AC units--Rooftop and ground level

[Pack12]

I will be wiring two 10 ton ac units, two 20 ton ac units and one 7.5 ton ac units in the next couple of weeks. I have purchased the raintight disconnects for all of the units, 4- 60 amp disconnects and one 30 amp disconnect for the 7.5 ton. I have decided to use fused disconnects since the breaker is quite a ways from the units and I thought it would be best to interrupt the fault as close to the unit as possible. There is currently no GFCI service recepticles near any of the units, so I have to install and wire those as well. My question is, where in the code does it state whether one should use a fused or non-fused disconnect? And also where is the article that requires a GFCI for servicing? I believe there must be one within 25 feet of each unit, right? I have not done many AC's this size mostly home units, and I just want to make sure I am not missing something. Any help appreciated. Thanks.

PS, these are 480 volt units.

 

[480sparky]

Whether or not you use fused disconnects is dependant on the unit it serves.

Many times, the wire size will be increased to minimize voltage drop, the a fused disco installed to protect the unit. But this is not an NEC requirement, only an engineering one.

As for the GFI recep, see 210.63.

 

[hillbilly1]

The nameplate rating on the unit may call for fused protection only, but most are rated for fuse or hacr breaker.

 

[bob]

Many times, the wire size will be increased to minimize voltage drop, the a fused disco installed to protect the unit. But this is not an NEC requirement, only an engineering one.

Sparky
I am not sure what you are saying. Are you saying a fused disconnect will protect the unit better that a breaker. This may be true in the case of a single phase condition. Bussmann has a table of fuses that closely match the FLA of the unit and during a single phase situation they will blow and take the unit off line.

 

[480sparky]

Sparky
I am not sure what you are saying. Are you saying a fused disconnect will protect the unit better that a breaker. This may be true in the case of a single phase condition. Bussmann has a table of fuses that closely match the FLA of the unit and during a single phase situation they will blow and take the unit off line.

In many commerical installations, the unit is quite a ways from the panel. The wires are upsized to minimize voltage drop, and a breaker (too large for the unit) is installed to protect the wires.

Then a fused disco is installed to protect the unit itself. But the unit must be rated to allow fuses for protection. But this installation is not an NEC requirement. It is typically specified by the EE or manufacturer.

 

[hillbilly1]

In many commerical installations, the unit is quite a ways from the panel. The wires are upsized to minimize voltage drop, and a breaker (too large for the unit) is installed to protect the wires.

I havn't seen this practice,(increase in breaker size for wire protection) must be a regional thing. Just because the wire size is increased, it does not require the ocp to be increased.:smile:

 

[480sparky]

I havn't seen this practice,(increase in breaker size for wire protection) must be a regional thing. Just because the wire size is increased, it does not require the ocp to be increased.:smile:

If this is done, the cost of a 100a breaker is the same as an 80a breaker. So why not?

 

[Pack12]

what article does it state the clearances in?

 

[480sparky]

what article does it state the clearances in?

110.26......

 

[iwire]

If this is done, the cost of a 100a breaker is the same as an 80a breaker. So why not?

Why? Because you will have to add a fused disco at the unit. (Just as you explained)

Would it not make more sense to run the wire sized needed for voltage drop and protect it at a level that matches the units requirements?

 

[Ace18]

Sparky
I am not sure what you are saying. Are you saying a fused disconnect will protect the unit better that a breaker. This may be true in the case of a single phase condition. Bussmann has a table of fuses that closely match the FLA of the unit and during a single phase situation they will blow and take the unit off line.

Thats just the opposite of what I have experienced. I have seen a few 3 phase compressors get single phased and burn up beause one of the fuses blew and the compressor would cycle on it's internal overload until it finally burned it up. Because the fuses were time delay, the internal overload in the compressor would kick out before the other two fuses would blow.

 

[480sparky]

Why? Because you will have to add a fused disco at the unit. (Just as you explained)

Would it not make more sense to run the wire sized needed for voltage drop and protect it at a level that matches the units requirements?

Not if you're protecting the unit with a fused disco.