A/C Rooftop Units: Fused vs. Non-Fused Disconnects

[rr]

I'm becoming confused as to when a non-fused versus a fused disconnect is required for a typical 480V/3PH rooftop AHU.

Typically, I size these units with a non-fused disconnect located at the unit and size the wire and circuit breaker according to the namplate MCA and MOCP respectivly.

However, another Engineer in my office insists that we design with a fused disconnect using the MOCP as the fuse size (located at the unit). The wire and circuit breaker are sized according to the namplate MCA and MOCP respectivly.

To me, it seems like you are oversizing your required protection by providing MOCP fuses at the disconnect AND circuit breaker.

I thought that proper MOCP sizing of the circuit breaker was enough to not have a fused disconnect.

I've looked all over NEC 2002, but nothing really jumps out at me.

Thanks in advance for any help!

 

[tom baker]

Re: A/C Rooftop Units: Fused vs. Non-Fused Disconnects

The mfgs AHU label will state the type or overcurrent device that can be used (fused or circuit breaker). A disconnect is required within sight, it can be unfused if the overcurrent device type is complied with elsewhere

 

[bphgravity]

Re: A/C Rooftop Units: Fused vs. Non-Fused Disconnects

It's not exactly a bad idea to interrupt faults as close to the equipment as possible, however I agree it is not a code requirement.

 

[steve66]

Re: A/C Rooftop Units: Fused vs. Non-Fused Disconnects

If the unit says "Maximum Fuse Size", then you have to use fuses. If it says "Maximum Overcurrent Protection", then an unfused disconnect can be used.

Steve

 

[luckyshadow]

Re: A/C Rooftop Units: Fused vs. Non-Fused Disconnects

Ditto what steve66 says. I was taught the if the nameplate states Max fuse / HACR Breaker, Thats what you must use.

 

[bphgravity]

Re: A/C Rooftop Units: Fused vs. Non-Fused Disconnects

I don't believe that is accurate. Why can't I use fuses as the branch circuit overcurrent device and still use an unfused disconnect? The disconnecting means and the required protection do not have to be integral.

 

[steve66]

Re: A/C Rooftop Units: Fused vs. Non-Fused Disconnects

I didn't intend to imply the fuses have to be at the disconnect or at the unit. They can be anywhere in the branch circuit. But if the distribution equipment is a breaker panel, the local disconnect at the unit is usually the easiest (cheapest) place to put the fuses.

Steve

 

[john m. caloggero]

Re: A/C Rooftop Units: Fused vs. Non-Fused Disconnects

This question can be answered by 110.3(B). I assume the roof-top unit is Listed. There should be instructions that come with it that specify what type of device must be used with the unit. It may state a "Fuse" or "HACR Breaker". This will be the ground-fault short-circuit protection that must be provided, either at the service panel or at the disconnect located within sight of the unit. If the fuse of breaker is provided at the panelboard, it is only required to provide only a disconnecting means within sight of the unit. It is inconvenient to have fuses or breaker located on the roof.