Hermetic Compressor Disconnect

[mike9593]

I am a little confused..I am going to install wiring for a couple of 4HP 208V 3 phase hermetic compressors.(Separate units)

**Branch circuit wiring is RLA & 125% No problem
**OCPD is 175% or 225% if needed No problem

**But the disconnect - 440.12 says 115% - However Since 440.12(A)(2) says if HP is not listed in table 430-250, then the next higher value must be used. That would be 5 HP - OK, table 430-251(B) says the disconnect required ampere rating must be 105.8 amps.

A Siemens 60 amp disconnect is rated for 7.5 HP at 208 V - I'm assuming that since the 60A disconnect is rated for 7.5 HP, then it is technically rated for > 105.8 Amps for LRC? It's hard to believe that I need to go to a 200Amp disconnect?

Am I reading this right?

 

[don_resqcapt19]

Does the equipment have a nameplate showing the current?

The disconnect is rated at 115% of the rated current not of the locked rotor current. Same thing with motors in 430.110.

 

[david luchini]

I am a little confused..I am going to install wiring for a couple of 4HP 208V 3 phase hermetic compressors.(Separate units)

**Branch circuit wiring is RLA & 125% No problem
**OCPD is 175% or 225% if needed No problem

**But the disconnect - 440.12 says 115% - However Since 440.12(A)(2) says if HP is not listed in table 430-250, then the next higher value must be used. That would be 5 HP - OK, table 430-251(B) says the disconnect required ampere rating must be 105.8 amps.

A Siemens 60 amp disconnect is rated for 7.5 HP at 208 V - I'm assuming that since the 60A disconnect is rated for 7.5 HP, then it is technically rated for > 105.8 Amps for LRC? It's hard to believe that I need to go to a 200Amp disconnect?

Am I reading this right?

I don't believe you are reading it right. Let's assume your compressor had an RLA of 12 and an LRC of 65.

You would need a disconnect with a current rartng of at least 12*1.15=13.8A.

You would need a disconnect with a HP rating of the larger of...

.......From 230.250, the next size up for a current of 12A, for a 208V motor is 5HP.

.......From 230.251(B), the next size up for a current of 66A, for a 208V motor is 3HP.

So you would need a disconnect of at least 13.8Amp rating and at least 5HP rating.