RTU nameplates....

[chevyx92]

Scenario: (7) 5-TON RTU's all 3 Phase 208V, nameplates MCA is 29.8 and the MCB/MFS is 45. The RLA (I assume this is "Running Load Amps") is 17.9. Do you calculate all the RLA numbers to determine the demands amperage? I have a 150A 3 Phase panel and this is design as you go so I am making sure these RTU's will be ok in this 150A panel. I also have a 175A panel available but its easier to feed out of the 150A.

 

[ike5547]

Reading through 440.6 and 440.7 I would tentatively say you should go by the "rated load currents" i.e. "RLA."

 

[augie47]

Good question.
440 does not seem to help a lot IMO.
440.33. although it is for branch circuits, states for more than 1 motor the branch circuit shall be rated the sum of the rated load or branch circuit selection current whichever is greater plus 25% of the highest.
If we use that as your guide (with the selection current) your 150 amp panel would be undersized.
If we used Art 430 and sized it per 430.28 for feeders you could use the FLA of the individual motors as your base and the 150 amp panel would be adequate.

 

[ike5547]

Are "branch circuit selection currents" marked on the nameplates of your units? Or is that synonymous with MCA (minimum circuit ampacity?)

 

[chevyx92]

Are "branch circuit selection currents" marked on the nameplates of your units? Or is that synonymous with MCA (minimum circuit ampacity?)

Yes it's in the OP.

 

[chevyx92]

Good question.
440 does not seem to help a lot IMO.
440.33. although it is for branch circuits, states for more than 1 motor the branch circuit shall be rated the sum of the rated load or branch circuit selection current whichever is greater plus 25% of the highest.
If we use that as your guide (with the selection current) your 150 amp panel would be undersized.
If we used Art 430 and sized it per 430.28 for feeders you could use the FLA of the individual motors as your base and the 150 amp panel would be adequate.

Im curious as to how engineers would compute this and which articles they would use. I see what you're talking about, but I wonder which is the "Intended" method?

 

[ike5547]

Yes it's in the OP.

I'm not sure that "branch circuit selection current" and "minimum circuit amps" are the same thing.

 

[ike5547]

It looks like... according to 220.50 (2005NEC) 440.6 is the section you should use to calculate the demand on that panel. And it looks like in the absence of marked BCSC then you can use the rated load current (RLA.) Unless MCA is the same as BCSC -- which in my opinion isn't the case.

 

[augie47]

440.2 Definitions.
Branch-Circuit Selection Current. The value in amperes to be used instead of the rated-load current in determining the ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices wherever the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current. The value of branch-circuit selection current will always be equal to or greater than the marked rated-load current.

 

[ike5547]

I decided to have a look over at an HVAC forum and it looks like they are using "minimum circuit amps" to calculate loads on feeders etc.

The RLA "rated load amps" appears to be related only to the compressor and does not include other motors. The actual load can vary and is not necessarily going to run at its optimal rating.

 

[chevyx92]

440.2 Definitions.
Branch-Circuit Selection Current. The value in amperes to be used instead of the rated-load current in determining the ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices wherever the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current. The value of branch-circuit selection current will always be equal to or greater than the marked rated-load current.

So is "Branch Circuit Selection Current" the MCA rating on the nameplate?

 

[augie47]

That would be my opinion.

 

[raberding]

Mca

Mca

we typically use MCA when sizing feeders and BCs

 

[chevyx92]

we typically use MCA when sizing feeders and BCs

Thats fine. I was just trying to determine the proper way to figure the demand load of all the RTU's together. From what I gather, you can add up all the MCA ratings and that should be the "Demand" amps.

 

[augie47]

Thats fine. I was just trying to determine the proper way to figure the demand load of all the RTU's together. From what I gather, you can add up all the MCA ratings and that should be the "Demand" amps.

I don't agree with what I read but it appears to me that 440.33 would require you to add 25% of the largest to that number.

 

[curt swartz]

The 25% of the largest motor is already included in the MCA marked on the units. There would be no reason to add it again. The nameplates should list the FLA for each motor. For sizing the feeder feeding multiple units add the FLA of all the motors plus 25% of the largest motor.

 

[chevyx92]

The 25% of the largest motor is already included in the MCA marked on the units. There would be no reason to add it again. The nameplates should list the FLA for each motor. For sizing the feeder feeding multiple units add the FLA of all the motors plus 25% of the largest motor.

Figuring the size of the feeder isnt in question. It was how to determine the actually demand amps of all the units to determine if a 3 phase 208V 150A panel was adequate.

 

[curt swartz]

Figuring the size of the feeder isnt in question. It was how to determine the actually demand amps of all the units to determine if a 3 phase 208V 150A panel was adequate.

I'm lost? What demand? Amps are amps. The panel needs to have an ampacity not less than the feeder supplying it.

 

[chevyx92]

I'm lost? What demand? Amps are amps. The panel needs to have an ampacity not less than the feeder supplying it.

You are lost. Read post #3, and answer the OP. Just because you have a MCA, does this mean its the actual amperage the unit is going to pull? Or is the MCA the amperage plus 125% figured in, so all you have to do is read the MCA and size your wire based on nameplate? If the 125% is figured in then the MCA of the unit is NOT the actual amperage it pulls therefor you cant determine the "Demand Amperage" you will need to size your panel correctly. IMO.

 

[curt swartz]

The name plate MCA includes the 25% of the largest motor for branch circuit selection. The name plates should also list the FLA of each motor in the unit (compressor, fan, blower) To size the feeder add the FLA of all the motors in all of the units plus 25% of the largest motor. Adding the MCA for each unit would result in a much larger feeder than necessary.