MCC Bus Sizing

[davidb123]

Gentlemen,

Would it be possible (in accordance to the NEC) to size a MCC bus below the value obtained by : 125% of largest motor FLA + all others FLA if I have the right OCPD ? Let's say :

I have a MCC that feeds : 15x 50hp + 1x 100hp = 15x65A + 1,25x124A = 1099A. However, I choose a 800A bus with a Main Fuse Disconnect Switch of 800A to protect that bus. Of course, If I do that it's because I know all the motors won't be full load at the same time. Am I violating the NEC ?

Thanks guys

 

[charlie b]

If I do that it's because I know all the motors won't be full load at the same time.

How exactly do you know that? Is there an electronic management system in place that selects which motor is on at a given time, and does that system make it impossible for more than a certain number of motors to run simultaneously? If so, then 220.60 gives you a way to reduce the calculated load below the 1099 amps. But without that fancy control system, you will need to be able to explain how the word "unlikely" applies to your installation.

 

[davidb123]

We know it by experience, system is not managed by any kind of electronic devices.

 

[Cow]

We know it by experience, system is not managed by any kind of electronic devices.

No offense, but "experience" isn't an exception to the pertinent code articles that require it to be sized as mentioned above.

If you had another existing MCC with the same loads you could put a recording meter on for a month and submit that recording with your plan review to the inspector to show your actual load is under 800 amps.

Or, like Charlie mentioned, if you had a control system in place to do load shedding over 800 amps, you could get away with it.

The way it looks right now, we'd use a couple of 600 amp MCC's or 800's if the other options didn't work out. Actually, we are doing a project right now where we went with two 600's just because we wanted the extra capacity as well as room for future expansion.

 

[don_resqcapt19]

I have never seen an industrial MCC that complied with that rule. There is a judgement made as to the actual load that would be running at any one time as compared to the connected load. There is no provision in the NEC to permit this, but it is a common practice in the industrial world.

There are lots of things in the industrial world that are done that do not comply with the rules in the NEC. There have been a number of code changes over the years to permit some of these things, with the only substantiation being..."we have always done it that way".

 

[Jraef]

Have you tried to buy that MCC yet? In the "olden days" of manual data entry and drawings, this was possible if you knew the right people with the right skills and authority to do it. But now with automated software systems configuring MCCs, it will automatically fault on this, telling you that the bus is too small or at best, just automatically increasing the bus size. When I did MCC workups, I had the authority to do what was called an "Engineering Override" to allow it, but the "kid" that replaced me does not have it (and they took my "powers" away when I changed roles). One reason they gave when I complained was that some user complained that we "allowed" this to take place, then they ended up getting tagged by an AHJ. So the solution was to not allow it to happen any longer.

 

[adamscb]

Based on my (limited) experience, I would have upsized it to 1200A, and then added room for spares, unless you were limited on physical space.

At the very least I would upsize it to 1200A. From what I've seen the price difference isn't that great. I'm not a fan of cutting corners like this to save money. Just my two cents.

 

[Jraef]

Based on my (limited) experience, I would have upsized it to 1200A, and then added room for spares, unless you were limited on physical space.

At the very least I would upsize it to 1200A. From what I've seen the price difference isn't that great. I'm not a fan of cutting corners like this to save money. Just my two cents.

Yeah, When one of those 800A fuses blow and the entire operation shuts down, the money saved on the bus bar gets eaten up in the first minute of lost production.