Journeymen exam question

F

FrenchyTech

Member
Location
Maine
Occupation
Electrician
Hello! I am preparing for my journeysman exam which I am taking soon and I purchased Mike holts examination preparation course and I came across something that gave me a hard time and I’ll post the question below:
c) 10 AWG
440.33
Conductor Ampacity = (25A × 125%) + 2A
Conductor Ampacity = 31.25A + 2A
Conductor Ampacity = 33.25A
Conductor Size = 10 AWG, rated 30A at 60°C, 110.14(C)(1)(a)(2) and Table 310.16
What I don’t get is why is the conductor a parity above the ampacity of 10 awg? I thought the ampacity always had to be bigger than the calculated amps. Thanks for your time and knowledge!
 
FrenchyTech said:
Conductor Size = 10 AWG, rated 30A at 60°C, 110.14(C)(1)(a)(2) and Table 310.16
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That sounds like it is your calculation is that correct? Did the question specify terminal temperature ratings? #10 AWG @ 75° C is good for 35 amps.
 
Dennis Alwon said:
Was the question about 60C conductor because that is incorrect. If the #10 was 75C then that would be okay because 240.4 (D) & (G) allows #10 to be more than 30 amps for motors.

It would be good to see the question
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Sure here’s the question!: What size minimum branch circuit conductor is required for a 25A refrigerant motor compressor with a 2A fan, terminals rated 75 C: A 15 AWG, B 12 AWG, C 10 AWG, and D 8 AWG, I think what’s more getting me is on the answer it said 10 AWG 60 Celsius but that’s 30 Amps but Kayden since the terminal is 75 I can pick from the 75 degree column
 
infinity said:
That sounds like it is your calculation is that correct? Did the question specify terminal temperature ratings? #10 AWG @ 75° C is good for 35 amps.
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It did! It said 75 degree terminals and if that’s what I use to pick the wire ampacity then that makes sense
 
FrenchyTech said:
It did! It said 75 degree terminals and if that’s what I use to pick the wire ampacity then that makes sense
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Yes so since it's a motor compressor with 75° C terminals you can ignore the 30 amp limit for #10 AWG conductors in 240.4 and use the 75° C ampacity of 35 amps from the table.
 
FrenchyTech said:
... I thought the ampacity always had to be bigger than the calculated amps. Thanks for your time and knowledge!
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Yep, and the ampacity of #10 copper AWG with 75C terminals is 35 amps, which is larger than the calculated amps of 33.25.

I am guessing what is throwing you off is the requirement of 2020 NEC 240.4 (D) (7), which is about the overcurrent protection of #10 copper being limited to 30 amps. But 240.3 references table 240.3 which references 440 for Air-conditioning and refrigerating equipment.

Article 440 Part VI "Motor-Compressor and Branch-Circuit Overload Protection" is where to look for the breaker size. BUT that is not part of the question. ;)
 
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