Yes, it's covered under 310.15(B)(1), here: (this same website will let you browse the whole NEC):
Thanks, Wayne. I wasn't sure if service conductors had different rules than 'general wiring'.
I have usually been going straight to the
NEC publication online... free access is available through a free NFPA account for those without a paid subscription.
But, ampacity correction for temperature (and adjustment for number of conductors) is not required to be done at the same time as using the temperature limits of the termination. That is, if you have 90C wire (you do), you can do the temperature correction from the 90C ampacity, compare that the uncorrected 75C termination ampacity, and take the smaller of the two results.
At one point I had read this, but since forgotten, so thank you for the re-education about the separate analysis and taking the lesser of the values.
So your 500 MCM Al has a 90C ampacity of 350A and a 75C ampacity of 310A. That means temperature correction will only matter if the correction factor is lower than 310/350 = 0.886. Looking at the 90C column of Table 310.15(B)(1)(1) (you need to use the 30C base table because the ampacity table 310.16 assumes 30C ambient), you are good to 104F.
For 105F to 113F, the table gives a correction factor of 0.87. But that's a simple approximation where you round up your temperature the next highest multiple of 5C. There's also a formula for a sharp answer, which I believe will tell you that you are good up to 109F. So does it regularly get above 109F outdoors where you are? E.g. for tens of hours per year?
We don't see those temps here in Colorado, so we should be good.
[What percentile outdoor temperature should be used for temperature correction is not defined anywhere in the NEC. ASHRAE 99.6% outdoor temperature would probably be fine, which implies that you'd expect to see the temperature for 3.65*0.4*24 = 35 hours per year.]
Cheers, Wayne
I think there are two more hurdles to clear, relating to feeder capacity. I saw a discussion over on mikeholt, but wasn't able to track it 100%. It seemed like in situations where feed-through lugs were used off the main breaker panel, there are instances where the conductors feeding the secondary may be considered a tap, which may have further conductor ampacity reductions. Now, I also understand, there are different definitions for what a feed-through lug is on a load center. In my case, these are trailer/ranch panels with the built in lugs at the bottom (end opposite main breaker) of the bus bars.
Do you think there is anything there that I have to consider around conductor ampacity for my case? To be honest, I get a little lost on the topic of taps, as I haven't really dug into them.
The second hurdle I feel pretty good about, which is thermal insulation surrounding a SER feeder for a secondary panel. If I understand correctly, per 338.10(B)(4) (for NEC 2017+), the SER can be encased in thermal building insulation without any conductor ampacity adjustments, provided a) there are two or less SER cables and b) larger than 10 gauge (not even sure they make SE 10 gauge or smaller).
Does this sound right?
Now, relating to the over all goal (determine if I can support a 50A electric heater, in addition to supplied loads), it seems like the answer is No, unless I use some type of energy management to reduced my load calculations sufficiently.
I put together this little table, including a summary of load calculations, circuits I felt were legible for adjustments through energy management, and some possible future uses requiring additional load.
For load calcs, I put an 2024 view of use and a 2027 view of use. In 2024, there isn't a well pump, second kitchen, 2nd mini-split,EVSEs, renters, etc. - even though these things are planned, and could change slightly. Also, in 2027, it's possible there isn't a hot tub, but not a given.What is clear is that the heater is probably best placed on the garage leg, as the main house leg is much more certain/solidified and nearer capacity in the near term.
Essentially, the question comes back to if I prefer:
a) leveraging energy management in 3 years to implement the electric duct heater now, which will reduce options in my back pocket for future electrical expansion without a service upgrade (which isn't going to happen). Practical energy management would be on one of the lines in green, below. Other than headroom built in to the NEC calculation, there isn't much headroom for any issues encountered.
b) using hydronics to heat the makeup airstream, and not have any energy management planned, thus preserving extra 'capacity' through energy management, should future needs arise. Also maintains a little more direct headroom over time.
For me, it's a tough call. If you have any experience/thoughts on this, would be great to hear.
I'd really like to make the electric heater work for reasons of reducing complexity (the hydronic solution will be more fiddly and potentially maintenance oriented) and dependence on fossil fuels. But, it seems that comes at the cost of not having many more tricks in the bag to support any future electrical needs.
I think tomorrow I will chat with a number of companies who have products around energy management, to get a sense of what will or won't work with my physical layout (lots of space between main and garage), and which ones seem simple+robust without crazy associated costs.