We have some misinformation regarding conductor and breaker sizing going on over in the Emergency forum and it needs to be cleaned up.
I’m looking for some of our resident expert electricians to chime in and answer the following questions based on the attached A/C condenser label. Your responses will be helpful to folks who stumble upon the Emergency forum thread at http://www.nachi.org/forum/f79/undersized-wire-99204/#post1305367 for those who have access to the members only section.
For the label below, please answer:
What is the smallest copper conductor size which can be used with this listed device?
What is the largest breaker size that can be used with this listed device?
Can the largest breaker size be used in combination with smallest conductor size for this listed device?
Should an inspector who sees an installation using a 10AWG copper conductor attached to a 50Amp breaker identify this as a defect/fire hazard and call for repair by a licensed electrician?
Please explain why for future inspectors who encounter this situation.
Why? The inspection was done in Florida. I was going to post it in the Aircraft Maintenance and Avionics section, but then I realized that they have absolutely no relevance to home inspections.
Chuck won’t take anyones opinion but his own on this one guys. He has told not only myself but also a CMI that we are wrong. I hold an AME in Electrical and Electronic aircraft systems but we are wrong and Chuck is right. I don’t think so, the CMI doesn’t think so. Quite frankly I already said that we would agree to disagree but he wants to take it to you. It is mainly about a 50 amp breaker attached to a 10 awg wire. Everything else in the disagreement is moot unless that is corrected.
Let’s keep our debate out of this. The other thread is already full of chest pumping. Let the professional electricians respond to the questions in the original post which were entirely objective and unslanted. Someone might actually learn something. At least do it for the sake of the future inspectors who will need good advice on the subject.
There are three aspects to overcurrent protection:
Short circuit protection
Ground fault protection
Overload protection.
The code simply states that you must allow the motor to start. This means that the 225% rule is not mandatory, but rather permissive.
Short circuit and ground fault protection are provided by the overcurrent protection device feeding the circuit (ie, the breaker), whereas overload protection is being provided by the heaters inside of the motor. The size of conductor need not be as large as the breaker, but it must be as large as the running load amps of the motor/compressor.
In a nut shell, the breaker will open the circuit in the case of a ground fault or short circuit, and the heater will open the circuit in the case of an overload.
This sounds like a compliant installation to me.
The only information on that nameplate that really matters in sizing the circuit is the Minimum Circuit Ampacity (MCA) and Maximum Over Current Protection Device (MOCPD). The conductors are sized to the MCA, in this case 29 amps so a #10 NM cable could be used because it has an ampacity of 30 amps @ 60° C.
The MOCPD can be any size from 30 amps (next standard size up from 29 amps) to 50 amps since it’s only needed for short circuit and ground fault protection it can be larger than the rated ampacity of the conducotrs because they’re protected from overload by the integral overload protection in the unit. A larger MOCPD may be needed to allow the unit to start.
So if the conductor is rated for the 30 amps or more and the OCPD is 30-50 amps the installation is code compliant.
Remember the minimum circuit ampacity which includes the conductors and the overcurrent device is 29 amps, a 30 amp conductor and a 30 amp OCPD meets the MCA.
The only information on that nameplate that really matters in sizing the circuit is the Minimum Circuit Ampacity (MCA) and Maximum Fuse or Circuit Breaker aka the Maximum Over Current Protection Device (MOCPD). The conductors are sized to the MCA, in this case 29 amps so a #10 NM cable could be used because it has an ampacity of 30 amps @ 60° C.
The MOCPD can be any size from 30 amps (next standard size up from 29 amps) to 50 amps since it’s only needed for short circuit and ground fault protection it can be larger than the rated ampacity of the conductors because they’re protected from overload by the integral overload protection in the unit. A larger MOCPD may or may not be needed to allow the unit to start.
So if the conductors are rated for the 30 amps or more and the OCPD is 30-50 amps the installation is code compliant.
Remember the minimum circuit ampacity which includes the conductors and the overcurrent device is 29 amps, a 30 amp conductor and a 30 amp OCPD meets the MCA.
#10 on a 50amp breaker would be compliant. Breakers are purposely over sized to allow for motor inrush. As a result the breaker only takes care of faults on the conductor (ground faults and line to line shorts). Over load protection is provided via the motor over loads built into the unit usually located within the motors themselves.
When reading those tags it goes like this:
Minimum circuit ampacity (max current x 1.25%) gives you what wire to use.
Max breaker size tells you what breaker to use.
Of note, 240.4D does not apply to 14, 12 and 10 gauge wire when sizing motors, rather the temperature rating of the terminals. and wire itself. Thus, if I had a MCA of 18amps 75*C on the breaker and all other terminals I could use #14 THHN in conduit and it would be complaint.
Romex is always restricted to 60C, however before table 310.15 B 7 came about table 310.16 was used to size wires in prior codes. Under the 60C column #14 was listed for 20amps and #12 was listed for 25 amps. Thus, under older codes I could have an MCA of 19amps, use #14 romex and put it on say a 35 amp breaker.
I can clarify more as the topic is a thesis course all on to itself lol, but in short breaker sizing is very different.
I guess we will remain at odds over this. I would look at the panel that had a 50 amp breaker with a 10 gage wire attached and see this as a potential fire hazard and call it out as a warning. My reasoning is that if the wiring at the other end shorts together it is going to heat the wire up more than the correctly rated breaker for the wire size. I got called on this nearly identical item on my own home by the electrical inspector when they were checking the addition I built several years ago. I had a 60 amp breaker to my original garage feeding a 8awg wire. The code (and I know we don’t talk code as inspectors) told him and me that either the breaker had to be downsized or the wire had to be upsized regardless of the load at the other end or he would not allow the power to go back on. My garage only had a car warmer plug and lights so no overload would occur.That is where I have been coming from. Thanks for all your answers here.
Maybe if the breaker is FPE or one without magnetic trip, but most (all modern) breakers have magnetic trip which trips the breaker immediately. The magnetic trip is what trips the breaker instantaneously during a line-line fault or ground fault. So the wire will see hardly any heating. Now, the wire will be subjected to more heating with a long chronic overload however that is prevented by the overloads within the motor.
No fire hazard exists since any possible contingency is stopped either by the breaker or the unit itself.
Of course if this was a regular circuit it would be an issue because other appliances do not have over load protection and nothing can stop a person from plugging in to much. In that case code mandates the OCPD be sized to the wire because its not only proving ground fault and short circuit protection but overload protection as well.
Thanks Martin, good explanation. I haven’t run across breakers with magnetic trip in the renos I’ve done so far. Most of the panels where I’ve had to add or remove circuits were older systems and I was putting like breakers back in. I learn more everyday since I joined Inachi to make the leap from contractor to inspector.
Motor and A/C sizing uses different rules than allowable branch circuit wiring due to the short term inrush currents. The inspector that red tagged a unit in compliance with the date plate sizing information was wrong and could not backup their claim with a code article.
Thanks again Martin,
I will look it up. The more I learn the better I can be.
Just finished taking a quick look at the information in NEC article 430 and 440 for and there was a Q and A that was almost identical to this debate. I read the question and answer and it now makes a bit more sense. I was incorrect in this instance and Chuck was correct. I do think all of this could have been avoided if someone had pointed to the NEC articles instead of just saying your wrong. I want to be the best inspector I can be and backed up information is nice to have.
I am now much better informed.
