Sizing Protection for Duplex Air Compressor

[pzw5054]

Hello all,
First post on this forum and greatly appreciate any help in advance for I have a few questions.

Let's say I have a duplex air compressor with two (2) 3-phase 10 HP motors each with its own magnetic starter (contactor and thermal overload), fed from a power distribution block where my three 480V conductors land, all within the same control panel enclosure and factory assembled.

I shall provide a 3/C #8 cable to the power distribution block as per section 430.24, the ampacity shall not be less than 31.5 A (14 A * 125% + 14 A; 14 A is the FLC from Table 430.250 for a 10 HP motor at 460V).

My 3/C #8 is fed from a 3-pole breaker in the upstream 480V panel. Since this is the last OCPD before reaching the control panel with the enclosed magnetic starters (motor controllers), I believe this would be a motor branch circuit instead of motor feeder and, therefore, Table 430.52 would apply in sizing the 3-pole breaker in my 480V panel.

Question: I can't exceed 250% of motor full load current for my inverse time breaker rating. Assuming both motors can run simultaneously, is this 250% of 28 A (14 A + 14 A)? 31.5 A (14 A * 125% + 14 A)? Let's say the maximum rating of the breaker is 250% of the 28 A, so that would be a maximum 70 A breaker. I know that the breaker only needs to protect against short circuit and ground faults and not need to protect the cable as the thermal overload has that job for motor applications. OR would I apply 250% to 14 A (a single motor and not for both motors) as this would equate to a maximum breaker rating of 35 A?

Another question: Can a single non-fusable disconnect switch be used as the controller disconnect for this application as I know 430.102(A) states that an individual disconnecting means shall be provided for each controller (magnetic starter in our case)? It doesn't seem like any of the exceptions apply to allow this single disconnect to be the disconnecting means for both motor controllers in that control panel.

As for the motor disconnect(s), I believe the controller disconnect(s) can serve as the disconnecting means for the motor also as the proposed disconnect(s) would be in sight in the same room as the motors and pumps and air compressor as allowed by 430.102(B)(2).

Any guidance would be greatly appreciated. Thanks!

 

[Cow]

I can't see how you're going to get around your motors needing individual protection according to 430.52. Even with a factory assembled (and listed?) control panel that should already have individual motor breakers/fuses installed, protecting two motors with just a feeder breaker/fuses is poor design.

I would probably add individual manual motor protectors to each motor, then size the feeder wires and breaker appropriately.

 

[kwired]

I agree each motor needs individual protection, but what you are asking how to do is still what is needed for selecting feeder protection.

Without looking at code I believe you can go with largest motor FLA x 2.5 and then add the rest of the motors FLA but still could increase that if it doesn't allow starting. Leaves you with a 50 amp breaker in this case.

 

[petersonra]

take a look at the group motor requirements found in 430.53.

I think you fit into the guidelines for such an install.

I always get a kick out of people who are perfectly happy to put two motors on two separate 250% breakers but think it is somehow unsafe to put both motors on a single 250% breaker. BTW, the 250% would be of an individual motor.

I am not saying I would do it that way, as I probably would not, but it is code compliant and just as safe.

Using a single breaker does introduce other design considerations such as being able to service just one of the contactors/motors that might be an issue, or being able to start them simultaneously.

 

[Smart $]

You mention factory assembled, but only in reference to the control panel. Is this compressor manufactured and delivered as a complete unit? As already asked, is it listed?

Assuming it is a complete multimotor unit, such is required to be marked on the nameplate with minimum supply circuit conductor ampacity and the maximum ampere rating of thecircuit short-circuit and ground-fault protective device, under 430.7(D)(1). However, those values are additionally required to determined in accordance with 430.24 and 430.53, respectively.

While it appears we make the determinations the same whether the unit is factory wired or not, I believe when factory wired and nameplate values are provided, if there is any conflict, the more restrictive value must be used (higher ampacity, lower OCPD rating).

If termination temperature limitations are 75°C, as is commonplace, you could use #10 75°C- or 90°C-rated copper conductors.

As Bob (petersonra) noted, the OCPD cannot exceed the 430.52 rating for the smallest motor [430.53(B)].

Then you must comply with the remainder of 430.53, i.e. (C) and possibly (D)...

...but getting back to where I started, if the unit is listed and has nameplate min and max, I would power it up using those values.

As for the disconnecting means, I believe your only option is to completely disconnect the unit rather than individual motors. To do individual motors requires revision of unit factory wiring. Warranty and liability issues arise in doing so.