ONE (1) OCPD FOR TWO (2) AC CONDENSER UNITS 2

[ED-GEE]

I HAVE TWO 480V CONDENSERS FED OFF ONE 60A MCB IS THIS CODE COMPLIANT? (THEY BOTH HAVE DISCONNECTS IN LINE OF SITE)

12 Ton Unit
MOP – 40A
MCA – 26.4A

14 Ton Unit
MOP – 40A
MCA – 31.3A

 

[jraef]

No, not as you describe here.

Assuming you meant MOCP, that stands for "Maximum Over Current Protection" (someone may have thought "Overcurrent" was one word?) and in your case that is 40A. 60A > 40A, so the concept of "maximum" has been violated. But if the "disconnects within sight" are FUSED disconnects with fuses that are 40A or less, then that's OK, so long as they both never run at the same time. If they do, then the FEEDER circuit must be 125% of the largest load, plus the remaining loads. so 125% of 31.3A makes that 39.13A + 26.4A = 65A, you would round that up to a 70A breaker and use conductors rated for 65.5A minimum.

MCA means Minimum Circuit Ampacity, i.e. conductor size for each unit. You might be OK there, again so long as they don't both run at the same time. If not, see above. The MCA only applies to ONE unit, as soon as they fed two units from one circuit, the feeder rules kicked in.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[rhatcher]

Hey Jeff,

I am familiar with the acronym 'MCB' being used for 'molded case breaker' but I am not familiar with the common use of the acronym 'MOP'. The only thing that I can think of in this context is 'motor overload protection', otherwise known as the 'heaters' (very old school) or the 'overloads'.

I read the question to describe one 40A breaker feeding two condensers that together require just under 60A. If the 40A MCB is 80% rated then a it can only supply 32A continuous current but a 100% rated breaker can supply 40A. In either case, I agree that only one condenser can run at a time.

If 'MOP' means 'motor overload protection' then neither motor is properly protected with 40A overloads. However, if 'MOP' refers to the 'disconnects in sight' then I agree that properly rated 40A fuses are probably adequate. It depends on the local requirements.


This brings me to the basis of the OP's question; neither of us can answer the question of whether this system, whatever it comprises, meets Code because the OP did not tell us which Code is applicable to their location.

 

[jraef]

Typically on packaged equipment, there is a mixture of load types that can complicate the way an electrician picks out protective devices and conductors, and/or they don't like to publish the actual motor ratings so that people are discouraged from trying to rebuild them themselves. But legally they are required to provide a nameplate that tells the installer the Maximum Over Current Protection (MOCP) and the Minimum Circuit Ampacity (MCA) of the packaged unit as a whole. What I have seen happen is that some suppliers, often foreign, will misinterpret the translation of "Over Current" as if it is one word, "Overcurrent", so the acronym becomes "MOP", not understanding that Overcurrent is not actually a word, it is two words (or sometimes hyphenated). Here is an example from someone's FAQ site:

What does MCA and MOP stand for?
The minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP) ratings provide guidance for safely connecting field-wired equipment to the building mains in North America. Understanding these ratings, and their relationship to each other, is critical to properly selecting wire and circuit breaker sizes.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ED-GEE]

Wow you guys are the best so much help thank you very much!!

yeah upstream one MCB main circuit breaker 60Amps controls (2) 40A unfused disconnects of which each feeds the AHU, I figured the 60AMCB is too small and said to upsize to 80Amps lets see what they say. Code to reference would be NEC.

 

[itsmoked]

We've also seen where a package company states what breaker to use. One major brand, I think it was Carrier, stated, "You shall use the included 43A breaker or the warranty is void".

Keith Cress
kcress -

http://www.flaminsystems.com