Control Panel Disconnect - "Main" fuse requirement? 4

Follow UL508A. That's what you'd have to follow if you wanted to UL mark the panel.

Can you use a breaker as the main disconnect in your panel?

Bill
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Ohm's law
Not just a good idea;
It's the LAW!

Hi Bill and Lionel,

Thank you for your responses.

I was planning to include a local (lockable) disconnect in my panel. My hope when I posted my question was to find out if it is acceptable for me not to have "main" fuses inside my panel between the disconnect and my individual component fuses.

To me, it seems reasonable that this could be justified by the facts that (1) all individual components inside my control panel will be protected by dedicated fuses and (2) the panel itself would be protected by 80 amp breakers located outside of the control panel.

Considering that I will have the 80 amp breakers outside my panel (upstream of my disconnect), I can't think of a reason why I would need 80 amp fuses inside my control panel (downstream of my disconnect).

When you asked "Can you use a breaker as the main disconnect in your panel?", I am not sure if you are asking if I might be able to forget about the local (lockable) disconnect and rely on an external circuit breaker as the main disconnect for my panel. This would save me even more space in my panel, but I'd wonder if for service purposes, it might be considered to be a safety issue not having a lockable disconnect on the control panel itself.

I will not be required to UL mark this control panel, but I do try to follow good practices that satisfy as many standards (UL, CE, etc) as possible. I do not presently have a copy of UL508A. The $716 price tag (at least on the UL standards sales site I found) will be tough on my budget at the current time, but this sounds like a purchase I should plan for.

Thanks again and best regards,
Paul

Without knowing the details it's hard to say for certain, but you probably don't need the main protection.

Fuses are generally required if you're trying to meet a fault current rating. This is because fuses can limit fault current unlike breakers. If you're not specifically trying to do that then no you don't need fuses AND breakers in series.

I've lately found disconnect enclosures to be a royal PITA with the dozen or so components that all have to precisely match your enclosure dimensions and then constrains you to specific disconnects or breakers. Specing just that wad of junk can take almost as much time as the rest of the panel and it can be the most expensive part of the build AND be fraught with availability issues. I've taken to using a simple stand-alone disconnect screwed to the enclosure in lue of a 'disconnect enclosure'. Much less expensive in time and money.

Keith Cress
kcress -

To Keith's point, even if your panel is not UL listed, it must still meet the requirements of the NEC, which has an entire section, Article 409, dealing with requirements for "Industrial Control Panels". Within that article is a requirement for your panel to have a label stating the Short Circuit Current Rating (SCCR) of the entire panel, which for the installer, must be equal to or greater than the Available Fault Current at the installation site. UL listing is one of the possible methods of determining the SCCR, but in general if you don't use fuses, you will run into problems with coordinating all of the various components in the power circuit. MOST power components will be UL listed with fuses though, so including the fuses will make that easier for you.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

Hi Keith, JRaef, and Lionel,

Thank you for your responses.

Keith's point about using a standalone disconnect screwed to the side of the enclosure is very interesting to me. I agree that it is a pain selecting and finding space for the components required to assemble a "built-in" disconnect for the enclosure.

For the enclosure I am trying to design now, I would require 80 amp 240 VAC 3 phase input power. What led me to post my original question was uncertainty as to whether I should include 80 amp "Main" fuses inside my panel. I'd have to choose either a door mounted or side mounted disconnect, and decide whether to get a fusible disconnect or to use a non-fusible disconnect with separate fuse holders. This can take up quite a bit of space inside the enclosure, likely forcing me to use a larger enclosure than I would otherwise need. I feel that keeping my enclosure size as small as possible is an advantage when it comes time for installation, as I am often faced with space constraints. I think using a smaller enclosure and screwing a stand-alone disconnect to its side, or even mounting a stand-alone disconnect on a nearby wall (if acceptable) seems like it would give me some much desired flexibility during installation.

This thought leads me to a question about JRaef's point about SCCR. On this project, I will not be required to state a SCCR, although I have been required to do this on one other project (where the equipment was installed in California). I did some research on how to calculate SCCR and found it to be confusing. I was not entirely confident that I did it correctly, but the electrical inspector did accept it.

For the future, I would like to make determining the SCCR as simple as possible. If having main fuses would simplify this, my next question is to whether I should expect any complications related to whether the main fuses are (1) inside my enclosure, (2) inside a stand-alone disconnect enclosure screwed to the side of my enclosure, or (3) mounted to a wall in close proximity to my enclosure.

Any advice or suggestions will be much appreciated.

Thanks again and best regards,
Paul