# MCA and MOCP



## Pete m. (Nov 19, 2011)

Looking at a set of plans that includes the installation of a new RTU. The plans indicate that the unit has a MCA of 17 amps and a MOCP of 19 amps.

What size breaker or fuse would you use?:blink:

Pete


----------



## macmikeman (Jan 23, 2007)

Pete m. said:


> Looking at a set of plans that includes the installation of a new RTU. The plans indicate that the unit has a MCA of 17 amps and a MOCP of 19 amps.
> 
> What size breaker or fuse would you use?:blink:
> 
> Pete


The one on the nameplate.


----------



## AK_sparky (Aug 13, 2013)

Mocp=19a


----------



## bkmichael65 (Mar 25, 2013)

Maybe you should ask the inspector if he'll approve using the next higher size breaker? :whistling2:


----------



## wildleg (Apr 12, 2009)

I'd send an RFI asking for verification of the numbers, or get the RTU info (and qualify the bid), before giving them the bad news.


----------



## Pete m. (Nov 19, 2011)

bkmichael65 said:


> Maybe you should ask the inspector if he'll approve using the next higher size breaker? :whistling2:


Sorry, should have clarified... I will be the inspector but right now I'm just doing the plan review.

And, yes, a call to the engineer is in order.

Pete


----------



## btharmy (Jan 17, 2009)

Misprint? Check the unit. What the plans say doesn't leave any room for a standard size breaker to be installed legally. More than 17 amps but less than 19 amps? There is no such beast as far as I know.


----------



## Wirenuting (Sep 12, 2010)

If it's a Carrier or Trane we go the next size up. 
They've written the same things for some of our newer units so we've called. 
But since we don't have or give warranties, get it in writing.


----------



## Kunolop (Feb 9, 2013)

We have come across this a lot over the past couple years. Most of the new RTU's have lots of little motors that cannot start at the same time as opposed to one or two bigger motors that can start at the same time. This happens even more at the higher voltages (480 or 600v). The way MCA is calculated is the largest motor at 125% plus all other smaller motors or loads, and MOCP is the largest motor at 225% plus all other smaller motors or loads. If the largest motor is only a couple amps and you have 6 other motors that are just a bit smaller then you would have a situation like this. The correct wire size is 12awg minimum on a 20a breaker in my opinion.


----------



## bobelectric (Feb 24, 2007)

Then you get into all the temperature deratings for conductors in conduit on roof applications.​


----------



## don_resqcapt19 (Jul 18, 2010)

You can't use a 20 amp breaker without creating a code violation. Last time I checked 20 was more than 19. 

If the maximum OCPD is 19, you have to find a 19 amp fuse or breaker or go to a size less than 19. I know that they make 17.5 amp fuses.

Apparently the manufacturer's electrical engineers really don't have any idea of what the information on the nameplate means to the installer.


----------



## Cow (Jan 16, 2008)

don_resqcapt19 said:


> Apparently the manufacturer's electrical engineers really don't have any idea of what the information on the nameplate means to the installer.


I didn't see any mention of manufacturers plans, I can only assume it's the building plans calling out the 19 amp MOCP. And since we know prints are never wrong....


----------



## Kunolop (Feb 9, 2013)

don_resqcapt19 said:


> You can't use a 20 amp breaker without creating a code violation. Last time I checked 20 was more than 19. If the maximum OCPD is 19, you have to find a 19 amp fuse or breaker or go to a size less than 19. I know that they make 17.5 amp fuses. Apparently the manufacturer's electrical engineers really don't have any idea of what the information on the nameplate means to the installer.


In this case you would use a 20amp breaker even though it is larger than 19amps. See link below describing the MOP calculation and interpretation.

http://www.apcmedia.com/salestools/KSIH-6Y6UU5/KSIH-6Y6UU5_R0_EN.pdf


----------



## Pete m. (Nov 19, 2011)

Kunolop said:


> In this case you would use a 20amp breaker even though it is larger than 19amps. See link below describing the MOP calculation and interpretation.
> 
> http://www.apcmedia.com/salestools/KSIH-6Y6UU5/KSIH-6Y6UU5_R0_EN.pdf


Perhaps in your jurisdiction you can install an OCPD that exceeds the nameplate values.

The NEC will not permit this as Don stated in his post.

Pete


----------



## Dennis Alwon (May 9, 2009)

I have never seen an a/c unit with a maximum overcurrent protective device that didn't match a standard size breaker. Now the RTU 's may be different but I doubt that info is correct


----------



## oliquir (Jan 13, 2011)

to be legal i would just put a 15 amp breaker with 20ga wire, if it trips, not my problem... :laughing:


----------



## Pete m. (Nov 19, 2011)

Dennis Alwon said:


> I have never seen an a/c unit with a maximum overcurrent protective device that didn't match a standard size breaker. Now the RTU 's may be different but I doubt that info is correct


I'm with you. I haven't seen either one with a MOCP that didn't match a standard size.

I've called the engineer and he is going to look into it. Initially I thought it was a typo but it's the same on the mechanical drawings as it is on the electrical.

Pete


----------



## NJWVUGrad (May 12, 2011)

Pete m. said:


> I'm with you. I haven't seen either one with a MOCP that didn't match a standard size.
> 
> I've called the engineer and he is going to look into it. Initially I thought it was a typo but it's the same on the mechanical drawings as it is on the electrical.
> 
> Pete


What did the Electrical Engineer call out in the panel schedule? When I do my power plans I get all my elec data for the units from the mechanical sheets and use that for filling out my panel schedules.

I usually have to do a bit of interpretation based on the manufacturer. For example, just had a Daiken unit with a unit with a min & max OCPD?

In this case, any semi-competant engineer/designer would not put 19A breaker on a panel schedule, but if it was a junior drafter, etc it happens.

Either a typo by the mech drafter that got carried through, or a goofy spec that didn't get any attention from the electrical designer. 

It likely should be 20A, but I would flag it to the contractor and have him submit an RFI to the engineer.


----------



## Pete m. (Nov 19, 2011)

NJWVUGrad said:


> What did the Electrical Engineer call out in the panel schedule? When I do my power plans I get all my elec data for the units from the mechanical sheets and use that for filling out my panel schedules.
> 
> I usually have to do a bit of interpretation based on the manufacturer. For example, just had a Daiken unit with a unit with a min & max OCPD?
> 
> ...


The panel schedule indicates a 3 pole 20. I spoke with the engineer earlier today and he is going to effort an answer and submit it as a revision.

He also had an issue with 250.122(B) for one of the feeders he specified.

Pete


----------



## aftershockews (Dec 22, 2012)

I always love it when I request the MCA on a condensor unit from the HVAC co and they come back and say " It's a # ton".


----------



## Roger123 (Sep 23, 2007)

Pete m. said:


> The panel schedule indicates a 3 pole 20. I spoke with the engineer earlier today and he is going to effort an answer and submit it as a revision.
> 
> He also had an issue with 250.122(B) for one of the feeders he specified.
> 
> Pete


Glad to see an inspector doing a good job!!


----------



## micromind (Aug 11, 2007)

don_resqcapt19 said:


> You can't use a 20 amp breaker without creating a code violation. Last time I checked 20 was more than 19.
> 
> If the maximum OCPD is 19, you have to find a 19 amp fuse or breaker or go to a size less than 19. I know that they make 17.5 amp fuses.
> 
> Apparently the manufacturer's electrical engineers really don't have any idea of what the information on the nameplate means to the installer.


I've seen 17.5 amp fuses too. Both 250 volt and 600 volt. 

If the figures are indeed correct, 17.5 amp fuses are the only compliant way to go.


----------



## erics37 (May 7, 2009)

If I saw this sh!t and the engineer verified the numbers I'd put in a 20 amp breaker anyway. Hair splitting.


----------



## ponyboy (Nov 18, 2012)

erics37 said:


> If I saw this sh!t and the engineer verified the numbers I'd put in a 20 amp breaker anyway. Hair splitting.


No kidding. While they're waiting for engineers and composing rfis I'd be long done with the pos.


----------

