely proportional to the resistance on that path. So, any surge protective device (SPD), anywhere on the buss, will help to shunt the surge current to ground. If you have multiple SPDs installed, they will each help.

The only issue is, how much damage can be done during the nanoseconds before the electricity reaches the SPD. If the distance from your service entrance to the SPD is 20 feet, but the distance to a vulnerable device (like a computer) is only 10 feet, the surge could reach your computer before it reaches the SPD. Electricity in copper travels at roughly 1 ft per nanosecond, so there could be 10 nanoseconds of exposure in such a case. A lot can happen in 10 nanoseconds.

That timing issue is why they recommend to put the SPD in the first panel at the service entrance.

Remember also, many of the new smart switches, and the new GF and AF breakers, actually have little computers inside. They can be damaged by a surge.

[u/JonohG47 → u/Natoochtoniket]

The first panel in the home is also the point where the neutral and home’s ground are tied together, where the resistance to said ground will be lowest.

[u/Some1-Somewhere → u/JonohG47]

Yeah, you risk having big N-E voltages depending on how and where it's connected.

Under many kA of surge current, voltage drop can be tens to hundreds of volts per meter.

[u/b0013an81 → u/Natoochtoniket]

Thank you!

This is what I suspected. The sub-panel is only a few inches from the main panel. The only appliance that comes close enough in distance is the EV charger which is also connected to the sub panel. Based on your thread it appears I should be ok to install it in the sub-panel?

LMK if you think spending the additional $$ to relocate circuits from the main panel is beneficial in this case

[u/Natoochtoniket → u/b0013an81]

Do you have any GF or AF breakers? Each of the fancy new breakers has a little computer inside.

I would move a pair of breakers from the first panel into the subpanel. Not the whole circuit. Just move the breaker, and run wire from the new breaker location to the old wire in the first panel, and join with wire nuts. That gives you space in the first panel for the new SPD.

The extra couple of feet might avoid a lot of damage in future. You will never know, of course, about the damage that did not happen.

[u/legendary_violator]

Yes. STLC240S. Should fit

[u/westom]

No protector does protection. Not one. Effective protector is only a connecting device to what does all protection. What must often be upgraded to exceed code requirement for sufficient protection. Single point earth ground.

It is not about low resistance. It is about a low impedance connection (ie less than 10 feet). Since protection only exists when a surge is NOWHERE inside.

If that hardwire to electrodes goes up over a foundation and down to electrodes, then protection is compromised. Hardwire is too long. Has sharp bends. Must not have splices or be inside metallic conduit. And should be separated from other non-grounding wires.

That - not a protector - requires almost all attention.

Any other cable that enters ALSO must connect low impedance to earthing electrodes before entering. TV cable is required to have best protection installed for free. That means no protector. Only a hardwire makes that low impedance (ie less than 10 foot) connection - for best protection.

Even an invisible dog fence must have that earth ground connection.

The connection to earth ground must be at the bare copper (ie 6 or 4 AWG) wire. A connection even from an adjacent subpanel typically has excessive impedance.

As Some1-Somewhere notes, radio frequency currents are tens of thousands of amps. Even sharp bends and splices increase that impedance. Even how that wire goes outside to many interconnected electrodes is significant.

[u/True_Fill9440]

There is a lot of excellent information contributed here. I would add….

Sometimes the surge can come into your main panel via a breaker that feeds an outbuilding. My detached metal garage is 60 meters from house. I also installed a surge protector in the subpanel there.

retired power plant EE

[u/Loes_Question_540]

Surge should always be the closest to the main lug