[tkrussell]

Interesting old thread.

Seems to be some confusion about the 80% rating, in general.

To begin with, NEC carry's over the 80% rule from Underwriters Laboratories as applies to circuit breakers.

From the 2008 edition of the NEC:

Article 100- Definitions
Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

II. Branch-Circuit Ratings
210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.

(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any
Adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

210.20 Overcurrent Protection.
Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies with 10.20(A) through (D).

(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

From a UL white paper, without purchasing the actual UL 489 :

http://www.ul.com/global/documents/o...BreakersMG.pdf


See Page 14:

38. 100 Percent Continuous Rated — Unless otherwise marked for continuous use at 100 percent of its current rating, a circuit breaker is intended for use at no more than 80 percent of its rated current where in normal operation the load will continue for three hours or more.


Some leeway is given by the NEC with this statement:

"the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load"


Next, how does this relate to residential. The NEC does not have any reference to if a circuit is in a home or anything that is not a dwelling.

A circuit is a circuit, no matter where it is located. The issue is, can the load on this circuit operate for more than three hours, if so, the breaker cannot be loaded more than 80%.

Now, I don't know where the concept that any circuit in a home will never be used more than three hours. I know I live in my home more than three hours.

I can easily have a general purpose lighting circuit, that can serve lighting fixtures and some receptacles. I can easily have a 15 amp general purpose lighting circuit with 18- 100 watt light bulbs in permanently installed lighting fixtures, plus any amount of table lamps and any amount of appliances, such as TV, stereo, etc. plugged in.

After wiring this circuit, and I leave the home, I would need to be very sure that all the 18 fixtures are never on more than 3 hours, otherwise this circuit is not Code compliant.

In a real situation, sure most homes would not use all 18 lights plus the appliances and table lamps all at the same time, let alone, for more that three hours.

But, could it? I chose not to take that chance, and would arrange the circuit so this amount of load would could not be on one circuit.

I don't get too excited by a true general purpose lighting circuit, with a combination of a few light fixtures and some receptacles.

But if there is a certain room, such as a den or family room with a large amount of lighting, I will treat that as a continuous load and assume it will be all on for more than 3 hours, and that circuit will not be loaded more than 80%.

[stanfortyman]

We have to remember the true definition of "Continuous Load" from above.

Article 100- Definitions
Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.


The word "expected" is a very important one that is often ignored or overlooked.
Some folks think if a load might be, or will ever be, on for 3 hours or more it should be considered a continuous load. This is not at all true.

I must say, erring on the side of caution is personal preference though that we all do for certain things.