Heat Pump Disconnect

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new_ee

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The heat pump falls under 440. The panel is not within sight so a separate disconnect will be required. Heat pump has MCA 17.7A and Max Fuse Size 25A. Does the disconnect have to be fused? Also since the nameplate says "Max Fuse Size" will a breaker in the panel be ok? Heat pumps have the overload protection built in usually, correct?

I guess what im trying to get at is that usually when you have a heat pump being fed from a panel that is not within sight, you have the disconnect at the heat pump as well as a breaker in the panel for the circuit. When this is the case I'm wondering if the disconnect has to be fused or not. If it doesn't, is it common practice to have double protection by the circuit breaker and a fuse in the disconnect? How do you size the breaker? Do you use max fuse size?
 
Re: Heat Pump Disconnect

If the nameplate of the heat pump says "Max Fuse" than you have to use a fuse. Most of the pumps I have installed say "Max fuse or circuit breaker" which allows you to choose the type of short circuit protection.

A disconnect does not need to be fused if the breaker feeding it is sized for the nameplate. It is permissible to fuse it but I don't see the need.

Chris
 
Re: Heat Pump Disconnect

440.4C requires the OC protection to be approved for use with the compressor. If the mfg has selected a fuse then that is required. Many mfg's approve breakers and fuses. Check it out.
440.14 requires the disconnect to be within site of the heat pump. It is usually at the site of the unit.
A disconnect does not need to be fused if the breaker feeding it is sized for the nameplate. It is permissible to fuse it but I don't see the need.
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Raider
If the mfg says fuses then the disconnect must be fused if the circuit is fed by a breaker.

[ November 16, 2005, 12:25 PM: Message edited by: bob ]
 
Re: Heat Pump Disconnect

Originally posted by raider1:
If the nameplate of the heat pump says "Max Fuse" than you have to use a fuse. Most of the pumps I have installed say "Max fuse or circuit breaker" which allows you to choose the type of short circuit protection.

A disconnect does not need to be fused if the breaker feeding it is sized for the nameplate. It is permissible to fuse it but I don't see the need.

Chris
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ok. for example, if it said max fuse size. you'd have a fuse in the disconnect. how would the unit be fed from the panel? through a breaker still? so then you'd have a breaker on a unit that called for a fuse. this is where my confusion lies.
 
Re: Heat Pump Disconnect

Bob,

I agree,if you look at the first part of my post I mention that if the nameplate says "Max Fuse" than you are required to fuse it. I did'nt make that very clear in the last part of my post, sorry :eek:

New_ee,

Yes you would have a breaker feeding a disconnect. But at the disconnect you now have overcurrent protection for the pump in the form of fuses. The wiring between the disconnect and the breaker is now a feeder and the breaker is only protecting the feeder.

Chris
 
Re: Heat Pump Disconnect

so then you'd have a breaker on a unit that called for a fuse
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I agree completely with the above post, but would like to add that if it says "max fuse size = 30", then somewhere in the circuit there has to be a 30A or less fuse. You can put any other disconnects or circuit breakers or relays or whatever else in the circuit. They are not telling you what you can't put in the circuit, only what you have to put in to protect the unit.

Fuses usually provide better protection from fault currents than breakers, so I doubt you will ever see anything marked "Maximum circuit breaker size = xx. But you will often see "Maximum Overcurrent protection = xx". In this case, you can install fuses or circuit breakers, or both.)

Steve
 
Re: Heat Pump Disconnect

Originally posted by raider1:
Bob,

I agree,if you look at the first part of my post I mention that if the nameplate says "Max Fuse" than you are required to fuse it. I did'nt make that very clear in the last part of my post, sorry :eek:

New_ee,

Yes you would have a breaker feeding a disconnect. But at the disconnect you now have overcurrent protection for the pump in the form of fuses. The wiring between the disconnect and the breaker is now a feeder and the breaker is only protecting the feeder.

Chris
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don't heat pumps usually have build in overcurrent protection in the controller?

see this is another point of confusion. you have the short circuit/ground fault protection from the breaker. then you need overload or overcurrent protection separately too, correct? so the fuse in the disconnect is providing overcurrent protection? if so, wouldn't you always need to use a fused disconnect? so you are providing overcurrent (fuse) and short circuit/ground fault (breaker).
 
Re: Heat Pump Disconnect

so if it says max overcurrent protection is XX amps and i have a panel (w/breaker) feeding a disconnect is that disconnect required to be fused or not?

does the max overcurrent protection number really apply to a breaker or a fuse? i always thought a breaker and a fuse were sized differently.

now, if you don't fuse the disconnect the breaker is providing only short circuit/ground fault proctection, correct? the breaker is sized 175% of the fla of the motor to allow for inrush current of motor starting. so if something goes wrong, the breaker is not going to protect the conductors which are only sized at 125%. where do you get overcurrent protection?

[ November 16, 2005, 01:47 PM: Message edited by: new_ee ]
 
Re: Heat Pump Disconnect

Originally posted by raider1:
Bob,

I agree,if you look at the first part of my post I mention that if the nameplate says "Max Fuse" than you are required to fuse it. I did'nt make that very clear in the last part of my post, sorry :eek:

New_ee,

Yes you would have a breaker feeding a disconnect. But at the disconnect you now have overcurrent protection for the pump in the form of fuses. The wiring between the disconnect and the breaker is now a feeder and the breaker is only protecting the feeder.

Chris
Click to expand...
would the fuse and breaker have the same amperage rating?
 
Re: Heat Pump Disconnect

Overcurrent protection is for the motor. Short circuit protection is for the conductors.

The overcurrent protection can be part of the motor controller or part of the motor itself in the case of thermally protected motors. This limits the current allowed to flow to the motor, to a acceptable amount.

The short circuit protection is to protect the conductors between the breaker or fuse to the motor overcurrent device.

The short circuit protection will work even if it is size above the rating of the conductors because a short circuit will cause a large fault current in a short time and trip the breaker or blow the fuse.

Chris
 
Re: Heat Pump Disconnect

Originally posted by raider1:
Overcurrent protection is for the motor. Short circuit protection is for the conductors.

The overcurrent protection can be part of the motor controller or part of the motor itself in the case of thermally protected motors. This limits the current allowed to flow to the motor, to a acceptable amount.

The short circuit protection is to protect the conductors between the breaker or fuse to the motor overcurrent device.

The short circuit protection will work even if it is size above the rating of the conductors because a short circuit will cause a large fault current in a short time and trip the breaker or blow the fuse.

Chris
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thanks. i understand what the meanings are but im trying to understand the "practicality" if you will.

so if it says max overcurrent protection is XX amps and i have a panel (w/breaker) feeding a disconnect is that disconnect required to be fused or not?

does the max overcurrent protection number really apply to a breaker or a fuse? i always thought a breaker and a fuse were sized differently.

now, if you don't fuse the disconnect, the breaker is providing only short circuit/ground fault proctection, correct? the breaker is sized 175% of the fla of the motor to allow for inrush current of motor starting. so if something goes wrong, the breaker is not going to protect the conductors or the motor because it is sized for short circuit/ground fault protection only. where do you get overcurrent protection (remember, we didnt fuse the disconnect)?


and for another case lets say you have the breaker and a fused disconnect. the unit calls for a 40A MOCP device. if the breaker and fuse are the same amperage how is the unit protected from overcurrent and short circuit/ground fault?

[ November 16, 2005, 02:18 PM: Message edited by: new_ee ]
 
Re: Heat Pump Disconnect

If the motor nameplate says max overcurrent xx, and the breaker feeding that disconnect is sized at that # then you would not need to have fuses.

The max overcurrent rating applys to either a fuse or circuit breaker, which ever you are using for overcurrent.

If the disconnect is not fused and we have overcurrent protection at the motor or controller, then the breaker will only need to protect the conductors for a fault. If the motor does not have overcurrent protection, and we are relying on the breaker for overcurrent protection, then it cann't be sized over 125% of the rated-load current. The breaker can serve both functions if it is sized to the motor name plate.

If you have both a breaker and fuse sized the same then the one closest to the motor will provide the overcurrent.

Chris
 
Re: Heat Pump Disconnect

Originally posted by raider1:

If you have both a breaker and fuse sized the same then the one closest to the motor will provide the overcurrent.

Chris
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And just how do these device know which of them is closest to the motor?
 
Re: Heat Pump Disconnect

Originally posted by petersonra:
Originally posted by raider1:

If you have both a breaker and fuse sized the same then the one closest to the motor will provide the overcurrent.

Chris
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And just how do these device know which of them is closest to the motor?
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The little guy named electron is in both the breaker and fuse. First guy there and back wins! He trips the breaker or fuse! I thought everyone knew that! :D :p :p
 
Re: Heat Pump Disconnect

I think Chris has it pretty much right, except the Rock -Paper Sissors thing :D If you provide an extra breaker or fuse, which one trips first isn't really a safety issue. So I wouldn't worry too much about that.

You are correct that heat pumps usually have an internal overload (maybe a thermal contact). That protects the motor for overloads in the 125% to 175% range (or to whatever range the breaker or fuse trips at).

I think this is a canidate for a FAQ.

Steve
 
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