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What NEC /Engineering guidance applies to cascading TVSS's

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Installer

Senior Member
Location
DesMoines, Washington
I'm installing a Sub Panel off of an existing Panel with a TVSS. I'm trying to locate engineering guidance for installing a TVSS on the subpanel.
I can't find anything in he NEC or any of my Engineering Handbooks.
Does such guidance exist?
The diagram below illustrates my concerns. In one case, the subpanel is only 5' away and in the other case the subpanel is 175' away. I don't think a panel 5;' away would require a TVSS.
Can anyone point me in the correct direction? Also does TVSS coordination apply here?
TVSS Question Mike Holt.png Thank you in Advance
Thank you in Advance
 
J

jim dungar

Moderator
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Location
Wisconsin
Occupation
PE (Retired) - Power Systems
This is a design decision. You may want to get assistance from a manufacturer's engineering team, not just a salesperson.
 
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GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
The 175' run definitely argues for a TVSS at the second panel. The protection at the primary panel is too far away in wire distance (impedance and propagation time) to be well protected against a transient originating other than at or upstream from the main panel.
The 5' distance probably needs engineering input With all circuit conductors in one metallic conduit or raceway, the justification for it is indeed reduced.
For the primary TVSS, lead length differences of inches can be significant for high frequency, such as lightning induced, transients. But as long as the transient does not originate downstream of the secondary panel, you are probably OK.

Coordination, and indeed placement of TVSS is not addressed in NEC. The only real coordination issue is the degree to which smaller TVSS downstream of the main need to be able to withstand large transients. It is more of an issue for point-of-use TVSS, as in power strips or at outlets.
 
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