Originally Posted By: jrabanus This post was automatically imported from our archived forum.
I came across a 30 amp breaker that had 12 AWG wire going into it while checking out a buddy’s electrical panel. I told him that this wire is rated for 20 amps not 30. He said that a “professional” installed it. What do you guys think? Also, does anyone come across this a lot?
Originally Posted By: jpeck This post was automatically imported from our archived forum.
Josh,
If that is for an a/c condenser unit, read the nameplate on the unit.
It may stated 'max overcurrent protection - 30 amps' and it may also stated 'min circuit ampacity - 20 amps', if it does, that would be correct (12 AWG on 30 amp breaker).
However, if it is not for an a/c, it is most likely incorrect.
Originally Posted By: rsummers This post was automatically imported from our archived forum.
I think your buddy should change the breaker to match the wire size or change the wire. Even the clerks at at the home improvement stores know how to size breakers to wire. Professional Electrician
Originally Posted By: Bob Badger This post was automatically imported from our archived forum.
rsummers wrote:
I think your buddy should change the breaker to match the wire size or change the wire. Even the clerks at at the home improvement stores know how to size breakers to wire. Professional Electrician 
Rolland I know you say that because you think it would be the right thing to do. However that may not be true.
Let's stick with Jerry's example of a AC unit.
Lets say name plate on this unit says the following.
Minimum circuit ampacity 24 amps.
Table 310.16 tells us 12 AWG is rated 25 amps
The name plate goes on to say
Maximum breaker 30 amps.
Bingo 30 amp breaker with 12 AWG conductor, you could use 10 AWG but the NEC would not require it.
There are times when feeding motor loads that the breakers are rated much higher than the conductors.
It is possible to have 12 AWG connected to a 60 amp breaker and still be code compliant. (You are unlikely to see this in a dwelling)
Bottom line before you say that 12s on a 30 are a violation, wrong or unsafe you have to know what they are feeding.
Originally Posted By: bkelly1 This post was automatically imported from our archived forum.
Not to get into a big “thing” with everyone as I do value all your opinions, but I am not sure I agree with some of the breaker size discussion. If the #12 wire is on a 30amp breaker, and the a-c only pulls 24 great, but what about when the a-c gets old and takes a little longer to start or bearings or something is going out and now its amp draw is at 30, long enough unitl hopefully the breaker trips before wire melts? I have personally never seen a #12 on 60 amp breaker in 10 years as a comm/ind. electrician. Maybe our local code is different. If the wire is good for 25, and the unit says 24, what about some de-rating?
We have always gone with breaker size determines wire size, of course there are some exceptions such as on larger motors that require more at start up...There is just too many things that could go wrong with a circuit to leave a oversized breaker on it, here that would be turned down by state inspectors very quickly.
Originally Posted By: Bob Badger This post was automatically imported from our archived forum.
Ben if your state adopted the NEC without changes they have to accept these installations.
There is an answer to why this is safe and allowed, the breaker is not the final overcurent device.
In AC units (not to sure if this applies to window units I will check) there are thermal overloads that will open before the circuit conductors are overloaed.
Same with motor circuit, the same motor overload units that protect the motor from smoking protect the circuit conductors from being overloaded.
In these cases the circuit breaker provides only short circuit and ground fault protection.
I will be glad to private message with anyone that wants more info on this or code references to back my position. 
Originally Posted By: rmoore This post was automatically imported from our archived forum.
Josh…
I guess we need to know if this was a dedicated circuit. Was the window A/C unit hardwired or was it just a "portable" 120 volt unit plugged into a regular room circuit outlet? If the latter, and that was what the 30amp breaker was for, then it's wrong. If the former...well...see the above posts.
-- Richard Moore
Rest Assured Inspection Services
Seattle, WA
www.rainspect.com
Originally Posted By: mpfeffer This post was automatically imported from our archived forum.
No matter what, #12 wire should not be protected with a 30a breaker. If the breaker allows the circuit to draw more than 12 wire is rated for (as a 30a breaker would), then it’s a fire hazard. NEVER undersize the wire. The code references are correct, but when the house is a 10 foot pile of glowing briquets, the code inspector will be at the bar sucking suds while you’re in court fighting for your kids’ college savings. (a little over the top, but i have seen this first hand).[/b][/i][/u]
Originally Posted By: Bob Badger This post was automatically imported from our archived forum.
Matt the wire is not protected by the 30 amp breaker it is protected by the motor overloads in the unit.
You say the code references are correct but yet say it is unsafe.   
Originally Posted By: mpfeffer This post was automatically imported from our archived forum.
If the motor OL’s fuse together quickly, which i have seen on a 20hp motor (very ugly) the wire is left unprotected, and will catch fire before the breaker trips if it is oversized. I was working in a packaging plant abotu 10 years ago, and a 20hp motor “single phased” fusing the overload into a nice little ball of steel, then the wire from the j box down to the moter, which was sized for the motor, not the breaker, melted and caught fire. the breaker never tripped because the wire never drew too much, until the insulation melted off and it contacted the conduit. By this time the it was too late, as the machine the motor was running caught fire.
Originally Posted By: mpfeffer This post was automatically imported from our archived forum.
I didn’t mean any disrespect, and i’m surely not trying to be sarcastic or put your knowledge down in any way. Since i saw this happen, i thought i’d share it with the person who wrote the question in the first place, i didn’t realize i’d be stomped into the ground for it. Geez, that’ll teach me. Sorry everyone, i’ll crawl back in my cage now, and Bob i apologize deeply and sincerely…really i mean it… really.
Originally Posted By: Bob Badger This post was automatically imported from our archived forum.
Matt keep on posting I edited my inappropriate comment.
I do apologize.
I should have said if we can not rely on the final overcurrent device to function we will have to run the branch circuits in a building with sufficiently sized conductors to handle the current of the next overcurrent device.
It gets messy running 3 AWG or 2/0s to receptacle outlets.
Originally Posted By: rsummers This post was automatically imported from our archived forum.
I think the problem would with over sizing the breaker comes when the compressor goes out and draws LRA(locked rotor amps) and if the IOL (internal over load) fails to open then what. I still don’t think you should over size the breaker.
Originally Posted By: roconnor This post was automatically imported from our archived forum.
I agree with Bob and Jerry on the requirements, but I see Matt?s point. It might be better practice to match the wire with the breaker, it?s just not what the ?minimum requirements? of the model codes require.
Note that there are special circuit provisions for both AC compressors and room units which allows a larger than normal breaker rating for a given wire size on a dedicated circuit to keep a very short start-up draw from constantly tripping a breaker ... although the two are treated a little different (NEC 440 and IRC E3602).
But about the original post, unless that 30A breaker with #12 wire is clearly for the room AC units (with the rated current and allowable 30A protection stated on the label) you have no idea if it is mismatched. Good pick-up, because that may need to be looked at a little closer by a professional. Then he can figure out the code requirements and make the call ? 
-- Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee
I am absolutely amazed sometimes by how much thought goes into doing things wrong
Originally Posted By: rpalac This post was automatically imported from our archived forum.
I agree with Jerry, Mr.O’cconor and several others…that the wire should dictate breaker size or visa-versa.
Mr. Bagger on this one I don't agree with the code. Since the 70's I've always felt uncomfortable running what appeared to be under rated wire to a unit that was going to be fused higher to compensate for start up draw. I adoppted a "personal" practice to change the wire to match the breaker.
What you say is correct in the rating of the breaker to the unit as far as derating or over rating to compensate. Can you tell me the code section that say's the wire does not have to reflect the breaker size?
This is a good exception to the rules that we are tought in the electrical industry betweeen theory and principle. Tough call here. I would rather error on the side of safety, but if it is not incorrect I can't call attention to it.
Educate me! Let me know the exact sections and and figure this out as a case.
Refer to a unit draw, wire size calc, and breaker size (asuming no line loss)
Originally Posted By: Bob Badger This post was automatically imported from our archived forum.
rpalac wrote:
Let me know the exact sections and and figure this out as a case.
Refer to a unit draw, wire size calc, and breaker size (asuming no line loss)
OK remember you asked 
Do we really want to do this, it will be long and the short story is that the breaker is not the final over current protective device in this application.
The motor overload device is the final overcurrent device protecting both motor and conductors.
WARNING intense code references ahead, skip ahead if you do not want to deal with it.
First code stop should be from article 240 overcurrent protection.
Quote:
240.4(G) Overcurrent Protection for Specific Conductor Applications. Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).
I can not post the Table but it tells us that conductors for Motor and motor-control circuit conductors shall be determined by article 430, Parts III, IV, V, VI, VII
Let's say someone has a 5 HP 240 volt single phase air compressor.
430.6(A)(1) Tells us to size the conductors, branch-circuit short-circuit and ground-fault protection from the tables at the end of 430.
Quote:
430.6 Ampacity and Motor Rating Determination.
The size of conductors supplying equipment covered by Article 430 shall be selected from the allowable ampacity tables in accordance with 310.15(B) or shall be calculated in accordance with 310.15(C). Where flexible cord is used, the size of the conductor shall be selected in accordance with 400.5. The required ampacity and motor ratings shall be determined as specified in 430.6(A), (B), and (C).
(A) General Motor Applications. For general motor applications, current ratings shall be determined based on (1) and (2).
(1) Table Values. The values given in Table 430.147, Table 430.148, Table 430.149, and Table 430.150, including notes, shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.147, Table 430.148, Table 430.149, and Table 430.150, interpolated if necessary.
Table 430.148 lists the amperage of this 5 HP motor at 28 amps
Quote:
430.21 General.
Part II specifies ampacities of conductors that are capable of carrying the motor current without overheating under the conditions specified.
430.22 Tells us to size the conductors for a single motor at 125% of the value determined by 430.6(A)(1)
Quote:
430.22 Single Motor.
(A) General. Branch-circuit conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor?s full-load current rating as determined by 430.6(A)(1).
Now we need a conductor rated 35 amps
430.6 tells us to size the conductors from 310.16
310.16 shows we will need 10 AWG
So now we know the required conductor size.
Now we need to determine the Motor Branch-Circuit Short-Circuit and Ground-Fault Protection.
Start at 430.51
Quote:
430.51 General.
Part IV specifies devices intended to protect the motor branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or grounds. These rules add to or amend the provisions of Article 240. The devices specified in Part IV do not include the types of devices required by 210.8, 230.95, and 527.6.
This is a good time to point out that this breaker, the one we have been talking about in this thread is not intended to provide overload protection to the motor or conductors.
This breaker is for two purposes short circuits or grounds, that's it.
Quote:
430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.
Now I would like to post Table 430.52 but depending on the motor and type of device used the values range from 150% to 1100% increase in amperage.
For my simple air compressor we will use a standard breaker.
Table 430.52 shows we can use a Inverse Time Breaker (normal breaker) up to 250% of the table 430.148 amperage
28 amps x 2.5 = 70 amps.
So we have determined a 10 AWG copper THWN conductor with a 70 AMP inverse time breaker is an allowable combination.
We still have not provided overload protection for the motor or the conductors feeding it.
Quote:
430.31 General.
Part III specifies overload devices intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start.
Overload in electrical apparatus is an operating overcurrent that, when it persists for a sufficient length of time, would cause damage or dangerous overheating of the apparatus. It does not include short circuits or ground faults.
These provisions shall not be interpreted as requiring overload protection where it might introduce additional or increased hazards, as in the case of fire pumps.
Quote:
430.32 Continuous-Duty Motors.
(A) More Than 1 Horsepower. Each continuous-duty motor rated more than 1 hp shall be protected against overload by one of the means in 430.32(A)(1) through (A)(4).
(1) Separate Overload Device. A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:
Motors with a marked service factor 1.15 or greater 125%
Motors with a marked temperature rise 40?C or less 125%
All other motors 115%
It is important to note a continuous duty motor is marked as such, this has nothing to do with continuous loads of 3 hours or more.
Quote:
430.6(A)(2) Nameplate Values. Separate motor overload protection shall be based on the motor nameplate current rating.
So once we choose the overload protection based on the motor name plate value we are now done choosing the conductor size, selecting the short circuit, ground fault protective device and the overload device that protects the motor and the conductors feeding it.
This is a safe and code compliant installation, it may not provide enough current for starting loads but that is not what the NEC is concerned with.
