MCC sizing question

[Acxpert]

Looking for some guidance on an MCC sizing/capacity issue. Here is the situation:

There is an existing 600 amp MCC with several loads, all HVAC units, fans and pumps. The MCC itself is fed by a 600 A breaker and (2) #400 wires per phase.

The plan is to remove most of the equipment and replace it with 2 large package units. It appears that the panel was overloaded already, per the loads below:

Existing Equipment	HP	MCA	Fuse size	Comments
AH-1	40		80	To be demo'd
AH-2	40		80	To be demo'd
EF-1	10		30	To be demo'd
EF-2	10		30	To be demo'd
PD-1	3		8	To remain
PD-2	3		8	To remain
ACP-1	3		8	Abandoned
CU-1		117	150	To be demo'd
CU-2		117	150	To be demo'd
CU-3		117	150	To be demo'd
CU-4		117	150	To be demo'd
AC-1		41	60	To remain

Electrician is telling me that the max load we can put on the MCC is 80% or 480 amps. Is this correct?

I was hoping we can feed our new AC units out of the MCC and re-feed the smaller loads that need to remain from elsewhere. Any thoughts?

Thanks!

 

[augie47]

The 80% would factor in if you have continuous loads.
In any event be sure to look at the actual load and not the overcurrent device size.
Di you know the MCA of the units you are adding ?

 

[Acxpert]

The 80% would factor in if you have continuous loads.
In any event be sure to look at the actual load and not the overcurrent device size.
Di you know the MCA of the units you are adding ?

The MCA of new new units is 260A x2

The other equipment that I am assuming will have to be re-fed has an MCA of 41A and (2) 3HP pump motors.

Everything is 460v.

 

[petersonra]

it seems pretty common to load MCCs and switchgear above what their calculated loads would seem to indicate is appropriate.

Not real long ago we had a similar discussion and several P.E.s stated here that they routinely do so in their designs.

It is not unusual for a calculated load to far exceed the real load.

I guess I am not all that worried about it. If the MCC is overloaded something will trip to protect it.

 

[augie47]

IMO, you could proceed with the plan and be Code compliant (keeping in mind the Code is the minimum standard). The MCA has a built-in factor that gives you a number higher than the actual RLA so it's not quite as bad as it may look initially.

 

[Smart $]

it seems pretty common to load MCCs and switchgear above what their calculated loads would seem to indicate is appropriate.

Not real long ago we had a similar discussion and several P.E.s stated here that they routinely do so in their designs.

It is not unusual for a calculated load to far exceed the real load.

I guess I am not all that worried about it. If the MCC is overloaded something will trip to protect it.

IMO, you could proceed with the plan and be Code compliant (keeping in mind the Code is the minimum standard). The MCA has a built-in factor that gives you a number higher than the actual RLA so it's not quite as bad as it may look initially.

When determining the compliance of an MCC, especially with Article 440 motor(s) involved, you can get lost following all the section references. Ultimately, the best place to end up if the typical load calculation exceeds the MCC rating is...

430.26 Feeder Demand Factor. Where reduced heating of
the conductors results from motors operating on duty-cycle,
intermittently, or from all motors not operating at one time,
the authority having jurisdiction may grant permission for
feeder conductors to have an ampacity less than specified in
430.24, provided the conductors have sufficient ampacity
for the maximum load determined in accordance with the
sizes and number of motors supplied and the character of
their loads and duties.

Informational Note: Demand factors determined in the design
of new facilities can often be validated against actual
historical experience from similar installations. Refer to
ANSI/IEEE Std. 141, IEEE Recommended Practice for
Electric Power Distribution for Industrial Plants, and
ANSI/IEEE Std. 241, Recommended Practice for Electric
Power Systems in Commercial Buildings, for information
on the calculation of loads and demand factor.​

In many cases I believe the AHJ don't even bother to require or check a load calculation for an MCC because an EE is typically involved.

 

[electrofelon]

The 80% would factor in if you have continuous loads.
In any event be sure to look at the actual load and not the overcurrent device size.
Di you know the MCA of the units you are adding ?

Doesnt the continuous load factor not apply to article 430 and 440 equipment?

 

[Smart $]

Doesnt the continuous load factor not apply to article 430 and 440 equipment?

Continuous loads are always what Article 430 refers to as "other loads". With respect to "normal" load calculations for determining service and feeder ratings, only the largest [FLA] motor is factored 125% per 430.24 (disregarding 430.26 for the moment).