OCP for Feeder serving multiple Condensing Units

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groundingelectrode

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Michigan
I am serving (2) condensing units with (1) feeder, then utilizing the tap rule to make separate connections to each unit.

Nameplate data for Condensing Units
CU1 - Rated Load Amps - 27.4A / MCA - 38A / MOP - 45A
CU2 - Rated Load Amps - 38.8A / MCA - 55A / MOP - 70A

How would I size the OCP for the feeder to these (2) units?
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Rule for multi-motor feeder is 125% of the larger plus sum of smaller.

In this case, we're dealing with Article 440 equipment. The MCA includes 125% of larger motor, so just add them together.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
jumper said:
Isn't that feeder size?

OP asked:

How would I size the OCP for the feeder to these (2) units?
Click to expand...

groundingelectrode said:
correct. It is feeder, not 440. What I'm really worried about is sizing the OCP for the feeder large enough to allow simultaneous start-up.
Click to expand...
My bad...

See 430.63(3). That'll take you to 430.62. For largest unit you'll go to 440.22(A), for which you'll use the MOP. You'll then add smaller unit FLA per 430.62, for which you'll use the MCA. The aforementioned is the easiest method. You can get nit picky and use actual load values for the smaller unit. Also note this is a minimum. You can go larger... but that may require a larger conductor [430.62(B)]. Unit SC/GFP will be per unit after tap.
 
T

topgone

Senior Member
groundingelectrode said:
I am serving (2) condensing units with (1) feeder, then utilizing the tap rule to make separate connections to each unit.

Nameplate data for Condensing Units
CU1 - Rated Load Amps - 27.4A / MCA - 38A / MOP - 45A
CU2 - Rated Load Amps - 38.8A / MCA - 55A / MOP - 70A

How would I size the OCP for the feeder to these (2) units?
Click to expand...

Take the MOP of the larger unit (70A) and then add the FLA of the smaller unit (27.4A), therefore, a 100A MOCP would suffice, IMO.
 
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GoldDigger

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I do not see why you use the MOP for this purpose and then add more.
The MOP tells you that it is not safe to put that equipment on a circuit protected only by a larger OPD.
And the MCA already tells you what the lowest possible protection level is.
I do not see that the rules of multiple motors on one branch allow you to override the specified MOP from the equipment label.
If you fuse each individual unit at its MOP, you could size the feeder OPD at the sum of the MCAs as a minimum, yes?
 
F

Fitzdrew516

Senior Member
Location
Cincinnati, OH
GoldDigger said:
I do not see why you use the MOP for this purpose and then add more.
The MOP tells you that it is not safe to put that equipment on a circuit protected only by a larger OPD.
And the MCA already tells you what the lowest possible protection level is.
I do not see that the rules of multiple motors on one branch allow you to override the specified MOP from the equipment label.
If you fuse each individual unit at its MOP, you could size the feeder OPD at the sum of the MCAs as a minimum, yes?
Click to expand...

I tend to be in your corner. I rely on the manufacturer's MOP (Maximum Overcurrent Protection) instead of trying to cobble something together myself. GoldDigger brings up an excellent point. If you were to combine the OCP for both these units into one breaker/fuse/etc. and one unit has an overload condition then the breaker won't necessarily trip when it's supposed to because you've sized it too high (and against manufacturer's installation requirements might I add). Now you're dealing with a burnt up motor and an angry customer. If this is your only option then do as GoldDigger suggests and fuse down at the units' local disconnects.
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
GoldDigger said:
I do not see why you use the MOP for this purpose and then add more.
The MOP tells you that it is not safe to put that equipment on a circuit protected only by a larger OPD.
And the MCA already tells you what the lowest possible protection level is.
I do not see that the rules of multiple motors on one branch allow you to override the specified MOP from the equipment label.
If you fuse each individual unit at its MOP, you could size the feeder OPD at the sum of the MCAs as a minimum, yes?
Click to expand...

I started to answer and for me, it is a no-brainer that you would have to put fused disconnects at the units to comply with the MOCP of each unit, but the reason I hesitated was the feeder size. I wondered whether you could size the feeder by adding the MCA's together as stated by Smart, and if you did, what size can the overcurrent protection on the feeder be?
 
T

topgone

Senior Member
GoldDigger said:
I do not see why you use the MOP for this purpose and then add more.
The MOP tells you that it is not safe to put that equipment on a circuit protected only by a larger OPD.
And the MCA already tells you what the lowest possible protection level is.
I do not see that the rules of multiple motors on one branch allow you to override the specified MOP from the equipment label.
If you fuse each individual unit at its MOP, you could size the feeder OPD at the sum of the MCAs as a minimum, yes?
Click to expand...

The MCA basically tells installers the minimum size of wires to be used in supplying the equipment. "M" here means "minimum" circuit amperes. On the other hand, MOP tells installers to choose an overcurrent protection no larger than the figure stamped on the equipment (MOP). "M" here means "maximum" overcurrent protective device. It follows that MOP will always be larger than the MCA.
 
F

Fitzdrew516

Senior Member
Location
Cincinnati, OH
Strathead said:
I started to answer and for me, it is a no-brainer that you would have to put fused disconnects at the units to comply with the MOCP of each unit, but the reason I hesitated was the feeder size. I wondered whether you could size the feeder by adding the MCA's together as stated by Smart, and if you did, what size can the overcurrent protection on the feeder be?
Click to expand...

Read Smart's post again. It's not simply adding the MCA's, but you must follow 430.62. To make it simple, he suggests to use the MCA plus the largest OCP (which I agree with - No sense in wasting time trying to add all the small loads up). After you figure this out you would just size the OCP for the feeder like any other feeder I guess. In this case it would be 70+38= 108. So I'd put a 110A breaker with #1's for the feeders (assuming 60 degree C).
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
topgone said:
Take the MOP of the larger unit (70A) and then add the FLA of the smaller unit (27.4A), therefore, a 100A MOCP would suffice, IMO.
Click to expand...

Fitzdrew516 said:
I was wondering about that. Do you have a code reference? It's not one of the 3 conditions listed in 240.4(B)
Click to expand...

430.62 Rating or Setting — Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on
430.24 shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group.
 
F

Fitzdrew516

Senior Member
Location
Cincinnati, OH
jumper said:
430.62 Rating or Setting — Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on
430.24 shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group.
Click to expand...

Got it. Funny how I read something and apply it during sizing, but I skip over arguably the most direct part because I'm so used to using that next size up rule.
 
T

topgone

Senior Member
jumper said:
430.62 Rating or Setting — Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on
430.24 shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group.
Click to expand...

To summarize the above citation:
The sizing of the overcurrent protective device for a feeder supplying specific fixed motor loads feed with conductors (125%+sum of the rest in 430.24) shall be no greater than the rating of the largest OCP rating in the branch circuits plus the sum of the full-load current of the other motors of the group in accordance with the provisions in 430.52 (Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices) or 440.22 (hermetic motors OCP rating 175%FLA but not greater than 225%FLA if using 175% will not be able to carry the starting current of the motor).

That was what we did. 70 + 27.4 = 97.4A. But you can't find a 97.4 OCPD. Knowing that 100 (+2.6A) is nearer the computed value compared to 90 (-7.4A), I considered 100A as compliant. Please remember that we don't split hairs here, the selection merely puts the setting in the right ballpark, IMO.
 
jumper

jumper

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david luchini

david luchini

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Staff member
Location
Connecticut
Occupation
Engineer
Using the MOCP of the larger unit isn't exactly correct. 430.62(A) is based on the MOCP of the largest single motor, plus the sum of the flc's of the other motors.

The MOCP of 70 may be based on a calculation that gives 74.2, so a 70A is required.
Combining the two units would then give 74.2 + 27.4 = 101.6, so a 100A OCP would be OK.
 
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