Buck Parrish
Senior Member
- Location
- NC & IN
Thanks.
Heat Pump MCA
- Thread starter infinity
- Start date
- Status
Thanks.
To all,
I learned something new today.
electrofelon and kwired,
Thank you for taking the time and your continued persistence, efforts, to explain where I was wrong in my understanding of the ampere rating for #14, #12, and #10 wire for motor branch circuit wire sizing.
I confess my ignorance on this one. I used 240.4(D)(5) for the ampacity rating for #14 (15A), #12 (20A), and #10 (30A) for motors. Yes I had read 240.4(D)(G) which sent me to 430 and 440, and it sent me back to 310.15(B)(16). **
kwired, I reread 110.14(C) a couple more times before it sunk into my old head. I kept getting hung up on the last sentence ampacity adjustment, correction or both. That made me think of 310.15, 310.15(B)(3) and the following tables. There again them ** next to #14, #12, and #10 in Table 310.15(B)(16).
>>
@ infinity,
I was so caught up on using #12 wire for, (21.7A), the current carrying branch circuit conductors, I just read over the branch circuit wiring used is #12 conductor MC.
Post #15
Table 250.122
EGC minimum size.
30 amp breaker, #10 cu wire. You are either going to have to step up to #10 wire MC, or use conduit.
EDIT:
Or drop down to a 20 amp OCPD, if possible.
Best regards,
Jim
I learned something new today.
electrofelon and kwired,
Thank you for taking the time and your continued persistence, efforts, to explain where I was wrong in my understanding of the ampere rating for #14, #12, and #10 wire for motor branch circuit wire sizing.
I confess my ignorance on this one. I used 240.4(D)(5) for the ampacity rating for #14 (15A), #12 (20A), and #10 (30A) for motors. Yes I had read 240.4(D)(G) which sent me to 430 and 440, and it sent me back to 310.15(B)(16). **
kwired, I reread 110.14(C) a couple more times before it sunk into my old head. I kept getting hung up on the last sentence ampacity adjustment, correction or both. That made me think of 310.15, 310.15(B)(3) and the following tables. There again them ** next to #14, #12, and #10 in Table 310.15(B)(16).
>>
@ infinity,
I realize the main purpose for your thread is for the temp terminal rating of the switch terminals. As I said in a post I agreed with you, on the switch.
I was so caught up on using #12 wire for, (21.7A), the current carrying branch circuit conductors, I just read over the branch circuit wiring used is #12 conductor MC.
Post #15
Table 250.122
EGC minimum size.
30 amp breaker, #10 cu wire. You are either going to have to step up to #10 wire MC, or use conduit.
EDIT:
Or drop down to a 20 amp OCPD, if possible.
Best regards,
Jim
Last edited:
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
For those mentioning "breaker" in this thread, I would like to point out that the nameplate in the image in the OP says "max fuse size" so fuses must be used in the circuit.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
Its a super common gotcha. I didnt know it for many years into my career. We get so used to "#12 is 20 amp". I recall several instances on this forum where someone complained an inspector didnt know it. Last week I wired 2 mini splits, low 20's MCA. HVAC guy said, "...so they need #10 wire...."
- Location
- New Jersey
- Occupation
- Journeyman Electrician
The EGC is not required to be larger than the circuit conductors so #12 is OK for the EGC and a 30 amp OCPD.
NO GOTCHA! I was sincere in post.
If I went back to work tomorrow I still wouldn't use the 25 amp rating for #12 wire! Nor would any electricians working on my job.
Just because bare minimum code says I can continuously load #12 cu wire up to 20 amps for a motor branch circuit doesn't mean I have too. Nor would I. (20 FLA X 125% = 25A)
Just for an example use a 1Ph 115V 1.5HP motor. Wire is THHN, breaker terminal rating is 75 degree.
NEC 115Vac 1.5HP = 20 amps FLA X 125% = 25amps. You can use #12 wire.
Length of branch circuit 80' one way. Motor starts under load. Forget figuring VD it's not mandated by NEC, only a suggestion.
Just figure everything bare minimum to meet NEC. How long do you think the motor is going to last? Do you care?
Technically it doesn't even have to the job it was intended for, according to Art 90.1(A) & (B)
Best regards,
Jim
.
.
- Occupation
- Licensed Electrician
I do care.
Since motors are manufactured to tolerate a supply voltage swing of plus or minus ten percent I don't get too hung up on worrying about voltage drop.
I also understand that code minimum is not an insult, it's a lot like buying a basic truck. It's still a good truck, has all the safety features of any other truck it just doesn't have all the extra cost for upgrades.
.
packersparky
Senior Member
- Location
- Wisconsin
- Occupation
- Inspector
The motor overloads provide the overload protection for the wire. The OCPD provides the ground fault and short circuit protection.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
Im not really sure what you mean, but just to be clear, i didnt mean "gotcha" as in I/we "got you" or anything like that. I meant it as in one of those subtle code things that you miss, and maybe have done it for a long time and never gotten called on it, and one day you get called on it. In this case I guess its an inverse gotcha because its something you could do but didnt know it :?:
Bigger wire does not mean better. Sometimes, yes, but it seems again you are generalizing and trashing code minimum. The fact is:
1. NEC FLA's are high
2. Your circuit will include an extra 25%
3. Most motors will never operate at their peak power and if so, it will be short in duration.
4. #12 is actually good for 30 amps, easy.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
As Electrofelon stated you have already added 25% to the size of the conductor and the NEC FLC tables have some additional headroom built in. Also most motors can operate normally within about 10% of their voltage rating so an 80' branch circuit with minimal VD isn't going to make any difference.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I mostly look at the merits of each installation. In my own home I have a heat pump with MCA of 29.5A. Little bit of a bad example because it is under 30 amps, but say it had MCA of 30.5 amps - then many would think they need to run 8 AWG because it is over 30.
Reality of this heat pump is I have never clamped an ammeter on it and seen more then 12-15 amps. In heating mode I don't think I have ever seen it draw more then about 6 amps. It also is short run to the panel and is in a raceway so 60C NM is not a factor.
Another thing that people don't always think about is MCA already has 125% of largest motor factored into it. So when you get that AC unit with MCA of 21 amps some just see it being over 20 and assume 10 AWG is needed. If you gave same person the RLA of the compressor of maybe only 15.6 amps - then they are thinking 12AWG is all that is necessary.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I noticed that but cannot believe that in 2018 this equipment would require fuses. Could be that it's because it made in Canada and they've used the word fuse to mean OCPD?
For the 115Vac 1.5HP motor example I used in my post #26, calculate the VD on the 80' branch circuit when the start winding of the motor is connected to the circuit. Keep in mind the average starting current for the single phase start winding is 3 to 5 times the FLA of the motor. For you calculation use 20 amps for the motor nameplate FLA.
I didn't say it's an insult. I said it is bare minimum electrical safety standards. I didn't write Article 90.1(A) & (B).
It clearly states this code is note intended as design specification or an instruction manual.
Read 90.1(B) Adequacy.
Read the fine print note below (B)
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Not in my experiences. Just had a service call yesterday involving a 10 HP single phase motor and starting issues. That start winding never drew more then around 15 amps when starting a high inertia blower wheel. Main winding however did draw around 187 amps when motor was accelerating, but dropped to 36 once full speed was achieved.
That aux winding is only there to give some "rotation" to the field, it don't take a whole lot of VA to do so, the main winding still does the majority of work both starting and running.
Note the FLA for the motor is 20 amps. Where is your 25% fudge factor?
(For the example, the 115V 1.5HP motor nameplate FLA is 20 amps.)
The 25% was built in the the sizing of the branch circuit wiring.
20A X 125% = 25 amps. NEC says you can use #12 wire. That means if the motor is loaded to its' 1.5HP rating the motor will draw 20 amps. #12 wire.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
That extra 25% is for the termination temp ratings - the conductor can handle up to the 90 C rating (if it has 90C insulation). The copper can handle even more current if we didn't have to consider insulation or termination ratings.
I agree. I should of said until the motor reaches its' normal operating speed. I know the run winding is always in the circuit. The start winding gives the motor direction and starting torque. The start winding remains in the circuit until enough RPM is developed for the centrifugal switch contact to open disconnecting the start winding from the circuit. What factors come into play that determines the length of time it takes for the motor to reach its' rated RPM?
So how did the math figure for the motor drawing 187 amps as the motor accelerated to its' normal operating,running, speed? Was it 3 to 5 times its' name plate rated FLA?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Aux winding itself doesn't necessarily draw much current though, like I said when main line measured 187 amps that aux winding was only drawing 12-15 amps, the rest was in the main winding.
This was a large blower that took several seconds to accelerate to full speed. A load that doesn't have such inertia would have accelerated to full speed seemingly instantaneously and one wouldn't have been able to measure starting current as easily. But yes FLA was about 42 AMPS IIRC, about 4.5 times FLA was what I did measure,
- Location
- New Jersey
- Occupation
- Journeyman Electrician
The fudge factor is that you have a 25 amp conductor with a 20 amp load hence the 25%.
To reiterate this thread is about using #12 AWG MC cable at 75° C ampacity for a unit with an MCA of 21.7 amps and whether or not it's code compliant. Although you have some valid points good/bad design is not the issue.
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