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For homeowners, do you need to follow the derating rule? Illustration:

https://www.youtube.com/watch?v=OLIHEiiY_Rs

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The point is conductors can be used that is 83% of the supposed size compared to service conductor.

But in the above example. What if the residential houses have so many guests and the wire amperage reach 190A. Then since the breaker is still 200A, and the wire is only rated at 175A (or 83%). Then the wire can burn before the breaker trips. So isn't the 83% kinda dangerous for those with many guests? The philosophy is the breaker should protect the wire. But in this case. It can't anymore.

For my actual use. I have service entrance breaker of 150A. 38mm^2 is within the 83% rule (according to Harper in related Can I do a 125A subpanel feeder with slightly-larger-than-#3 wire?). But 38mm^2 is only 122A max at 75C. What if my house load is 130A (with many guests), then the 38mm^2 wire can burn before the 150A breaker trips.

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    I have to admit this rule never really set right with me, it just doesn't seem like the logic is consistent with the rest of the code. But I'll admit I doubt it will cause any problems. It never hurts to look at the code as a minimum, nothing wrong with being safer than the code requires.
    – batsplatsterson
    Feb 8, 2019 at 10:40
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    By the way - it's not a derating, it's actually the opposite - you could call it an uprating factor.
    – batsplatsterson
    Feb 8, 2019 at 10:46

2 Answers 2

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It gets kind of weird, but in general residential loads just don't have high enough duty cycle for it to be a concern. Add up every load in your house, and I doubt you'll be able to draw the full 200 amps (48kw assuming 240V service) for any length of time, even with a house full of guests. Indeed, I have 200amp service (48kw), but the transformer that feeds my house and about 5 others is only rated to 25kw. The power company asked what loads I was going to power with an upgraded service, and deemed it unlikely that I and all my neighbors would be using more that 25kw for more than a few seconds at a time.

Ampacity charts also have a significant safety margin. Drawing more than 175A over a 2/0 wire will get things hotter than desired, but won't cause the cable or its insulating jacket to break down.

Commercial/Industrial loads are different. The Code assume that the duty cycle of devices is much higher, and therefore commercial/industrial rated breakers actually trip at 80% of their load rating for continuous draws. (They allow spikes above that for shorter lengths of time.)

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  • The 200A is just example used in the video. In my actual. The breaker at service entrance is 150A, yet my actual wire from main panel feeder becomes mere 107A. So if my load is 120A, then can't the 107A wire burn?
    – Jtl
    Feb 8, 2019 at 5:37
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    @Jtl If that 120A (or even 150A) is just a short spike now and then, everything will work fine (that's the "safety margin"). Heck, even a 200A spike wouldn't do much. Drawing 120A continuously from such a service sounds unlikely, because then any spike would immediately trip the main breaker. Sounds like a miserable experience which would likely prompt a service upgrade. Anyway, that wire of yours is rated for 122A 24x7, so drawing 130A all the time would still just make it a bit hotter than desired, not set it on fire.
    – TooTea
    Feb 8, 2019 at 11:00
  • Actually, the thing with the 80% derate for continuous draws is that UL 489 breakers are not guaranteed to not trip if run at 100% for a long period of time.
    – ThreePhaseEel
    Feb 8, 2019 at 12:41
  • I maintain industrial equipment including load testing on breakers and have not seen one trip at 80% in fact on motor controll breakers they usually can be set to 5 - 10x the rated value looking at a 600 amp set on h setting the instantinous trip is up to 6000 amps so I am not sure where you got or think they trip at 80%.
    – Ed Beal
    Feb 8, 2019 at 18:03
  • According to random selection of industrial the breaker rating charts I looked at online you are correct. Most were rated almost indefinitely at 100%. My experience with fuses has been different. Regular (not slow-blow) fuses used at 85% power will eventually blow, although sometimes it takes days. (It may be something where the higher power draw keeps the fuse hot, and so it becomes sensitive to a momentary spike and opens. I've experienced a few datacenter failures caused by fuses used above 80%.)
    – mfarver
    Feb 21, 2019 at 19:35
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The main reason according to the code handbook that the 83% is allowed is because based on thousands of evaluations this reduction in the feeder or service is still more than sufficient to carry the load. There are large safety factors built into the code. An example of this is #12 wire is limited to 20 amps but according to table 430.72.b the limit is 60 amps with the conductor exiting the motor control enclosure. With this you can see there is a big safety margin. Most of the homes that I have checked prior to adding a sub panel are drawing 50% of the main and with LED lighting high efficiency water heaters, mini split HVAC systems being installed in older homes I think the draw may become smaller. Eventually the NEC will probably change the load calculations because of this especially on lighting (supervised and monitored lighting loads already have been reduced). So to answer the question you can use larger wire sizes you do not have to use the 83% of the main. 83% of the main is the minimum size allowed but you can use larger wire. The NEC testing and evaluation shows that 83% is a safe value to use.

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