Industrial Control Panel vs Motor Control Center

[xguard]

Currently we have cabinets that contain relays, breakers, starters, etc, that aren't built with NEC Article 409 in mind. They may meet some or all of the requirements, but not necessarily on purpose. These are existing old installations, going forward I'm investigating what the new installations should be.

I'm curious if at minimum, they should be built in accordance with Article 409.

and also,

If it would be better, potentially safer, to use a combination of a Motor Control Center with a separate controls panel. From a safety standpoint I'm wondering if arc flash potential or risk could be mitigated with individual buckets vs multliple starters and control equipment in one enclosure. Also the manual operation of the starters is sometimes necessary for maintenance, the start/stop on MCC buckets seems useful in that regard as well.

Any guidance would be greatly appreciated; recommended reading, code articles, IEEE standards, OSHA, etc.

Thanks

 

[tom baker]

I don't see Art 409 referenced at all in job specs, but rather UL508A.
The advantage of an MCC is it can be less expensive, as its more of a commodity item and more competitive.
What I often see is an MCC modified under UL 508A.
I prefer an MCC as it can up easily updated

 

[Jraef]

As with most things, it depends on the circumstances.

1. An MCC gets you the ability to do LO/TO on individual motors without exposing the workers to arc flash hazards by opening a door. If you have a control panel with multiple combination starters inside, you must kill power to the entire panel in order to then turn off one unit, lock it out, then close the door and re-energize. The only alternative is for a "qualified electrical worker" to suit up to the available arc flash hazard in order to open the panel live. But if a control panel serves one machine or group of motors that must all operate together, shutting off power to work on only one of them becomes irrelevant since they all must shut down anyway. So that benefit of an MCC might be wasted.

2. MCCs provide a distributed power infeed using factory built bus bars, which if there are lots of motors controllers, can significantly reduce you installation time. It also makes future expansion very easy (if planned for properly). But if you have only 1-5 motors on a machine, an MCC might be too expensive.

3. MCCs tend to offer higher SCCR (Short Circuit Current Rating) capabilities than you might be able to attain with a control panel; 65kA is now common in MCCs, 100kA is possible in most cases. A good panel shop who knows what they are doing can usually get to 65kA, so long as they use series listed components, which means everything from the same mfr. But the reality is that a lot of panel builders don't pay enough attention to that and buy the cheapest parts they can find. Then you end up with an SCCR of only 5kA, making it all but impossible to connect it in an industrial setting.

To that point, Article 409 is in the NEC, so if an AHJ judges your equipment to be "industrial", you have no choice but to follow it. But 409 is light on details, preferring to defer to NRTL listing for that (NRTL = Nationally Recognized Test Lab, i.e. UL, ETL etc.). It does so by requiring the SCCR, which you get by using something that has been tested and listed. If you do it on your own, the documentation you may need to produce for that issue can be onorous. Many states however will require an NRTL listing of control panels anyway.

 

[petersonra]

The last two or three years I have found it fairly easy to get to 65k a short circuit current rating in an industrial control panel. It isn't even a lot more expensive anymore. You just have to know what to do and how to do it.

MCCS make more sense when you have Motors that are not associated with other more Motors. As another poster mentioned if you have a machine or you shutdown a motor and it stops the machine as a whole it really doesn't matter whether you put the motor starters in an industrial control panel or motor control center.

Cost-wise an industrial control panel with the same short circuit current rating can be half the price of an MCC. That is a big inducement. The MCC usually takes a lot more space too.

Another issue with MCCS is that they don't deal with vfds very well. None of the designs from the major manufacturers is very well-thought-out and unfortunately you can only put the MCC manufacturers drives in. If you want to use Allen Bradley motor starters and Siemens drives you're not going to get them in the same MCC.

The real answer to dealing with article 409 in my opinion is to specify industrial control panels that are listed to ul508a. It is not that big a deal to do so and the additional cost is not that big a deal either. Just make sure you tell them what short circuit current rating you need.

I would also point out that if you just have a box with some relays and other control components in it you don't need a short circuit current rating because short circuit current ratings only apply to power circuits and not control circuits. This seems to be a major issue with some people how to get all excited when their ul-listed control panel shows up in the short circuit current rating says n / a.

By the way, you can buy an industrial enclosure that is the same form factor as an Allen Bradley MCC and bolts on to the end of the MCC that you can use as the enclosure for an industrial control panel. These are industrial enclosures listed by UL.