Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Control Panel Disconnect - "Main" fuse requirement?

Control Panel Disconnect - "Main" fuse requirement?

Control Panel Disconnect - "Main" fuse requirement?


My company builds equipment for which I design the control panels. For every control panel, I always include a main disconnect. I also individually fuse every component inside the control panel.

What I have not been consistent with through the years is whether or not I include "Main" fuses just downstream of the disconnect and upstream of all the individual component fuses.

Last year was the first time I had one of my controls panels CE marked. To do this, I paid for support from a 3rd party consulting company that helped me with the requirements. They did not ask me to include main fuses, but this was a very small/simple machine in which my main "disconnect" was a power cord that plugs into a wall receptacle.

I have never gone as far as getting one of my control panels officially UL listed, although I do my best to follow recommended practices.

I am now designing a control panel that will require 240 VAC 3 phase 80 amp service. No individual component within this panel will require more than 30 amps. I have a space constraint that will make it beneficial if I can minimize my panel size to whatever extent is possible. I know that the 80 amp service will come from a breaker panel that will be close to the machine / control panel location. As we will have 80 amp breakers in this panel, it seems redundant to also have 80 amp fuses inside the control panel. Unless these Main fuses are required by applicable codes, I personally do not think they would add any benefit that would justify the panel space they would require. This being said, I don't want to make this decision if it would violate any applicable codes.

This particular machine / control panel will be installed in the USA. Any advice or suggestions will be greatly appreciated.

Thanks and best regards,

RE: Control Panel Disconnect - "Main" fuse requirement?

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

RE: Control Panel Disconnect - "Main" fuse requirement?

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

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

RE: Control Panel Disconnect - "Main" fuse requirement?

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,

RE: Control Panel Disconnect - "Main" fuse requirement?

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

RE: Control Panel Disconnect - "Main" fuse requirement?

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 - http://www.flaminsystems.com

RE: Control Panel Disconnect - "Main" fuse requirement?

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

RE: Control Panel Disconnect - "Main" fuse requirement?

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,

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close