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Old 04-20-2012, 09:00 AM   #1
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Central Air Condenser Disconnect


Hello All,

I'm a DIY'er for most things except things that I want perfectly done and I know my limits. My brother is a licensed Master electrician but has really only dealt with Commercial last several years - his company are commercial electricians who setup new building contruction etc..
Anyway, my wife and I are getting Central Air from our tax money this year. That project starts in two weeks. My brother is going to tie everything together for me (electrical) but I had to do the research and buy everything. The HVAC installer said I need a 30A 2P GFI breaker to a 30A disconnect on the side of my house. I have the breaker specific to my panel, 10-2 wire and PVC conduit, etc.

Question:
I did purchase a 30A fusible pull-out disconnect box and 2x 30A NOS type H fuses. Will this work? The condenser is a new Trane XB13 and I can't find any documentation stating it needs fusible over non-fusible.

Thanks,
~S

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Old 04-20-2012, 12:33 PM   #2
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Central Air Condenser Disconnect


If you have that circut protected at the breaker panel then almost all codes require you to have a disconnect at the unit.
This does not require a fused disconnect and most contractors use a non fused disconnect,usually a 60 amp one.
That is how I did it for 30 plus yeatrs.

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Old 04-20-2012, 01:58 PM   #3
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Central Air Condenser Disconnect


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Originally Posted by REP View Post
If you have that circut protected at the breaker panel then almost all codes require you to have a disconnect at the unit.
This does not require a fused disconnect and most contractors use a non fused disconnect,usually a 60 amp one.
That is how I did it for 30 plus yeatrs.
I agree breaker protects conductors... dissconnect can be 100 amp it doesnt matter as long as its rated for at least the breaker size.....
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Old 04-20-2012, 06:51 PM   #4
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Central Air Condenser Disconnect


Most common non fused disconnects are 40 and 60 amp. You can use a 40.
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Old 04-20-2012, 07:54 PM   #5
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Central Air Condenser Disconnect


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Originally Posted by Skelleyman View Post
Hello All,

I'm a DIY'er for most things except things that I want perfectly done and I know my limits. My brother is a licensed Master electrician but has really only dealt with Commercial last several years - his company are commercial electricians who setup new building contruction etc..
Anyway, my wife and I are getting Central Air from our tax money this year. That project starts in two weeks. My brother is going to tie everything together for me (electrical) but I had to do the research and buy everything. The HVAC installer said I need a 30A 2P GFI breaker to a 30A disconnect on the side of my house. I have the breaker specific to my panel, 10-2 wire and PVC conduit, etc.

Question:
I did purchase a 30A fusible pull-out disconnect box and 2x 30A NOS type H fuses. Will this work? The condenser is a new Trane XB13 and I can't find any documentation stating it needs fusible over non-fusible.

Thanks,
~S
I would use non fusible dissconnect save a service call for blown fuse......
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Old 04-21-2012, 05:02 AM   #6
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Central Air Condenser Disconnect


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I would use non fusible dissconnect save a service call for blown fuse......
WHAT. I make a lot of money changing fuses.

OP, make sure you have a set of spare fuses if your going to use a fused disconnect. And that the disconnect isn't mounted behind the condenser.
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Old 04-21-2012, 12:09 PM   #7
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Central Air Condenser Disconnect


I don't use a breaker for OL protection of an out side unit. over time the mechanism and linkage can hang up and react too slowly.

I put a fused disconnect at every unit with the min fuse size. Been to a lot of Copeland refer seminars and know that even HVAC/R rated breakers are bunk. In most cases the solder links that holds the breaker in the closed position is not uniform in their melting point and will open the ac power circuit at different time intervals...far in excess of comp design standards.

A one time-time delay fuse will melt dead nuts on its amp rating.

how many times have you observed amp draw on a comp in excess of the breaker rating but the breaker won't open? Or the breaker opens during normal running condition.

If i install a second utility meter both ends of the ac conductors get a fused disconnect.

Nope. Breakers just don't react fast enough and you are depriving the condenser of valuable protection by not fusing at the unit.

Anybody in the Tri-County area who buys ac packages will tell you that the package comes with a fused disconnect.
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Old 04-21-2012, 12:49 PM   #8
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Central Air Condenser Disconnect


a straight knifed non-fused disconnect it cool with the CB covering the load the AHU goes right into the CB with a service toggle on the side of the air handler section.tip add a fuse to the TR within the furnace if it doesn't have one auto inline is cool 3A
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Old 04-21-2012, 12:50 PM   #9
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I don't use a breaker for OL protection of an out side unit. over time the mechanism and linkage can hang up and react too slowly.
Split system air conditioner and heat pump outdoor units don't actually need OL protection. Since the compressor and fan motor both have it built into them.

Fuses or breakers are to protect the wire, not the compressor or condenser fan motor.
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Old 04-21-2012, 03:57 PM   #10
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Split system air conditioner and heat pump outdoor units don't actually need OL protection. Since the compressor and fan motor both have it built into them.

Fuses or breakers are to protect the wire, not the compressor or condenser fan motor.
You think the thermistor buried in the windings is sufficient OL Comp protection? Boy are you wrong.
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Old 04-21-2012, 05:45 PM   #11
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You think the thermistor buried in the windings is sufficient OL Comp protection? Boy are you wrong.
Then so is NEC.

I've seen the 30 amp fuse at a package unit not blow from a dead short, and the 100 amp sub panel service fuse blow instead.
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Old 04-21-2012, 08:51 PM   #12
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Central Air Condenser Disconnect


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Then so is NEC.

I've seen the 30 amp fuse at a package unit not blow from a dead short, and the 100 amp sub panel service fuse blow instead.
That has nothing to do with comp protection.

If a power supply is fused at both ends the one that blows depends on the impedance of the circuit. Does not matter which as long as one does, it's done what it was supposed to.

But I guarantee you that a dead short will NOT cause the internal thermistor to open. Since the operation of a thermistor is dependent on a resistance change to open the windings, it would not not react fast enough.


And, yeah, Been I know thermistors function on resistance change.
The thermistor OL was introduced to save money on an elctro-magnet or Klixon style inherent overload that is both amperage and heat sensitive.


Two types of overloads exist ..Thermal and thermal-current type. The second type is called an inherent OL because it senses amp draw and temperature.

Thermal type OL responds strictly to heat and provides POOR over amperage protection (as the comp gets hotter the winding slowly draw more amperage, slowly rising to thermal cut out).

Thermal-Current type OLs are designed to be current AND heat sensitive. They respond to heat and current changes and spike much more quickly than thermal protectors only.

The type thermistor we find in compressor is strictly heat responsive and not current sensitive. Short cycle a comp and it will draw locked rotor (assuming no start cap and potential relay) for several seconds before tripping on the internal OL.

Precious seconds that the windings are cooking away.

If it were a current sensitive OL it would trip almost immediately. But it is not.

They Comp mfgs throw the OL protection on the contractor. We have to provide properly sized fuses to protect BOTH comp and and conductors.

Thermal OL thermistors are just another way to save money on the manufacturing process.
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Old 04-21-2012, 09:20 PM   #13
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Central Air Condenser Disconnect


No additional protection is need or required, after the circuit is protected at the panel. Compressors have their own protection, and it works fine. Any contractor that puts in fused disconnects to protect the compressor, isn't doing any harm. But is under a false sense of doing anything that will really protect the compressor.

Min circuit amp is the largest motor's RLA times 1.25 plus the RLA/FLA of all other motors. So on a condenser with a compressor that has a RLA of 12 amps. 12X1.25=15. Plus the FLA of the condenser fan of say 1.4 amps. Gives a min circuit of 16.4 amps. Which means your using a 20 amp fuse on a motor that isn't suspose to be drawing more then 12 amps. So the fuses don't protect the compressor from an over load. They can only give additional protection to the wires between them and the compressor and condenser fan.
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Old 04-21-2012, 09:43 PM   #14
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No additional protection is need or required, after the circuit is protected at the panel. Compressors have their own protection, and it works fine. Any contractor that puts in fused disconnects to protect the compressor, isn't doing any harm. But is under a false sense of doing anything that will really protect the compressor.

Min circuit amp is the largest motor's RLA times 1.25 plus the RLA/FLA of all other motors. So on a condenser with a compressor that has a RLA of 12 amps. 12X1.25=15. Plus the FLA of the condenser fan of say 1.4 amps. Gives a min circuit of 16.4 amps. Which means your using a 20 amp fuse on a motor that isn't suspose to be drawing more then 12 amps. So the fuses don't protect the compressor from an over load. They can only give additional protection to the wires between them and the compressor and condenser fan.
I agree ...breakers or fuses do little to protect compressors in split systems...
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Old 04-21-2012, 10:07 PM   #15
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Central Air Condenser Disconnect


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Originally Posted by beenthere View Post
no additional protection is need or required, after the circuit is protected at the panel.
road apples. Most homes use breakers and they do not respond as fast to an overload condition that could harm a comp. compressors have their own protection, and it works fine. double road apples. Contact any manufacturer and they will tell you not to trust the comp's ol for complete compressor over-heat and over amperage protection. thermistors do not react quick enough to amp-draw changes to be effective... w h y d i d n 't y o u a d d r e s s t h a t i n y o u r r e p ly?

Especially since it's common knowledge in manufacture circles.


any contractor that puts in fused disconnects to protect the compressor, isn't doing any harm. But is under a false sense of doing anything that will really protect the compressor. we do it it because it is code and it protects the equipment from allowing a ho put an over sized breaker in the main while using a no fused disconnect.

min circuit amp is the largest motor's rla times 1.25 plus the rla/fla of all other motors. So on a condenser with a compressor that has a rla of 12 amps. 12x1.25=15. Plus the fla of the condenser fan of say 1.4 amps. Gives a min circuit of 16.4 amps. Which means your using a 20 amp fuse on a motor that isn't suspose to be drawing more then 12 amps. So the fuses don't protect the compressor from an over load. They can only give additional protection to the wires between them and the compressor and condenser fan.the fuses are sized i n p a r t to prevent a nuisance failure of a fuse as the unit starts and draws lra.


The fuse provides protection from voltage spikes low voltage conditions and repeated short cycling. The thermistor can not provide that with out severe temp rise (and endangering the windings) because it reacts so slowly .

so are we done?

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