Upsizing breaker for motor load, necessary?

[mbrooke]

Ok, maybe I don't understand the code all that well... but why do AC units and motors need an upsized breaker if the current never goes near the handle rating? I can understand larger units, but why smaller ones?


Molded case trip curves:


http://static.schneider-electric.us...it Breakers/QO-QOB Circuit Breakers/730-4.pdf

http://static.schneider-electric.us...it Breakers/QO-QOB Circuit Breakers/730-5.pdf

http://static.schneider-electric.us...it Breakers/QO-QOB Circuit Breakers/730-6.pdf

 

[jim dungar]

... do AC units and motors need an upsized breaker if the current never goes near the handle rating...

Up sizing is optional not a requirement.

 

[GoldDigger]

Up sizing is optional not a requirement.

For an appliance with MCA and MOCPD on the nameplate, the OCPD can be anywhere from MCA up through MOCP. But the smaller it is the more likely to get nuisance trips on starting. Particularly on restarting without enough down time to relieve the system pressure across the compressor.
Usually the built-in overload will terminate the latter condition without blowing the OCPD, but if the OCPD is at or near MCA, that might not happen.

 

[mbrooke]

But why upsize for inrush when breakers trip magnetically around the same point regardless of amperage?

 

[GoldDigger]

But why upsize for inrush when breakers trip magnetically around the same point regardless of amperage?

Because you are upsizing the thermal trip, not the magnetic.
The starting surge and inrush (two different things) are both short term, but are still on the thermal curve, not the instantaneous magnetic trip curve.

 

[mbrooke]

Because you are upsizing the thermal trip, not the magnetic.
The starting surge and inrush (two different things) are both short term, but are still on the thermal curve, not the instantaneous magnetic trip curve.

Ok, that makes more sense then. I always assumed the issue with ACs was the magnetic trip part hence my apprehension.

 

[don_resqcapt19]

But why upsize for inrush when breakers trip magnetically around the same point regardless of amperage?

That is not correct, the instantaneous trip point moves up as the breaker rating moves up.

 

[mbrooke]

That is not correct, the instantaneous trip point moves up as the breaker rating moves up.

I disagree, look at the trip curves.

 

[kwired]

I disagree, look at the trip curves.

they look like they are different to me.

Shape of the trip curve is similar, values at borders of the shape are different in each one, plus each horizontal axis on the graph represents "multiples of rated current, so a multiple of 10 for a 20 amp breaker is 200 amps but is 350 for a 35 amp breaker.

 

[mbrooke]

they look like they are different to me.

Shape of the trip curve is similar, values at borders of the shape are different in each one, plus each horizontal axis on the graph represents "multiples of rated current, so a multiple of 10 for a 20 amp breaker is 200 amps but is 350 for a 35 amp breaker.

At the beginning the thermal portion stays the same for all listed amperages (simply multiply by handle rating), however towards the bottom in the magnetic region the curves actually begin to shift to the left as the amperage increases?

 

[iwire]

At the beginning the thermal portion stays the same for all listed amperages (simply multiply by handle rating), however towards the bottom in the magnetic region the curves actually begin to shift to the left as the amperage increases?

What specific breaker does that apply to?

 

[mbrooke]

What specific breaker does that apply to?

The trip curves posted in my first post, Square D molded case breakers.

 

[kwired]

What specific breaker does that apply to?

Upper right hand part of page (in the documents he linked) tells which breakers the graphic applies to - there are multiple breakers listed on each page - but each page is not the same. They pages he linked to are all in the QO line though.

 

[iwire]

The trip curves posted in my first post, Square D molded case breakers.

I am not a mind reader.

If you post a graphic it helps to identify it.

 

[mbrooke]

I am not a mind reader.

If you post a graphic it helps to identify it.

My mistake :ashamed1:

Anyway, why does the magnetic trip go down as the breaker rating goes up? :blink:

 

[kwired]

My mistake :ashamed1:

Anyway, why does the magnetic trip go down as the breaker rating goes up? :blink:

The actual "trip point" isn't necessarily going down, you need to factor in the "multiples of rated current" to come up with what the actual trip rating is.

Like I said earlier if the instantaneous trip point is 10 times rated current it would be 200 amps for a 20 amp breaker, but if you go to a different graphic for a 30 amp breaker and the instantaneous trip is only 8 times the rated current then you have 240 amps trip point, but if you look at both curves and not look close at the values represented - they look like nearly the same graphics.

(I did not use numbers directly from the graphics you linked to - just tried to use easy to calculate numbers for the purpose of the example, I think actual IT points were higher then my example)

Looking at the 30-40 amp selection - the first 1 cycle trip point is the same for all three breakers, but after that they start to vary, the other pages have similar shape to curve as well, but maybe not same current or time at same positions of the shape.

 

[Mgraw]

So what is the answer, Thermal or Magnetic?

 

[mbrooke]

The actual "trip point" isn't necessarily going down, you need to factor in the "multiples of rated current" to come up with what the actual trip rating is.

Like I said earlier if the instantaneous trip point is 10 times rated current it would be 200 amps for a 20 amp breaker, but if you go to a different graphic for a 30 amp breaker and the instantaneous trip is only 8 times the rated current then you have 240 amps trip point, but if you look at both curves and not look close at the values represented - they look like nearly the same graphics.

(I did not use numbers directly from the graphics you linked to - just tried to use easy to calculate numbers for the purpose of the example, I think actual IT points were higher then my example)

Looking at the 30-40 amp selection - the first 1 cycle trip point is the same for all three breakers, but after that they start to vary, the other pages have similar shape to curve as well, but maybe not same current or time at same positions of the shape.

My understanding from the trip curves:

15-25 magnetically trip at 225

30-40 magnetically trip at 300

45-70 magnetically trip at 350 amps

It seems the magnetic trip is actually adjusted to remain the same across several amperages and increase on slightly as a whole relative to say having each increase by 10x, ie 15 amps trips at 150 while 70amps trip at 700?

 

[Mgraw]

This is what Square D has to say:

Cause:Circuit breakers trip

Resolution:
1. First, determine if the breaker trips on startup or if it trips after running for a while.* If it trips on startup, it is tripping magnetically; if it trips after a while, it is tripping thermally.
2. If it is tripping magnetically, evaluate the load to see if the inrush current is above the instantaneous trip level of the breaker.* If the breaker has adjustable magnetics, increasing the setting may solve the problem.* If not, the breaker may be inadequately sized, or there could be a problem with the circuit such as a short circuit
3. If the breaker is tripping thermally, determine if there is truly an*overcurrent.* Overcurrents may be caused by inadequate breaker sizing for the load, single-phasing (on a 3p system), worn or damaged equipment, or too long of a run feeding a motor (a motor will draw more current at the end of a long circuit than a short one.*

 

[mbrooke]

This is what Square D has to say:

Cause:Circuit breakers trip

Resolution:
1. First, determine if the breaker trips on startup or if it trips after running for a while.* If it trips on startup, it is tripping magnetically; if it trips after a while, it is tripping thermally.
2. If it is tripping magnetically, evaluate the load to see if the inrush current is above the instantaneous trip level of the breaker.* If the breaker has adjustable magnetics, increasing the setting may solve the problem.* If not, the breaker may be inadequately sized, or there could be a problem with the circuit such as a short circuit
3. If the breaker is tripping thermally, determine if there is truly an*overcurrent.* Overcurrents may be caused by inadequate breaker sizing for the load, single-phasing (on a 3p system), worn or damaged equipment, or too long of a run feeding a motor (a motor will draw more current at the end of a long circuit than a short one.*

That's true, but if I increased a 15amp breaker to a 25 amp breaker for an AC unit the magnetic trip threshold would remain the same. But that leads me to look at it from a different perspective, why do they go through the trouble of keeping the magnetic trip the same across several ranges current ratings? :?