Apparently today I learned something

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ManyHats

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Hey all, I'm new to the board and figured I'd ask some pros.
Apparently today I learned something. Not sure I like it though. It has been my understanding that a motor circuit can be rated as high as 125% of the circuit rating. I know the standard is FLA is 80% of the breaker rating. I learned today that you only have to upsize the breaker leaving the wire too small. I see occasionally a 30a breaker w/12 AWG or a 40a w/10AWG. Aren't thermo breakers designed to handle inrush loads. I typically see this on A/C condensers and well pumps. 240v/1Ph. The breaker is supposed to protect the wiring. Now, not only does the current have to rise from say 16a to 20-24a to trip a 20a breaker it now has to rise an additional 10a for a total of 14a to trip a 30a, I would think that would let some smoke out. Can anyone explain this. I have been talking to others and the answers are every which way. They are citing code that seems to go in circles, or is that my head. Anyway, I've lined up those 6 brain cells I got. I'm ready to listen.
 
Re: Apparently today I learned something

Originally posted by ManyHats:
I learned today that you only have to upsize the breaker leaving the wire too small.
Click to expand...
The conductor if sized properly to the load is will not be to small.

Originally posted by ManyHats:
I see occasionally a 30a breaker w/12 AWG or a 40a w/10AWG.
Click to expand...
You might even see 14 AWG with a 60 amp breaker.


Originally posted by ManyHats:
The breaker is supposed to protect the wiring. Now, not only does the current have to rise from say 16a to 20-24a to trip a 20a breaker it now has to rise an additional 10a for a total of 14a to trip a 30a,
Click to expand...
This is the key, the current can not rise above the level of the conductor assuming the conductor was properly chosen.

Motor loads are required to have motor overload protection this is not the breaker. These overloads are usually installed at the motor controller.

If the motor draws more current than it's rating these overloads will open before damage to the conductors happens.

The breaker is only there to protect the wire against short circuits or ground faults.

[ February 15, 2005, 06:14 PM: Message edited by: iwire ]
 
Re: Apparently today I learned something

ManyHats

Ever look at 6/2 NM and wonder why the grounding conductor is only a #10? It's because in a short circuit the current can be as high as 10,000 amps. This will open the breaker so fast that it doesn't have time to even heat up and thus it is protected. With motor loads you have a overload that will be set to just above the FLA this will protect the conductors from a overload, Then all the breaker at the panel has to do is protect the circuit from short circuits Like what Bob has said. One of the reasons you might not see this as much is because you still have to contend with voltage drop and if a run of #12 is to long the motor might have a hard time starting. :)
 
Re: Apparently today I learned something

I've seen these overloads at the motor starter (contactor). But those were on motors at or above 5hp. These are typically 1-3hp used for residential well water systems and A/C's w/no real motor controls. Guess I should have explained better.
 
Re: Apparently today I learned something

Many Hats

As the previous posts (iwire +hurk27
)
have indicated Motor branch circuits are a combination of two types of protections that are generally considered in a standard overcurrent device.
Over load (motor running protection 430.31 and 32)
And Short circuit protection BIG BOOM. See 430.52 for short circuit and ground fault protection.


Take a look at Table 430.151 02 or 251 in the 05 for LRC.


The fuse at the beginning of the circuit appears to be too large to protect the selected conductors , 430.22 125% of motors FLC. But by design The conductors are protect at there selected ampacity by the Running Protection 430.31,32

The fuse protects the circuit from large current flow hundreds or thousands of amps but it still must be large enough to allow the motor to start 6 X FLC.

See 240.4 G and it's table.This method has been around for a long time and works good and lasts a long time. If you can get a hold of a hand book the explanatory print does a nice job.

Charlie
 
Re: Apparently today I learned something

Manyhats,
If these are 1-3 hp single phase motors, they almost always have oerload protection in them in the form of a thermal overload. Look on the motor nameplate and it should say "thermally protected". That is your OL protection in lieu of a heater type of OL as used on 3 phase (or really big 1 phase) motors.

Mike
 
Re: Apparently today I learned something

Duh... I knew that. Thanks Mike. I got hung up on the wire and breaker size and forgot about the thermo. Now I understand the whole wire size issue and be cool with it.
Thanks everyone for the help.
 
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