need to downgrade breaker (from 40amp to 15amp) for new hvac?

Read article 440 and 430, then get back to me. You are citing 240.4(D) and is says

(D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

HVAC units are one of the exceptions in (G)

Reply to
gfretwell
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Exactly right Bud. In fact there is a question on every inspector exam I have taken about a 115v 1HP motor with internal overload protection (16a FLA) and the correct answer is "40a breaker, 14 ga wire". I understand this is an anomally using all the exceptions and "round up" rules but it is NEC compliant.

Reply to
gfretwell

40A to 15A more efficient???? Now THAT is one efficient system...

I ain't buyin', sorry.

--

Reply to
dpb

Really?

That's plain nuts!

Your run of the mill CB takes seconds to trip just from FLA currents. There just isn't any need to put a 40 amp breaker on a motor circuit.

If there is something about the motor the keeps it from coming up to speed in a second or two then fix that. But don't put a 40 amp breaker on #14 wire.

Reply to
John Gilmer

I suppose I could walk you through it all but this is what the code allows The short answer is

16a FLA from the nameplate is 20a, Article 430 (motors) is one of the exceptions to 240.4(D) (the 14=15a rule) in 240.4(G) 310.16 (the wire ampacity table) lists 14 ga copper at 20a in the 60c column. If you have internal overload protection you can use table 430.52 to size the branch circuit O/C device. If that is an inverse time breaker it can be 250% of FLA (16 x 2,5 =40a)
Reply to
gfretwell

Normally I'd agree, but if a resistive heat air handler was changed out for a variable speed heatpump only air handler, it would be completely logical. Or maybe the original breaker was oversized.

Reply to
Travis Jordan

I just checked Trane's documentation and all of their 1.5T and smaller heat pump systems state a MCB of 15A. It is quite possible the old system (1987 vintage, probably twice the load) had an oversized breaker installed also.

Reply to
Travis Jordan

FLA (full load amps) are not the problem. Starting current/LRA (locked rotor amps) are. I presume this was a typo on your part.

As a rule of thumb a motor draws a current (LRA) of about 6x its FLA when it starts.

Looking at a SquareD current - trip-time curve, at 6x the rated breaker current a breaker may be in it's "instantaneous" trip region - no time delay.

If the motor was rated 80% of the breaker rating (more likely) the starting current is (0.8)x(6) = 5.4x the breaker rating. The trip time can be 0.3 to 2.5 seconds - not reliable.

Obviously the motor current goes down as the motor accelerates. But that doesn't help in the instantaneous trip region. And in the 0.3 to 2.5 sec range there is no guarantee the motor will start. If turn up a thermostat then turn it down a refrigeration compressor it will likely not restart immediately. That causes the LRA until the motor protector opens the circuit. You would generally not want the circuit breaker to open. And high inertia motor loads increase the time to start.

The NEC is really quite pragmatic. Rules like this come from field experience as well as engineering considerations. Not only can a circuit breaker be 250% of FLA, as in gfretwell's example - if the breaker does not permit the motor to start the breaker can be increased to 300 or

400% of the FLA.

And as has been a couple times, the breaker provides short circuit protection for the wire, not overload protection.

-- bud

Reply to
Bud--

I have one issue with your otherwise excellent post -- this last sentence:

The breaker provides *overcurrent* protection for the wire -- IOW, the breaker is sized to open if more current is flowing through the wire than it can safely handle. This does not occur solely through short circuits, but can result from overload as well (e.g. fifty 60W incandescent light bulbs on a single 15A circuit is supposed to pop the breaker, even though there is no short).

Reply to
Doug Miller

Except OP specifically spoke of "emergency heat" kicking on which seems to preclude that -- or, of course, could be he just does not know what he just bought and paid for and had installed...

In about the same category as the above...

imo, ymmv, $0.02, etc., etc., ...

Reply to
dpb

Note that gfretwell and I agree on this.

This is a motor circuit, not light bulbs. The rules are in article 430.

The circuit breaker provides *short circuit* protection - 430 part 5.

*Overload protection* is typically provided by thermal overload units in, or attached to, the motor. - or motor starter (contactor) overload units - 430 part 4. (Note that the overload protection is near the motor, not the panel.)

Wire size is 430 part 2.

gfretwell's nice 'worst case' example gave code citations for his calculations - would be a good place to start.

IMHO article 430 is of similar difficulty to article 230 - grounding. Article 230 is the most difficult of the commonly used articles. As I wrote earlier, there is a reason why electricians are required to have licenses (in many jurisdictions).

If you (and John) are appalled by the 'oversizing' of the breakers for motors make sure you don't read article 630 - welders. Particularly for low duty cycle welders.

-- bud--

Reply to
Bud--

Right, I know -- I'm objecting to what appeared, at least, to be a general statement that breakers provide only short-circuit protection. In the specific cases you cited, that's correct, but as a general rule, it's not.

Reply to
Doug Miller

Doug I think the point was that in the case of motors on dedicated circuits the rules are different. You are right that in general lighting circuits the breaker provides overload protection as well as overcurrent protection. The installer has no control over what the user will plug in. That is the reason why we have atrticle 240.4(D) that gives us the familiar rule of thumb 14ga=15a, 12ga=20a, 10ga=30a This includes the other general rule of thumb that you shouldn't load a circuit more than 80%. The 80% is built into this limit when you look at what 310.16 says the wire is really good for. (14ga=20a) NFPA knows the typical user will keep loading a circuit until the breaker trips and then unplug the clock if that is all it takes to make it hold.

Reply to
gfretwell

Right, I know that. I just wanted to make sure it was understood that as a _general_ rule, breakers are there to provide overcurrent protection generally, which is not the same as short circuit protection.

Reply to
Doug Miller

If the unit has aux heat, it may very well need 40 amps. Read the manual.

If you have to ask, you don't have the skills. I wouldn't reccomend to open a panel box unless you've seen it done several times, and have someone experienced working with you.

Reply to
Stormin Mormon

See if the paper work mentions the word amp, or ampacity.

Since this is greek, please leave the panel box closed. For your safety, you see.

Reply to
Stormin Mormon

My post was in the context of motors.

Your response was general - it would have helped if it was limited to non-motors.

-- bud--

Reply to
Bud--

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