Sizing Calc for Expanding an existing MCC

[Howlin Dave]

I have a client with an existing 480V, 3 ph, 600A MCC, fed from an existing 600A (80% rated) breaker in a remote panel, using 2 x 350 Kcmil (2 x 310A = 620A) sets. The existing MCC is Main Lug Only (No main breaker). The largest motor is 75 HP.

I want to add a couple of MCC sections, but I think I may need more power than the code will allow.

Normally, when I've sized an MCC, I take the largest motor FLA x 1.25 then add the sum of the other motor FLAs. (per 430.24)
When I do this I get 584 Amps.
(464A + 1.25 x 96A = 584)
So I believe that my MCC is properly sized. All good right?

But when I check the MCC feeder sizing, I run into some confusion. (I assume the conductors feeding the MCC are "feeders", no?)
Article 215.2 (A)(1) feeder sizing, says two things:

First, use article 220, (I find 220.50, which sends me back to 430.24) which is all good.

Second, use 100% of non-continuous loads + 125% of continuous loads. Here I have a problem. Since my loads are all continuous I get:
(464 + 96) x 1.25 = 700A Uh-Oh.

It looks to me like the normal MCC sizing procedure in 430.24 will almost never apply to the sorts of industrial projects (continuous HVAC loads & Process loads) I typically encounter. I'll be needing to size the feeder first, because the requirements are more stringent, and then make the MCC match.

Is this so?
Am I doing these calcs correctly? Am I overlooking anything?

The kicker is that I know when the system is all running, it'll be drawing something on the order of 400 Amps. The bigger motors are on VFDs, the break horsepowers are probably less than 85%, the typical operating diversity for these facilities is around 70%. I can confidently say that this MCC will never see more than 450 Amps. But I don't have a month or a year's worth of data as required by 220.87.

I'd prefer not to put in a new feeder to a new MCC for this application, but my understanding is that the AHJ is rather strict in this location.

Any advice?

Thanks in advance,
Dave

 

[bob]

Have you checked to see if the utility can provide the max kw or kva that has occured at the plant?

 

[Howlin Dave]

Utility Info

Utility Info

Have you checked to see if the utility can provide the max kw or kva that has occured at the plant?

That info is not available.
The Utility provides power at 13.2 KV. The client owns the substation, which feeds multiple buildings on the campus. The Switchboard that feeds this MCC also feeds other loads. There's no meter on the feeder to this MCC. The existing process that is powered from this MCC is in startup now.

 

[mull982]

Second, use 100% of non-continuous loads + 125% of continuous loads. Here I have a problem. Since my loads are all continuous I get:
(464 + 96) x 1.25 = 700A Uh-Oh.

I could be wrong but I dont believe that you have to do this step since you've already accounted for 125% of your largest motor load. This step is only required for non-motor loads.

 

[buddhakii]

I agree with Mull. 420.24 states 125% of largest motor load plus sum of full load current ratings of remaining motor loads. Nothing in there about continuous, non-continuous.

 

[weressl]

I have a client with an existing 480V, 3 ph, 600A MCC, fed from an existing 600A (80% rated) breaker in a remote panel, using 2 x 350 Kcmil (2 x 310A = 620A) sets. The existing MCC is Main Lug Only (No main breaker). The largest motor is 75 HP.

I want to add a couple of MCC sections, but I think I may need more power than the code will allow.

Normally, when I've sized an MCC, I take the largest motor FLA x 1.25 then add the sum of the other motor FLAs. (per 430.24)
When I do this I get 584 Amps.
(464A + 1.25 x 96A = 584)
So I believe that my MCC is properly sized. All good right?

But when I check the MCC feeder sizing, I run into some confusion. (I assume the conductors feeding the MCC are "feeders", no?)
Article 215.2 (A)(1) feeder sizing, says two things:

First, use article 220, (I find 220.50, which sends me back to 430.24) which is all good.

Second, use 100% of non-continuous loads + 125% of continuous loads. Here I have a problem. Since my loads are all continuous I get:
(464 + 96) x 1.25 = 700A Uh-Oh.

It looks to me like the normal MCC sizing procedure in 430.24 will almost never apply to the sorts of industrial projects (continuous HVAC loads & Process loads) I typically encounter. I'll be needing to size the feeder first, because the requirements are more stringent, and then make the MCC match.

Is this so?
Am I doing these calcs correctly? Am I overlooking anything?

The kicker is that I know when the system is all running, it'll be drawing something on the order of 400 Amps. The bigger motors are on VFDs, the break horsepowers are probably less than 85%, the typical operating diversity for these facilities is around 70%. I can confidently say that this MCC will never see more than 450 Amps. But I don't have a month or a year's worth of data as required by 220.87.

I'd prefer not to put in a new feeder to a new MCC for this application, but my understanding is that the AHJ is rather strict in this location.

Any advice?

Thanks in advance,
Dave

I do believe that 430.26 also applies to MCC assemblies. Motors are seldom, if ever operate at their FLA in industrial type application and depends on the operational requirements the simultaneous operation of ALL equipment in an MCC is rare. The FPN referenced material enables you to make intelligent decision not necessarily based on hard historical data. I think this is an area where the NEC is overstepping its defined boundary of intent and purpose.

 

[Howlin Dave]

I agree that this seems sensible, but 215.2 (A)(1) appears to have 2 separate requirements. Is that the intent? I can't find a reason to ignore the sentence:
"The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an alowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."

Why doesn't this requirement apply?

 

[Howlin Dave]

I do believe that 430.26 also applies to MCC assemblies. Motors are seldom, if ever operate at their FLA in industrial type application and depends on the operational requirements the simultaneous operation of ALL equipment in an MCC is rare. The FPN referenced material enables you to make intelligent decision not necessarily based on hard historical data. I think this is an area where the NEC is overstepping its defined boundary of intent and purpose.

I agree, this makes good sense. I looked at 430.26, and its FPN.
"The AHJ MAY grant permission..." I find this less than comforting. The implication is that he MAY NOT just as easily.

 

[JWCELECTRIC]

Dave,

Check to see if you have a spare conduit between each MCC, run another set of 350 kcmil in spaqre conduit, install new lugs on exisitng MCC to handle the three sets of 350 Kcmil, then replace the 80% 600a breaker with new one sized to handle added loads for new sections along with existing loads. If there is no spare replace existing cable with larger cable as needed. I would not risk installing new loads on the existing mcc without upsizing cables and breakers, sounds like you are at max as is without added new loads.

- JWC