Tripping breaker

[cityhick]

I recently ran a new service line and put up a sub panel in the garage. I ran 12-2 w ground in the garage. My air compressor is a 110 Craftsman oil less model 30 gallon rated at like 3hp. When it fires up and puts the most strain on the electrical it is fine but after it runs a while the breaker for that plug will trip. It also happens with my welder. Miller 110 volt. If I weld with any consistency say 20-30 seconds without stopping it will trip the breaker. The welder is on a different plug but on the same breaker. I have never used the welder and compressor at the same time to overload the breaker. I have not used any different outlets to see if it happens on a different circuit. Could it be that I have a bad circuit. The box and circuits are new. Square D. Any thing I should be checking that Im not thinking of?

Mike

 

[raiderhillbilly]

Could be a loose screw on the breaker or in the recepticle. It could also just be a bad breaker.

 

[MrMark]

You should try a different circuit, but the best way to know for sure is to put a current clamp on the circuit to see what is going on. The compressor is probably pulling more than 20 amps when it runs. Perhaps it has motor issues.
The inverse time delay breakers work on a thermal trip curve and it could take 20-30 seconds to trip if the draw is say 25 amps.

 

[PRH44]

A true 3 HP motor roughly 34 full load amps. It will trip your 20 amp circuit. You really need to wire the compressor for 240 volt if possible. Otherwise you will need 40 amp 120 volt circuit.
My guess is that compressor is a 2 HP in disguise that will run at 24 amps on a 30 amp circuit requiring # 10 wire

 

[MrMark]

If it came with a 120V cord and plug, it is probably less than a 2 hp motor and designed to max out a 20 amp 120V circuit. I have a compressor that says "5 HP" but comes with a plug for a 120V receptacle and it is designed to pull about 15 amps. It is amazing the marketing nonsense that these compressors use with regard to HP ratings.

What does the rating plate on the motor state? I'll bet it either says SPL for "special" or is blank where the HP rating is stated. Then look for the amps on the rating plate. I'm guessing it is 15.

 

[pattenp]

You said you used 12-2 wire, but what size breaker did you use? The Craftsman 30 gal oil less compressor should only require a 120V 20 amp circuit.

 

[MrMark]

So on the 12-2 nm cable he cannot upsize the breaker as otherwise permitted by the rules to avoid nuisance trips (up to 250 percent of FLC) because 12-2 cannot be protected in excess of its 20 amp rating per the 60 degree ampacity column. Correct?

 

[rustedoutdodge]

You do not want to put #12 wire on higher than a 20amp breaker. You are either drawing too much current or there is a loose connection on the breaker & heating the breaker or the breaker is bad.

 

[PRH44]

So on the 12-2 nm cable he cannot upsize the breaker as otherwise permitted by the rules to avoid nuisance trips (up to 250 percent of FLC) because 12-2 cannot be protected in excess of its 20 amp rating per the 60 degree ampacity column. Correct?

OP please get the specs on you compressor?


MrMark how have you been?

To answer your question Mark.
NEC 430 will allow a larger overcurrent protective device on a motor circuit.
I caution this for A DIYer. There are a few things to consider
The motor would need to be thermally protected integral or external.
The circuit could not feed convenience outlets, it would feed the motor only not the welder or other loads not really a good option for the OP

The proper way to approach this issue is find the compressor specifications.
Rule of thumb is 1 HP on a 20 amp branch circuit at 120 volt. Sometimes a 1HP will claim to be a 1.5 or 2 HP
A 20 amp circuit will carry 16 amps continuously (3 hours or more) with out tripping.
It will carry up to 20 amps at short intervals or what is known as intermittent duty.
A compressor is an intermittent load BASICALLY, it can be rode hard and run continuously resulting in an overload condition.
A breaker that has tripped several times will be weak and needs replaced
The length of the run can cause a voltage drop causing overload. (2KID/CM=VD)
A wore out receptacle can cause arcing increasing the resistance of the connection causing heat build up.
My guess is this compressor is pushing the limitation of the branch circuit.

 

[MrMark]

Good, Phil.

Good info. I was really wondering about 240(D)???. I'll have to look it up. The section that limits nmb to 60 degree C ampacities and does not permit upsizing breakers as permitted in other sections such as 430 for motors.

Just trying to nail this down. My understanding is that if the conditions you detailed above were satisfied (even adding hardwired into your requirements) that he could upsize with 12 thhn but NOT with 12 nmb. Same issues are going to come up on AC circuits where romex is rated lower than thhn.

If this were me, the first thing I would do would be to verify the actual draw of the machine to see what we have going on. Is this machine pulling in excess of the rated current as stated on the motor? Do we have a machine problem or a circuit breaker/connection problem?

 

[MrMark]

Apparently, this 1HP is claiming to be 3HP and my 1HP is claiming 5HP!

 

[MrMark]

NEC 334.80 does not allow nm cable conductors ampacity to exceed the stated ampacity in the 60 degree column, but does it allow the overcurrent protection device to be upsized under 430, for example, as permitted by the exception of 240.4(G)?

Phil?

 

[PRH44]

NEC 334.80 does not allow nm cable conductors ampacity to exceed the stated ampacity in the 60 degree column, but does it allow the overcurrent protection device to be upsized under 430, for example, as permitted by the exception of 240.4(G)?

Phil?

Bingo Mark

 

[cityhick]

It is a 20amp breaker and like I said in the original post, It will also trip if I weld for more than 20-30 seconds continuously. The welder is a Miller 140 110 volt mig welder. The compressor says 5hp on the tank but in smaller print is says 5hp max output 2hp running. Before when the garage had 15 amp replaceable fuses it would blow a fuse every now and then when the compressor would first fire up but never blow one after it had been running. The motor on the compressor is under a plastic housing and its too damn cold to be farting around with it right now. Heat is the next project. I have extra breakers so I can check all the connections to make sure all is good and then change it out and see whats up. Thanks guys.

 

[MrMark]

Bingo Mark

Phil, so it DOES allow upsizing of overcurrent protection device under 430, even with romex? That is the way I read it anyway.

 

[PRH44]

AS stated before I caution DIYers when using this method.
One needs to understand that an inverse time breaker has two components.
1. Thermal that responds to heat over time. This portion protects against heat/overloads but will ignore short term in rush
2. Magnetic portion that will respond instantaneously when its magnet threshold is reached. This portion is for ground fault/short circuit protection where current can reach thousand of amps in a matter of cycles.

 

[PRH44]

Phil, so it DOES allow upsizing of overcurrent protection device under 430, even with romex? That is the way I read it anyway.

Yes it does you are correct

 

[PRH44]

It is a 20amp breaker and like I said in the original post, It will also trip if I weld for more than 20-30 seconds continuously. The welder is a Miller 140 110 volt mig welder. The compressor says 5hp on the tank but in smaller print is says 5hp max output 2hp running. Before when the garage had 15 amp replaceable fuses it would blow a fuse every now and then when the compressor would first fire up but never blow one after it had been running. The motor on the compressor is under a plastic housing and its too damn cold to be farting around with it right now. Heat is the next project. I have extra breakers so I can check all the connections to make sure all is good and then change it out and see whats up. Thanks guys.

Ok understood. I do believe your breaker could be weak. You are operating this branch circuit on the upper end of its capability. Do have a Digital Volt meter?
What size service in your garage?
Is it an attached garage with the main service for the house?

 

[PRH44]

AS stated before I caution DIYrs when using this method.
One needs to understand that an inverse time breaker has two components.
1. Thermal that responds to heat over time. This portion protects against heat/overloads but will ignore short term in rush
2. Magnetic portion that will respond instantaneously when its magnet threshold is reached. This portion is for ground fault/short circuit protection where current can reach thousand of amps in a matter of cycles.

When installing a motor sometimes we need to up size the overcurrent protective device to keep the motor start up current from tripping the Magnetic portion of the breaker. Known as NUISANCE TRIPPING.

The motor is supplied with overload protection typically in the form of an overload relay. This is set in the range of 110 to 125 percent of full load amps depending on service factor on name plate.
This overload will protect the motor from over current as a result its protecting the branch circuit also.

 

[cityhick]

Garage is detached with 2-2-2-4aluminum running from 60 amp breaker at the house to a sub panel in the garage. The garage panel is a Square D 100 amp panel with a main shutoff. Welder, compressor, and metal lathe are all in one corner of the garage all plugged into separate outlets but on the same 20 amp breaker. One is only in operation at any given time. I could see if I was welding and the compressor kicked on and tripped the breaker but whenever I am not using the compressor it gets switched off. If the plug outlet is only rated at 15 amps, could that cause an issue. My Pop gave me a box of plugs that said 15amp on the box but he said they were the same ones he used and I got the welder from him and had used it plenty of times at his house and never tripped a breaker and we have nearly identical set ups.

 

[PRH44]

Garage is detached with 2-2-2-4aluminum running from 60 amp breaker at the house to a sub panel in the garage. The garage panel is a Square D 100 amp panel with a main shutoff. Welder, compressor, and metal lathe are all in one corner of the garage all plugged into separate outlets but on the same 20 amp breaker. One is only in operation at any given time. I could see if I was welding and the compressor kicked on and tripped the breaker but whenever I am not using the compressor it gets switched off. If the plug outlet is only rated at 15 amps, could that cause an issue. My Pop gave me a box of plugs that said 15amp on the box but he said they were the same ones he used and I got the welder from him and had used it plenty of times at his house and never tripped a breaker and we have nearly identical set ups.

Sounds like you are OK on voltage drop but I would measure your voltage at the garage. I would be curious to see what it is. You could have low voltage from your utility at certain time of the day.
20 amp receptacles would be needed to maintain a full 20 amp rated circuit
Has the compressor tripped the breaker several times? Fairly new breakers?
Please do check your connections and if you have a spare breaker change it out.
If you have access to an amp probe. Take current readings on that compressor.
And if you can read voltage when it running also.