Disconnect Switches for MAU's

[jkim780]

I have (2) 460v 3ph 7.5hp(11 FLA) Make-up air units within 15' distance. The unit came with control center w/ disc.(I am not sure they have a fused or non-fused disconnect switch. I am checking on that with mfgr.) & starter installed as a listed factory assembly. Per NEC 430.53(D), they were fed with one branch circuit with same ampacity conductors. ( I am not sure each motor overload device & controller are listed for gruoup installation. I am checking on that, too.)

Electrician brought up the question saying that we have to have fused disconnect switch for these units because multi-motors share one branch circuit even though disconnects are factory installed unit. He is referring 430.28 Feeder tap rules.

Is he correct? Do I have to have additional fused disc. swiches for each unit if mfgr furnished disc. switch is non-fused? Units are already on site so we can't have mfgr change their disc. switches?

Thanks for your answer in advance.

[ March 03, 2004, 04:54 PM: Message edited by: jkim780 ]

 

[steve66]

Re: Disconnect Switches for MAU's

If you are complying with 430.53(C)(4), I don't think you have a feeder tap. Tap conductors are reduced ampacity conductors (i.e. smaller than the feeder and smaller than would be allowed by the NEC without tap rules), but it sounds like your branch circuit condutors are the same size as your feeder.

Aside from your installation not including taps, I'm not sure if you need a disconnect at each motor.

Steve

 

[jkim780]

Re: Disconnect Switches for MAU's

Originally posted by steve66:
Tap conductors are reduced ampacity conductors (i.e. smaller than the feeder and smaller than would be allowed by the NEC without tap rules)

I thought same ampacity conductors are also tap conductors per 430.28(3). Am I wrong?

 

[steve66]

Re: Disconnect Switches for MAU's

I think you are right, they are tap conductors. But,I don't see anything that requires separate disconnects if one disconnect is in sight of both units. See 430.53(D) and (D)(1) and the exception to 430.112.

Steve

 

[jkim780]

Re: Disconnect Switches for MAU's

Steve, thanks for your response. But what I am concerned is if I have to have "fused disc. switch" or not and the reason is

First, as I mentioned earlier, mfgr. furnished disc. as a part of control center and it was non-fused disc. But the name plate on the unit shows "maximum fuse size". Should they have to have fused disc. per NEC 440?

Second, feeder tap per 430.28.

As you pointed it out, 430.53(C)&(D) allows me not to have a fused disc. but what about 440? On my handbook, it says

"Where an air conditioner is listed by a qualified electrical testing laboratory with a name plate that reads "maximum fuse size", the listing restricts the use of this unit to fuse protection only and does not cover its use with circuit breakers."

You think I still don't need a fused disc. for this units?

 

[steve66]

Re: Disconnect Switches for MAU's

Sorry, but apparently I didn't understand your original question. I thought you were asking about disconnects requirements, not overcurrent protection requirements.

Yes, if the units are marked with a "maximum fuse size", somewhere you have to have a fuse of that size or smaller protecting the units. Although you may be able to use a single fused disconnect on the feeder (depending on the size of the units and what the max. fuse size is), it sounds like a fused disconnect at each unit would be the best way to go.


Steve