Conductor/OCPD sizing for single point equipment connection

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greenspark1

Senior Member
Location
New England
Hi,
I am wondering about how to size the conductor and OCPD for a single point power piece of equipment such as a large air dryer or chiller. They usually consist of several small pumps, heaters, controls, etc and come labeled with a full load current. For example, a dryer with122 full load amp. Do you have to take 125% of this for the conductor and breaker size, or is there another way to handle this load? It seems tricky since it acts like a branch circuit but has a number of loads on it so is also like a feeder. Thanks for any feedback.
 

kwired

Electron manager
Location
NE Nebraska
Hi,
I am wondering about how to size the conductor and OCPD for a single point power piece of equipment such as a large air dryer or chiller. They usually consist of several small pumps, heaters, controls, etc and come labeled with a full load current. For example, a dryer with122 full load amp. Do you have to take 125% of this for the conductor and breaker size, or is there another way to handle this load? It seems tricky since it acts like a branch circuit but has a number of loads on it so is also like a feeder. Thanks for any feedback.
What exacty does the nameplate say? Often something like this will have the calculations done for you and will be marked with something like "minimum supply circuit" and "maximum overcurrent protection" or similar.
 

greenspark1

Senior Member
Location
New England
What exacty does the nameplate say? Often something like this will have the calculations done for you and will be marked with something like "minimum supply circuit" and "maximum overcurrent protection" or similar.

This dryer has a one-line that shows the feeder to it as 480/3ph and labels the incoming power requirements as 122 FLA. I have a chiller with labeled 501 Min Cir Amp/ 700 MOP.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I would guess for an appliance that's not a continuous load you could just size to the FLA without adding anything. When an MCA and MOCPD is given you must use them. For the chiller in question size the conductors to the MCA and the OCPD can be any size from the MCA to the MOCPD.
 

kwired

Electron manager
Location
NE Nebraska
This dryer has a one-line that shows the feeder to it as 480/3ph and labels the incoming power requirements as 122 FLA. I have a chiller with labeled 501 Min Cir Amp/ 700 MOP.

The chiller is an art 440 application and the nameplate should definitely be followed on that one.

The dryer is not so straight forward with what the FLA actually means. It could be a figure that includes 125% of largest motor, or it could just be 100% of connected load, or 100% of maximum load that can run at any given time.

If you know what is all inside for loads, you could do some calculations of your own and see how they compare to the marked FLA and then maybe it will become more obvious just what their marked FLA really is all about.

You can do the same with the chiller using calculations in art 440, but the results of those calculations should come very close to the marked MCA.
 

greenspark1

Senior Member
Location
New England
OK I have another good example. A motor starter panel serving a large 300HP 480V motor plus two small 480V accessory motors.
Main motor FLA = 365A
fan motor= 21A
small motor = 1A

The conductor size needs to be 125% of largest motor plus all others = 478A min. I'd run two sets of 350kcmil in one conduit (310A at 75C, plus 0.8x derating for >3 current carrying conductors= 496A).

Overcurrent protection: NEC allows up to 2.5x of the largest plus 100% of all others = 912A (rounded up to 1000) plus 21 +1 = 1022A (rounded down). This is a max and I'd use say a 800A breaker with a 700A trip.

I could use some feedback, any agreement/disagreement with this design? Any conflict using 620A rated conductors protected by a 700A trip unit? Thanks!
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
What' the insulation type of your conductors? If you use THHN then the derating can be from the 90? C column or 350 amps per set.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
also, for motor calculations 430.6 states Table values shall be used for all but O.L.
A 350HP @ 480 has a Table value of 361 amps, unsure about the others as HP was not given. In this case it will not change your end result, but you need to keep that in mind as in some cases it will.

also you could use a 1000 amp breaker.
 

kwired

Electron manager
Location
NE Nebraska
I could use some feedback, any agreement/disagreement with this design? Any conflict using 620A rated conductors protected by a 700A trip unit? Thanks!
Even without the motors a 620 amp conductor could be protected by a 700 amp overcurrent device as long as the load is 620 or less - next standard size rule applies here.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Even without the motors a 620 amp conductor could be protected by a 700 amp overcurrent device as long as the load is 620 or less - next standard size rule applies here.

This is correct, but the OP made a mistake. He is has a 496A conductor with the 700A OCPD, rather than a 620A conductor.
 

kwired

Electron manager
Location
NE Nebraska
This is correct, but the OP made a mistake. He is has a 496A conductor with the 700A OCPD, rather than a 620A conductor.


You are correct. I didn't look at his math or figures all that closely, just that last sentence happened to catch my attention. Not sure how he come up with the 620 mentioned there.
 
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