air compressor

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AE-29

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I came across an air compressor 7 1/2 hp 230v sized with 30 amp ocpd. It's flc rating 22 amps but runs at 26. It was tripping the 30 amp ocpd because it would run at 31-32 amps depending on the load. My question is i believe it should be sized at 40 amps but if i'm reading the nec correct it says motor compressors ocpd are to be sized at 125% of flc. Am I correct in sizing at 40 amps?
 
You have three general sizing requirements for motors.

1. branch circuit size - 430.22
2. motor overload protection size - 430.31
3. motor short-circuit and ground-fault protection size - 430.51

Based on the information you have provided,

1. 26A x 125% = 32.5A = #10 AWG. conductors
2. 26A x 115% = 29.9A overload
3. 26A x 250% = 65A breaker

Now, this based on your information. But according to table 430.248, the FLC of this motor would be 40A. This changes the entire calculation.
 
Now, this based on your information. But according to table 430.248, the FLC of this motor would be 40A. This changes the entire calculation.
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And per 430.6(A)(1) you must use the table value for everything except the sizing of the overload device.
Don
 
Well, 430.6, reference the sizing of conductors, branch short ckt and gnd fault when flc is not listed. art.440.52(3) says 125% of rated load current for bc short ckt. But it must have time delay to allow for motor compressor start up. It is my understanding that air compressors and a/c refrig motors are not rated the same like synch, induct. etc, motors. If I'm reading this right and thats how the air compress. is already wired, FLC listed @22amps(125%)= sizing for 30 amps ocpd. but the running flc is more than 80% of the ocpd then it's not leaving much room before the breaker trips. Am i justified in sizing up to 40a. or 175% FLC.
 
AE,
First an air compressor in not covered by Article 440...it is an Article 430 application. Table 430.52 says that the branch circuit OCPD can be rated at 250% of the current shown in Table 430.250. The FLA is 22 amp and you are permitted to use a breaker rated at 60 amps. The conductor is required to be 125% of the FLA from the table or 27.5 amps...a #10 on a 60 amp breaker is permitted by the code for this installation. Note that the motor will also require some form of overload protection...most likely an overload relay that is part of the motor controller.
Don
 
Thank you Don, I was unsure as to article 440 because it is speaks of motor controllers, and I could not find anything specifically referencing air compressors. Thanks again. This is a great site.
 
I do have an issue with 60 amp breaker on #10's..the type of compressor he is installing is used in small shops and I have never had the problem he is having..I install on 30 amp breakers because that is what the manufacture states in the installation instructions, at-least for the ones I have installed..The next issue is when the motor goes out the owner almost never gets the motor from the manufacture and just gets it from like northern tool and the cheap one with out the built in circuit breaker..Now you have an unsafe installation on #10's with a 60 amp breaker..I would check the compressor out something is wrong..I would not up size the breaker but that is just me..I will have to read on this one (I think the electronic version is coming this code book) but I believe in order to use the 60 amp on #10's you must have a motor controller with overload protection..I do not believe the overload on the motor counts..
 
cschmid,
I do have an issue with 60 amp breaker on #10's..
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This is a motor circuit and the motor requires some type of overload protection. The 60 amp breaker only provides ground fault and short circuit protection. For those types of faults there is very little difference in the protection provided by a 30A or 60A or even larger breaker. As far as someone making a non-compliant replacement at some future date, that can be said about any installation.
Don
 
don_resqcapt19 said:
220/221,

Why? Do you have a problem with a #10 on a 60 amp breaker for a motor circuit? If so why?
Don
Click to expand...


Because since 1973 I have been thinking that 10 wire needs to be protected by a 30 amp breaker.


I come her to try and learn and I just get reminded that I obviously don't understand anything.

I give up.
 
Because since 1973 I have been thinking that 10 wire needs to be protected by a 30 amp breaker.
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The wire is still protected at its ampacity in a circuit like this, just in a manner that is different. The motor overload device protects the conductors from overload. Overload protection can be installed at any point on the circuit. It does not have to be installed at the supply end of the circuit. In the case of a motor circuit it is often the overload relay in the motor starter, but it could be the overload device that is built into the motor itself. This device will open and stop the flow of current when the motor is overloaded. The motor conductors are required to be sized at 125% of the rated motor current and so is the overload protective device, so the conductor cannot not be damaged by excessive current. The ground fault and short circuit protection is provided by the breaker. If you look at the time trip curves for the breakers, you will find that for these types of faults which cause a large current flow, there is very little difference between the trip times for a 30 and 60 amp breaker. The breaker will act to open the circuit if there is a ground fault or short circuit. The conductor is protected, but just not in the same way as a standard circuit where the breaker is providing the overload, short circuit and ground fault protection.
Don
 
220/221 said:
Because since 1973 I have been thinking that 10 wire needs to be protected by a 30 amp breaker.
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See the note at the bottom of the table that tells you about the 30a-#10 specs.

Hint: it says "See 240.4(D)"
 
So...a fault in the wiring between the breaker and the motor overload protection will be detected and cleared by the motor overload protection?



LarryFine said:
See the note at the bottom of the table that tells you about the 30a-#10 specs.
Click to expand...



No. I'm a plumber now.
 
So...a fault in the wiring between the breaker and the motor overload protection will be detected and cleared by the motor overload protection?
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No...that fault will be cleared by the breaker.
Don
 
220/221 said:
Because since 1973 I have been thinking that 10 wire needs to be protected by a 30 amp breaker.


I come her to try and learn and I just get reminded that I obviously don't understand anything.
Click to expand...

Don't give up, hang out here.

A while back I would have felt as surprised as you that 10 AWG may be on a larger breaker.

Normally a breaker or fuse is installed to provide both overload and short circuit protection.

With a motor circuit things change, the breaker or fuse is only installed for short circuit and ground fault protection.

If there is a short circuit (Line to line direct connection) or a Ground fault (line to ground direct connection) the 60 amp breaker will open quick enough so the 10 AWG will not be damaged.

As an example, have you ever run a 60 amp feeder?

What size EGC would you run?

The code requires a 10 AWG, if there is a ground fault that 10 AWG will be large enough to open the 60 amp breaker.

Short circuits are very different then overload, overload is another issue all together.

The only way for motor circuit conductors to be overloaded is if the motor starts to malfunction.

So by installing the code required motor overload protection we also end up protecting the conductors for overload.

If the motor draws to much current the motor overloads will open stopping the overload condition
 
I thought I put a post on here last night..Must have been a dream..Yep you can put a 60 amp on #10's but when the motors overload fails the breaker will trip and then they will call another electrician to fix all the damage..
 
220/221 said:
:-? That's it. I'm done.:confused:


I am going to become a plumber.
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Buddy, you ain't the only one! I started an 80 week "Professional Electricians Program" 10 weeks ago and I've learned WAY MORE at this site. I'm a bit of a hero amongst my school mates, as I've called out the teacher twice... Once on where fault current most wants to go. He and the rest of the class had never heard of this "source" I was talking about.;) A few code references and Ohm's law later a bunch of light bulbs went off.

Fortunately the teach is a good sport.

I wired houses for 7 years and cringe to think how much money I wasted wiring AC's correctly. I'm guessing tens of thousands!:cool:
 
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