Non-Fused Disconnect Switches

Scherer

New User
Location
Brazil
Occupation
Electrical Designer
I'm starting to work on building panels that need to comply with UL508A.
My panel is designed for one or more motors and any other loads.

However, I'm struggling to understand whether the main disconnect of the panel should comply with Section 30: Disconnect Switches or Section 32: Overcurrent Protection of Feeder.

The term 'switch unit' is unfamiliar to me, but could this be considered a non-fusible switch? For example, something like the 194R-NU line?

I expanded my research to NEC Article 670.4 (B) Disconnecting Means (which seems unrelated to me), but it mentions:
"...The disconnecting means shall not be required to incorporate overcurrent protection."

Could anyone share their experience regarding the main disconnect of an industrial control panel?
 
Section 30 would apply to your main disconnect. UL508A does not dictate whether you need to have a disconnect or not, or what it needs to be or not. But IF YOU decide to include one, whether that is a breaker, non-fused disconnect or fused disconnect, or some other switching device, it's up to you and this section just tells you that it has to meet other UL listing requirements for whatever it is.

Section 32 is for feeders, meaning feeders going OUT of the panel to feed other external loads. Again, dealer's choice, but if you do it, it has to be one of the listed types.

Nothing in UL508A or the NEC dictates whether or not you must include a disconnect in your panel. The only spec that does is NFPA 79, the "Electrical Standard for Industrial Machinery", and possibly OSHA (depending on the machinery). NFPA 79 is widely adopted by a lot of industrial users, but is not "enforced" by anyone. OSHA is more of an "after the fact" requirement, meaning IF someone gets hurt and they find that a main disconnect SHOULD have been there under an OSHA rule, then the user can be found in violation (which may come back to you as the panel builder).

The NEC says that the circuit is REQUIRED to be protected but that can be the FEEDER to the panel, but then other issues come into play regarding LO/TO and whether the Feeder is within sight etc. So in my practice, I always spec a local main disconnect in a panel. Setting aside circuit breakers as a main disconnect (because they are easier in this regard), whether or not a Main Disconnect in a panel is fused or not is going to depend on other facts not in evidence, such as the requirements for the SCCR of the panel as an assembly. The trouble with non-fused disconnects is that they almost (as in I have never seen one) never have a rating of more than 10kA without fuses. So IF your upstream feeder is a fused disconnect, and the fuses in it are the ones that your main panel non-fused disconnect is listed with for a higher SCCR, you can use that in determining the SCCR for the entire panel (so long as you stipulate the upstream fuses on a sign on the panel). This to me all seems cumbersome just to save a few bucks on fuses...

Bottom line, I find it simpler and easier to just use a fused disconnect or circuit breaker as the main for a panel, because that way you don't have to know ANYTHING about any upstream devices when doing your SCCR calculations and listing.

And since we are talking about that, PLEASE, for the love of all things related to electrical contractors, do NOT just slap the "courtesy" 5kA SCCR label on your panel and expect some poor EC in the field to deal with it...
 
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