508A panel needs to switch a 120v hot from outside the cabinet

3 posts in this topic

Hi from Colorado, Panel Builders @ MRPLC~ I bought a custom 508A assembly from a UL shop (277/480v 3 phase). The prototype didn't work out very well so I'm revising the design for next time. Before I present my "red line" revisions to the fabricator, could someone advise if I'm OK to simplify part of the work in a particular way? I just need to make sure my simplification doesn't run afoul of the 508A standard.

My clients requested that a ten watt 120vAC amber LED (distant from the 508A cabinet) should light up whenever the 3 phase bus is energized. The panel fabricator provided an SSR solid state relay to close whenever 277v shows up across the "input" terminals & they fused the 277v hot for the input so that's all good. The "output" terminal is "single pole normally open", rated for 10 amps. The power that I'm sending in from outside the cabinet is from a small UPS system where the inverter is rated for 250VA's--so it can't deliver more than two amps @ 120vAC. 

It seems to me that I don't need the series fuse that the shop provided to protect the 10 amp rating on the SSR--since my input power is already current limited. It also seems that I don't need to bring the associated single phase neutral wire into the 508A cabinet at all--since the neutral is never switched & it's doing nothing inside the 3 ph. assembly. If I revise the drawings along these lines, I'll eliminate two terminal blocks, a cc fuse/fuse holder & some wire...and this part of the assembly will be simpler & more intuitive. 

Basically, I want to treat the SSR like a switch leg for the light, running only a 120v hot wire to & from the SSR output terminals. I don't want to revise the drawings, however, unless I'm sure the fabricator won't have to change them back to comply with UL 508A. I don't own the UL "standard" document & I don't know the peculiarities...but I'm sure some of the forum members do. 

Should I add signage to explain that the 120v hot must be current limited or OCPD protected before it enters the 3 phase cabinet? Thanks in advance for any coaching or suggestions.




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In general, the U.S. National Electric code requires conductors to be protected from overload at the power source (external to your panel).  UL cannot waive this requirement, but might add more requirements.

More importantly, most codes require externally powered conductors to be yellow or have a yellow stripe, to signify that they are not cut by the panel's own disconnect.  And, above 60V, will need separate arc-flash treatment.

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So if I understand the circuit in question you have a Hot Wire from a 250 VA Inverter in Cabinet #1, to an SSR normally open contact in cabinet #2, to ten-watt LED in either Cabinet #3 or Cabinet #1 and the the Neutral from the 250 VA Inverter comes to the LED as well.

You will as I understand the NEC need to fuse the wire leaving the Inverter to protect it at the amp rating of either the wire or the SSR whichever is less.  So an AWG20 conductor if used is rated for 3 amps and you'd need to fuse for that.

It's not that the inverter can't produce the 10 amps that's the issue, in the event of a malfunction and one of the higher ampacity wires is shorted to your 10 amp ssr you want the fuse to blow before the SSR or the wire fails.

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