FrankC,
you'll need to clarify this a tad, what sort of TVSS do you have?

It installs in a 1/2 inch KO in panelboard. It is connected to a two pole 20 amp circuit breaker. The service panel is: 200Amp., 120/240V, single phase. There are three conductors on the TVSS, two black and one green.
When looking through catalogs I've often seen the same type with two black, one white and one green conductor. I guess my question is: What is the difference if any.
Frank

FrankC,
I've asked the fella's @ the ECN BB to look into the TVSS question you have. Go Here please and feel free to participate, there are many good people there that can help.

Frank

The TVSS with only three conductors is suitable for use in service equipment as the grounded conductors (neutrals) and the Equipment Grounding Conductors are all attached to the Grounding Electrode Conductor at that point. Since the neutral and EGCs are common in service equipment there is no need for two conductors and two sets of MOVS in the service equipment. In panel that was not service equipment the grounded buss and the grounded conductor buss bar would be separate and one lead to connect each to the TVSS would be needed.
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Tom

Thanks for the reply Tom. I was worried I bought the wrong TVSS for my service equipment panel. I understand what you mean. If I was installing it in a sub-panelboard with an isolated neutral I would then need the white (neutral) wire.
Frank