80% rule

[pa electrician]

You know how the NEC wants only 80% of the total current draw on a wire.(--just to make a long story short.)
For instance 12A on a 15A breaker= w/ #14 AWG
Or 16A on a 20A breaker= when using #12 AWG
What if...the wire was a #12 and the total circuit demand was only 6 Amps?
Is there a requirement to protect the circuit with the closest rated breaker possible?
AND also how about if the wire was #10 AWG, and the current draw is 14 Amps.
Can you use a 15Amp breaker? Or does the 15A breaker have to have only 80% of what it is rated for?

 

[George Stolz]

Re: 80% rule

Pa, welcome to the forum!

Originally posted by pa electrician:
You know how the NEC wants only 80% of the total current draw on a wire.(--just to make a long story short.)

To make a long story short, this is not true.

In 210.23, it states that any one cord and plug connected appliance shall not exceed 80% of the circuit's rating.

This is probably why vacuum cleaners advertise drawing "12 amps". That is the max current that a cord and lug connected appliance can draw when plugged into a 15 amp circuit.

Other than that, you are free to use the full rating of the OCPD (Overcurrent Protection) protecting the circuit.
[/quote]Edit to add: Provided the loads take a break once every three hours. If the loads continue for more than three hours, then the conductor must have an ampacity 125% of the continuous load per 210.19(A)(1).

[ July 29, 2005, 07:52 AM: Message edited by: georgestolz ]

 

[infinity]

Re: 80% rule

In 210.23, it states that any one cord and plug connected appliance shall not exceed 80% of the circuit's rating.

George is correct with one exception. A circuit with a single receptacle can have a load connected up to 100% of the circuit rating. The 80% that George mentioned is for a circuit with two or more receptacles.

 

[jim dungar]

Re: 80% rule

The branch circuit OCPD is there to protect the wiring not the load, 240.3, and 240.4. Supplementary protectors are used to protect the load, 240.10.

 

[charlie b]

Re: 80% rule

The purpose of the breaker is to protect the cable, not the load. If the load drew more current than the cable can handle, the breaker should open and terminate the event.

You might point out that if a device that should only draw 6 amps has an internal fault that causes it to draw 12 amps, and if it is fed by a #12 and a 20 amp breaker, then the load might be destroyed. So be it. The NEC is not the guardian of the devices you plug into a circuit. Protection of the devices is the responsibility of their manufacturers. The NEC is the guardian of the building's permanent wiring system.

Edit: I see Jim and I agree, and that he beat me to the draw on this comment. Great minds drink alike!

[ July 29, 2005, 10:31 AM: Message edited by: charlie b ]

 

[jim dungar]

Re: 80% rule

Charlie B, Great minds drink alike!

ya sure, you betcha