Best thread title of the year so far 
The derating only makes sense to me if it’s done in the original design and installation. Derating by the homeowing user is nonsense.
To derate a circuit where a lot of cables are in the same conduit, a circuit with a 15 Amp breaker would use #12 wire vs. the usual #14, a circuit protected to 20 Amp. would use #10 wire vs. the usual #12, etc.
For the user, if the circuit is protected to 15 Amp. go ahead and load it up to 15 Amp. If the breaker keeps tripping that’s an annoyance for the user and not a safety hazard that the NEC is concerned with.
Of course, but you (we) don’t want the homeowner to be calling us back because the breaker trips everytime they’ve turned on the tenth light in the home because all the lights in the kitchen/dining room/living room/garage are on the same circuit and they’re having a party so all the lights are on at the same time.
The same goes for the common usage of household appliances. The circuits are designed so that you sort of limit the number of radios, tv’s, electric heaters, computers, etc., can be plugged in at the same time (on the same circuit). If they estimate that the average outlet may at some point have 180 W (average per device) plugged in at the same time then you have a harder time pushing the limit when normal usage occurs. We can’t plan for the homeowner that plugs in 6’ electric baseboard heaters into every outlet, that’s clearly excessive. We do plan for the average usage.
If the homeowner wants to max out his circuits then that’s his business, we just try to make it harder on that person by giving him more circuits to use and more room on each circuit to play with. What possible objection would anyone on this planet have with that??
Sure, but I don’t think that’s the sort of derating that the NEC is concerned with.
Sorry, I meant to start the last post by saying you are correct here.
There is no law that says the owner can’t plug in whatever it takes to trip the breaker. It’s stupid, but not illegal.
And when I quote the nubers 1440W and 1920W those are the anticipated maximum numbers that we design the circuits for. Not what the homeowner has to follow.
What are you getting at?
Most homes are wired with romex, so the basic ‘conductors in the conduit’ scenario doesn’t apply. I think the NEC is concerned with the homowner overloading circuits. What if the breaker doesn’t trip?
The NEC seems to be asserting more control over residential wiring with every code change. Most of the changes are based on personal safety and convenience, it seems.
What I’m getting at is that the homeowner of the OP can put as much light in the room as he wants up to the breaker amperage. He or she doesn’t have to derate the circuit having a 15 Amp. breaker to 80% of that just because of the NEC.
Oh, right, I see where you’re going.
Might the 80% derating also be in there to account for voltage drop? I realize that most household circuit runs aren’t usually long enough to cause too much of a problem, but there is certainly a chance that a longer and perhaps (forgive me) less-well-planned circuit could create enough impedance to puch the amperage up to the breaking (breaker!) point.
The potential for voltage drop is probably a part of the “just for good measure” theory that **Uncommon Sense ** was referring to in response to QED…
- OlPeculiar.
Not when the load is resistive as with light bulbs. When the voltage goes down, the current goes down. The relationship isn’t linear because the bulb resistance also decreases as current decreases and so the current is a little higher than it would be with a constant resistance. But the current still goes down with voltage.
For example, a 60 W bulb at 120 V. takes .5 Amp and has a resistance of 240 Ohms. At 0 Amp. the resistance measures 17 Ohms. The resistance is proportional to the temperature (for a constant temperature coefficient) and the temperature for a given size wire is proportioal to the square of the current. If you take all that and work it out, for a voltage decrease to 105 (0.875 of 120) the current decreases to 0.45 (.95 of 0.5)