Sizing Conductors for Continuous Loading

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Re: Sizing Conductors for Continuous Loading

This is always an interesting topic, and as one of the responders answered, one must go back into the history of the rule. As staff liaison to Code-making Panel 6 that was involved in the current carrying capacity of conductors, I was fortunate to be present and record the actions for the rules. First of all, the ampacities listed in Table 310.16 are the amount of current the conductor can carry continuously 24/7, 365 days a yr without exceeding the temperature rating of the conductor. However, the restrictions are those indicated in the title of the table.
 
Re: Sizing Conductors for Continuous Loading

This is always an interesting topic, and as one of the responders answered, one must go back into the history of the rule. As staff liaison to Code-making Panel 6 that was involved in the current carrying capacity of conductors, I was fortunate to be present and record the actions for the rules. First of all, the ampacities listed in Table 310.16 are the amount of current the conductor can carry continuously 24/7, 365 days a yr without exceeding the temperature rating of the conductor. However, the restrictions are those indicated in the title of the table.
Conductors that carry current continuously can approach temperatures close to the temperature of the insulation rating. The conductor connects to terminals at both ends, these terminals are tested by testing laboratories for specific size conductors in the specific terminals. It is the testing lab's that place the limitation on the amount of continuous current a device can safely carry. By using the 125 percent factor for the load forces the size of the conductors to be increased thereby decreasing the amount of heat developed in the conductor and transferred to the terminations. The conductor serves as a heat sink to extract heat from the terminations.
 
Re: Sizing Conductors for Continuous Loading

John,

Thanks so much for the insightful and informative response. I appreciate you taking the effort to help me understand this issue. If I might ask a follow up question:

If a continuous rated 20 amp breaker was available, wouldn't I be within the guidelines of the code (T310.16) if I landed a #12THHN on the breaker if it was feeding a continuous load?
 
Re: Sizing Conductors for Continuous Loading

Gee, he sure sweetens up to the names he recognizes, hmm? :p

Rlee, yes. If you had a 20 amp continuous load, with a 20 amp continuous-rated breaker, with 12 AWG conductors, you would have a compliant assembly.

See 210.20(A), with exception. While it lasts. :p
 
Re: Sizing Conductors for Continuous Loading

Originally posted by john m. caloggero:
...I was fortunate to be present and record the actions for the rules...
Click to expand...
Mr. Caloggero, it's an honor to not only to be in the presence of your experience, but also of your humility. :)

For the record, I still haven't purchased a handbook. :D
 
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