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Old 07-20-2014, 11:40 AM   #61
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Quote:
Originally Posted by beenthere
They weren't burnt up. They were discolored/browning. Other NM in the same panel box were not discolored or browning.

Properly wired, it's a non issue, you seem to believe every unit should be wired to the maximum over current device rather than the minimum circuit ampacity.

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Old 07-20-2014, 11:42 AM   #62
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They weren't burnt up. They were discolored/browning. Other NM in the same panel box were not discolored or browning.
Were any of these pre 1984 NM cables?
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Old 07-20-2014, 11:59 AM   #63
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Properly wired, it's a non issue, you seem to believe every unit should be wired to the maximum over current device rather than the minimum circuit ampacity.
I won't say non issue. but your welcome to your opinion.
Not always to the max. But I don't believe in using the min.

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Were any of these pre 1984 NM cables?
Yes, late 70s, and mid to late 80.
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Old 07-20-2014, 01:51 PM   #64
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I won't say non issue. but your welcome to your opinion.
Not always to the max. But I don't believe in using the min.



Yes, late 70s, and mid to late 80.
NM from that era was only rated 60 degrees in both insulation and jacket. So it might make sense in terms of discoloration if the wire saw much higher than normal current for some time. I have never an AC discolor NM (not saying it didn't happen), but more than once I have seen the breaker trip before the compressor. did However, in properly sized separately mounted overloads for motors in commercial/industrial or those in MCC, should never trip a breaker first regardless of condition. The heaters in the overload always kick in first. The same should go for resi AC, but as I mentioned doesn't always. No idea why.
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Old 07-20-2014, 03:51 PM   #65
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http://forums.mikeholt.com/showthread.php?t=162987

That thread covers the same ground and arguments but with even more code and UL references, and with some additional field experience.
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Old 07-20-2014, 03:52 PM   #66
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Even at 60 degrees, we are still required to add 125% to the minimum ampacity, as far as the molded circuit breaker tripping instead of the internal thermal overloads would be based on whether it was actually a overload vs ground fault or short circuit, but the internal thermals will protect the branch circuit and is designed this way on purpose. On the smaller residential units, it wouldn't surprise me if the trip curves are very close to being he same since we are not exactly starting up 200 HP motors...
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Old 07-20-2014, 04:21 PM   #67
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Quote:
Originally Posted by stickboy1375
Even at 60 degrees, we are still required to add 125% to the minimum ampacity, as far as the molded circuit breaker tripping instead of the internal thermal overloads would be based on whether it was actually a overload vs ground fault or short circuit, but the internal thermals will protect the branch circuit and is designed this way on purpose. On the smaller residential units, it wouldn't surprise me if the trip curves are very close to being he same since we are not exactly starting up 200 HP motors...
Please clarify. Are you stating adding 125% to the minimum circuit ampacity on the nameplate?

Overloads (heaters) protect the motor. OCPD's protect the Bc conductors.
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Old 07-20-2014, 06:45 PM   #68
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Quote:
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Please clarify. Are you stating adding 125% to the minimum circuit ampacity on the nameplate? Overloads (heaters) protect the motor. OCPD's protect the Bc conductors.

I'm saying the nameplate already includes the 125% in the minimum circuit ampacity. And with AC units, the internal thermals protect the branch circuit wiring, the circuit breaker is only providing ground fault and short circuit protection, if the motor can't run, it's pretty hard to burn up the branch circuit wiring.
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Old 07-20-2014, 08:02 PM   #69
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Not when a winding shorts/shunts to ground it isn't.
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Old 07-20-2014, 08:04 PM   #70
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Quote:
Originally Posted by stickboy1375
I'm saying the nameplate already includes the 125% in the minimum circuit ampacity. And with AC units, the internal thermals protect the branch circuit wiring, the circuit breaker is only providing ground fault and short circuit protection, if the motor can't run, it's pretty hard to burn up the branch circuit wiring.
Ok. We are on the same page.
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Old 07-20-2014, 08:45 PM   #71
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Not when a winding shorts/shunts to ground it isn't.

If that happens. The OCPD will provide the proper ground fault protection, if the motor overloads, then the internal thermal protection will protect the motor and branch circuit wiring. Not really sure what you don't understand.
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Old 07-20-2014, 09:05 PM   #72
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If that happens. The OCPD will provide the proper ground fault protection, if the motor overloads, then the internal thermal protection will protect the motor and branch circuit wiring. Not really sure what you don't understand.
The motors overload doesn't open as quick as you think. It does not really protect the wiring, despite what NEC says. The NEC hopes it will open quick enough.
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Old 07-20-2014, 09:57 PM   #73
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Quote:
Originally Posted by beenthere
The motors overload doesn't open as quick as you think. It does not really protect the wiring, despite what NEC says. The NEC hopes it will open quick enough.
It's not designed to open quickly, if it did, we wouldn't be able to start motors because of the high inrush current at startup, this is why thermal overloads are used. by UL standards, which coincide with the NEC, depending on the actual amperage and trip curve would take anywhere from 5 seconds to 5 minutes to 5 hours, but in either case the branch circuit is still protected because of the overkill of the NEC, the amperage ratings of the conductors are much higher than we think, the NEC limits our use as best suited for general safety. No matter how you look at it, UL knows the trip curves are suited for protecting branch circuit wiring because they work with the NEC, this isn't a big mystery.

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Old 07-20-2014, 10:34 PM   #74
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People curious about motor protection, check out http://ecmweb.com/content/nema-stand...ed-closer-look
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Old 07-21-2014, 04:31 AM   #75
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If that happens. The OCPD will provide the proper ground fault protection, if the motor overloads, then the internal thermal protection will protect the motor and branch circuit wiring. Not really sure what you don't understand.
I think what Beenthere is trying to say is that nobody is denying the NEC allows for the motor's overload to protect the wiring or that the theory exits, but rather when it comes to it, real world experience has shown that the internal motor overload protector doesn't always respond fast enough. The theory nor intention is being denied, but rather what has been observed in the real world proves otherwise.

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The motors overload doesn't open as quick as you think. It does not really protect the wiring, despite what NEC says. The NEC hopes it will open quick enough.
I agree. Seen it first hand. The motor overload is supposed to protect the wiring. And Im sure that's what the engineers have in mind when designing them; but more than once I have seen and certainty heard the same from others that the breaker trips before an overload opens. Certainly not in all cases but its definitely not a rarity either.

FWIW Techs on HVAC forums frequently bring up a running compressor tripping a breaker before an over load opens it. Its not a short circuit since an amp clamp shows its pulling excessive current with half the time a dirty condenser or blocked capillary/TEX being diagnosed putting strain on the compressor. Granted newer units are more likely to have high and low side pressure cutoffs that drop the contactor, but millions of older units do not. And still those don't stop overloads from scrolls beginning to seize up.




Quote:
Originally Posted by stickboy1375 View Post
It's not designed to open quickly, if it did, we wouldn't be able to start motors because of the high inrush current at startup, this is why thermal overloads are used. by UL standards, which coincide with the NEC, depending on the actual amperage and trip curve would take anywhere from 5 seconds to 5 minutes to 5 hours, but in either case the branch circuit is still protected because of the overkill of the NEC, the amperage ratings of the conductors are much higher than we think, the NEC limits our use as best suited for general safety. No matter how you
look at it, UL knows the trip curves are suited for protecting branch circuit wiring because they work with the NEC, this isn't a big mystery.
But UL does not always anticipate failures or manufacturing defects. And what looks good on paper doesn't always come true in reality. You cant tell me that in the history of OCPDs one has never failed nor are they 100% reliable thorough out. UL certainly knows what they are doing and Im sure engineers who design these overloads are told build them with a trip curve that not only protects the motor within reason but also protects the wiring (ie compressor overload shell open within x amount of time when pulling more than the minimum circuit ampacity or something to that effect). And The CMP knows about this and thus makes a code allowance.


But, real world something isn't going as planed. It could be that a large number of these overloads are inadvertently deviating from intended time current curves as they age or are just defective from the start. To be honest I don't know reason why they are, but experience has shown that to be my case.

Case in point: Had an AC unit once with a condenser coil packed full of debris. Liquid line (small pipe) leaving the unit toward the air handler was so hot I swear you could've fried eggs with it. By all means the compressor was being overworked. Breaker would keep tripping half hour. Unit had a minimum circuit ampacity around 12 amps #14 on a 20 amp breaker as per max breaker size. The unit started normal but as it ran current draw ran up to 25 amps and kept steadily rising until the breaker tripped around 30 amps. No overload ever tripped. I accidently reclosed the breaker in less than 3 minutes, the lights dimmed for a second and then the overload clicked open with just the fan going. Breaker held as it should. (For those who don't know when you shut down a refrigeration or HVAC compressor and then turn it back on in a short time the rotor wont turn from the pressure difference in the lines. Motor will pull locked rotor amps) In this scenario its safe to assume the overload worked like it should on a hard overload, but was out of range for a soft overload. A 15 amp breaker would fail under UL tests holding over 25 amps for half an hour. The overload should've mimicked the trip curve of a 15 amp breaker but failed to do so. Under code its allowed to upsize the breaker several times over the rating of the wire. If this was a 30amp breaker you would've automatically hit the 90* rating of the wire.


Its true wiring ampere ratings are all based on worst case scenarios (under insulation, bundled, carrying full load...) but if CMP or UL knew overloads weren't working like they should something would change. Maybe the issue has been noted and newer ones are more reliable. Who knows.


I am certainly not disagreeing with you, rather agreeing with a known concern.

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