Whether to replace the breaker?

[MAXIWANG]

My mechanical people is going to downsize a condenser, which is fed from a 70A breaker in a panel, and with a local disconnect switch. the motor protection is integrated inside the unit. The new condenser only needs a 40A breaker. Now my question is do I really need to replace the 70A in panel or I can just leave as is and provide a new location 40A fused disconnect?
thank you in advance.

 

[itsmoked]

You don't have to remove the existing breaker but I would as would most people. Someone could later assume that the panel has 70A of capacity left in it even though you've just consumed 40A. Also the chances of the 70A breaker being correct for any subsequent task is a long shot.

The NEC states something along the lines of "neat and orderly". Extra breakers doing nothing might not fit this criteria in some cases.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MAXIWANG]

Sorry I may not make it very clearly in my pervious post. I mean can I use the 70A breaker for the new condenser? my understanding is the breaker is only protecting the downstream wiring, which is already sized to suit 70A. all the motor protections are inside the unit itself. so, as long as I provide a 40A fused local disconnect switch, it should be fine.
Am I correct?
anyway, thanks a lot Itsmoked.

 

[wareagle]

MAX
You are correct. The main thing is to provide the 40 amp
protection to the condenser. I assume that is the mfg specs.
You say a fused disconnect. The mgf will provide a recommended over current rating. If a fuse is recommended
then it is ok to use it. However a breaker may be required
if the mfg says to use a 40 amp breaker.

 

[itsmoked]

I see now MAXIWANG; Thx for the clarification.

Fuses for a compressor... very bad juju! You will blow one fuse someday and lose the compressor. I've seen it a million times. Like clockwork. Use a breaker.

It is so bad a lot of compressor makers will void any and all warranties. Also make sure it is a breaker rated for HVAC.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MAXIWANG]

Thanks a lot guys.
Itsmoked, can you please tell me more detail about why i can not use fuse for compressor? I thought of fuse because i thought disconnect wouldn't do any protection and the upstream breaker is 70A. So I thought maybe I could use a fuse to provide some protection.
also 'a breaker rated for KVAC', is there anything special for that breaker?
thanks again for your helps.

 

[jraef]

<Jumping in>
itsmoked's answer has to do with the fact that fuses allow for single phasing of 3 phase motors, because if only one fuse blows, the motor will get 1 phase power and may burn up before another fuse opens. Phase Loss Relays and many IEC style overload relays will protect against this however, so it depends on what else you have in the circuit. Your idea about adding fuses is sound from the standpoint of having closer protection for the motor windings against a short circuit, just make sure that you have supplementary phase loss protection.

Assuming you made a typo and the breaker is rated HVAC (not KVAC), that simply means that the breaker was designed to be used on motor loads in HVAC systems. But really that's just a marketing thing now, all breakers are rated the same and have been for years. That label is just a holdover from a bygone day.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[RalphChristie]

or use striker-pin fuses that activate a mechanical limit switch if any fuse blow.

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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