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I would like to install a SquareD HEPD80 whole house surge protector. My electrical panel has the main line feeding in from the top, and all accessible knockouts are on the bottom of the panel -- the knockouts on the side are either in use or blocked.

I understand that it is important to put the breaker with the surge protector attached as close to the line feeds as possible (i.e. in the top slot), and that it's also important to keep the wires to the surge protector as short as possible. But in this case, I can't do both, because the surge protector has to be mounted on a knockout in the bottom of the panel.

Is it better to put the breaker in the top slot and run long wires down to the surge protector? Or would it be better to put the breaker in the bottom slot and run short wires down to the surge protector?

(This is a small panel with only 6 rows of breakers, so the difference in wire length would be about 4 inches.)

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  • If you are using it for protection of the circuit breakers at the top of the buss and short wires for the house it won’t matter as the devices will be much farther away.
    – Ed Beal
    Jun 29, 2022 at 22:29
  • BTW, though an entrance-panel surge protector is an excellent idea, also use surge protectors at outlets, particularly those with valuable devices, such as televisions or computers. Nearby lightning can induce heavy currents along wires, not completely blocked by the main surge protector.
    – DrMoishe Pippik
    Jun 29, 2022 at 23:02
  • 1
    Have you considered that you can use a NEMA 4X hub or fitting with a field-punched KO to come out the upper part of the panel sidewall?
    – ThreePhaseEel
    Jun 30, 2022 at 1:12

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The bus bars in the panel will have a lower impedance to surge energy than longer wires.
So in general, rather keep the wires shorter and connect the surge protector to lower slots on the panel than extend the wires to make them reach a higher slot.

In your case where there's only 4" difference in wire length it's unlikely to make a significant difference either way.
But whatever you do, make sure you follow the instructions for the (presumably UL listed) surge protector - if it calls out a specific location then that's what you do.

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