Indoor unit

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codequestion

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I have gas furance indoor unit which uses 3/4 hp 120v fan for heating. No MOCP is provided. Is it because this is not hematric motor or compressor for heating? What would be properly size overcurrent protection be for that fan nec 2014?

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A disconnecting means would be required. It probably would not contain an OCPD, unless fuses are speced.

The OCPD for short circuit and ground fault protection would prolly be the panel breaker.

OCPD type and multiplier for start up current would be chosen per 430.52 , motor ampacity would be chosen from motor tables at end of 430, not the nameplate.

overload protection is probably built into the motor given the size. nameplate should say or have “TP” as part of motor identification.
 
jumper said:
A disconnecting means would be required. It probably would not contain an OCPD, unless fuses are speced.

The OCPD for short circuit and ground fault protection would prolly be the panel breaker.

OCPD type and multiplier for start up current would be chosen per 430.52 , motor ampacity would be chosen from motor tables at end of 430, not the nameplate.

overload protection is probably built into the motor given the size. nameplate should say or have “TP” as part of motor identification.
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Does not tell me percent to us based on RLA or FLA. Also what about table 430.248 i cannot use that?

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codequestion said:
Ok i got it nevwr mind percents are right there on table. So what about size of non fused disconnect itself for the fans. 20 amps non fused disconnect ok for the fans?

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What code section would or would not allow this? Look for motor disconnecting means in 430.

What does it tell you?

We can discuss your answer after you present one.
 
jumper said:
What code section would or would not allow this? Look for motor disconnecting means in 430.

What does it tell you?

We can discuss your answer after you present one.
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Would it be sized pwr nec 430.110 (C)(2)? 13.8x1.15 = 15.87 so 20 amps non fused disconnect?

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codequestion said:
Would it be sized pwr nec 430.110 (C)(2)? 13.8x1.15 = 15.87 so 20 amps non fused disconnect?

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I would think 430.110(A) is more applicable. C is for combination loads.

Same math though. HVAC loads are also usually considered continuous loads so I would multiply by 125% also, but a 20A disco still works.

And you correctly used table amp value not nameplate.

Nameplate value would have been used if one needed to size overloads/ heaters but small motors usually have internal thermal overload protection.
 
I'm not an engineer. I am an HVAC technician. I can tell you that almost all of these (incl. 3/4 HP 120V) are on a 15A circuit breaker in the panel. The local disconnect is sometimes a cord-and-plug (NEMA 5-15P), and sometimes a 15A rocker switch.

I guess I never saw the need to do the math to verify something that's been done a certain way for decades.
 
jumper said:
Sorry if this sounds rude, but I find that philosophy akin to being a lemming or such.

“That’s the way we have always done it” does not work for me.

Common does not mean correct.
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I agree with that principle. But I literally see several hundred of these a year. I've never not seen one installed on something other than a 15A circuit. :?

Obviously, a lot of people have already engineered this, including every manufacturer and local home builder.

I'm not referring to just some local custom of "old guys."
 
MAC702 said:
I agree with that principle. But I literally see several hundred of these a year. I've never not seen one installed on something other than a 15A circuit. :?

Obviously, a lot of people have already engineered this, including every manufacturer and local home builder.

I'm not referring to just some local custom of "old guys."
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The question noils down to what would you do for next installation you see continue to use 15amp?

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codequestion said:
The question noils down to what would you do for next installation you see continue to use 15amp?
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I'm certainly not going to tell an engineer what to design. So maybe it's best he doesn't know what I would do when I've never heard of a reason why every existing installation I see on a daily basis might be questionable. All I'm telling him is what the real world is doing.

I'd be very interested if the math says everyone is doing it wrong, though. :ashamed:
 
MAC702 said:
I'm certainly not going to tell an engineer what to design. So maybe it's best he doesn't know what I would do when I've never heard of a reason why every existing installation I see on a daily basis might be questionable. All I'm telling him is what the real world is doing.

I'd be very interested if the math says everyone is doing it wrong, though. :ashamed:
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And the AHJ is in agreement with the engineer design?

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codequestion said:
15 amp breakers never nuisance trip i.e when motor turn on or other conidtions?
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Not that I've ever seen. The only issues I've seen somewhat related to this have been when the installers used the backstab connections on the 15A switch, and the switch fails from overheating. I replace with another 15A switch, using the screws.
 
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