MCA & MOP - Design Phase - Discussion

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bradbb2005

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South Carolina
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Electrical Designer
Hello,

I am having a discussion with our Mechanical Engineer on how things are shown on design drawings. Obviously, circuit sizing for HVAC equipment is driven by nameplate ratings per 440.4(B). When I am doing a design, I expect the mechanical engineer to provide these values based on a selected design basis, schedule them, then I, as the Electrical Designer show a schematic and circuit breaker matching their values.

He is basically saying he ends up calculating these values sometimes because he is unsure which model may get chosen in the end and cannot just pick one (we work in government work, most design-build (no our contractors don't get us selections ahead of time as you'd imagine)) and is hesitant to schedule the numbers because they may be incorrect. He ends up just scheduling a HP rating and is done with it.

On my end, without having an MCA or MOP design basis, I go back to sizing the circuit based on motor and load sizes, which also, in the end, may not match a selected piece of equipment's nameplate, which in turn shows incorrect sizing on the drawings compared to the actual selected unit.

It is my opinion that on HVAC equipment, Mechanical schedules a piece of equipment utilizing the MCA and MOP for the selected model and I design around that, at least the design drawings will be consistent and if the contractor selects a different piece of equipment, they would size accordingly or submit an RFI.

What are your thoughts on the subject and how have you come to an amicable agreement with Mechanical Designers / Engineers?
 
I agree with you.

Our Mechanical guys will select generic units on jobs we don't know full details about. This way we at least can size our panels, service size, disconnects... all that jazz. And yes, when a contractor gets out in the field and sends a submittal with a different sized unit, submit an RFI.

If you're worried about change orders, I would say to make several notes that breaker, conduit, and conductor sizes shall be based on equipment supplied by others.
 
In my opinion the engineer should pick the equipment as a baseline so you can do your work. If the Mechanical decides he wan'ts different equipment the it should be in his spec to pick up the difference if he needs more power.

This will cause some infighting he has to make it crystal clear in the specs
 
I think as a general rule if you know you need a certain size unit you can generally figure worst case scenario for minimum conductor size needed. The problem still remaining is exactly what size OCPD will be needed when you finally know exactly which model will be installed, though you likely will want to order those OCPD's with your gear which often will be long before you see the actual units.
 
kwired said:
I think as a general rule if you know you need a certain size unit you can generally figure worst case scenario for minimum conductor size needed. The problem still remaining is exactly what size OCPD will be needed when you finally know exactly which model will be installed, though you likely will want to order those OCPD's with your gear which often will be long before you see the actual units.
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Which is one reason I think a lot of EE's just specify fusible disconnects for all the HVAC equipment. If the breaker is too large, the fuses can just be reduced in size. And I think most EC's buy fuses much closer to the end of the project.
 
What I see a lot, and recommend, is that the ME pick a design rating, give it to the EE, who designs accordingly. Then in the project / tender specifications, make the contractor / supplier responsible for the costs of any required design changes if they chose to use equipment other than specified. In other words if the ME designs around 40HP, and a supplier comes in at the last second and says "We'll do it with 30HP", then the SUPPLIER gets a bill for the cost of the EE having to redo the calculations. What that really does is to help lock down the specs so that suppliers don't play games.
 
Jraef said:
What I see a lot, and recommend, is that the ME pick a design rating, give it to the EE, who designs accordingly. Then in the project / tender specifications, make the contractor / supplier responsible for the costs of any required design changes if they chose to use equipment other than specified. In other words if the ME designs around 40HP, and a supplier comes in at the last second and says "We'll do it with 30HP", then the SUPPLIER gets a bill for the cost of the EE having to redo the calculations. What that really does is to help lock down the specs so that suppliers don't play games.
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If ME figures he needs 40 tons but the supplier/installer decides to go with 30 tons then ME is off the hook if the 30 tons can't keep up with demand, right?

I could understand a little more if you needed a certain capacity but supplier/installer decides to go with single phase instead of three phase (usually going to be less than 5 maybe 10 ton units with dual compressors in that case) but that kind of change can make a big difference for the EC, and not just two pole vs three pole breakers and branch conductors but can mess with service/feeder calculations as well. Enough smaller units can balanced across phases but just one or two larger units can mess load balancing up.
 
steve66 said:
Which is one reason I think a lot of EE's just specify fusible disconnects for all the HVAC equipment. If the breaker is too large, the fuses can just be reduced in size. And I think most EC's buy fuses much closer to the end of the project.
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Back in my EC days I did that routinely. Suddenly it's inspection time and the MOCP is different from the print. Odd size fuses are a lot easier to come by than breakers.
 
augie47 said:
Back in my EC days I did that routinely. Suddenly it's inspection time and the MOCP is different from the print. Odd size fuses are a lot easier to come by than breakers.
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Until covid messed up supply system I never had much trouble getting Square D breakers, whether it be QO, HOM, NQ, I-line, etc. in any standard size in accordance with 240.6. Since then common sizes are sometimes the ones harder to get. Some sizes maybe not stocked at supply house, but usually not much of a wait to get them, when things were normal.
 
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