Math Behind 83% Rule for Residential Feeder

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tp_spark

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Hauppauge
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Electrical Engineer
Is anyone aware of the math behind the 83% rule for residential feeders in 310.15(B)(7)? Or if there's any supporting documentation floating around out there for how it came about?

Im going down a rabbit hole after coming up with a 98A load calculation (100A service) for a family cabin after adding some electric heaters. Just wanted to read up more for my peace of mind.
 
The majority opinion of many here is that the NEC load calculations are bloated and do not actually reflect real world loads due to the diversity of how loads are used within a dwelling. The NEC has allowed the 83% rule for many decades without much evidence of problems in doing so.
 
Interesting, thats kind of the sentiment I was getting from reading this forums and other forums on the same topic.

I think I'm probably fine but just had the nagging worry that i'm sitting on an edge case since this is a cabin that sits powered off aside from the fridge. Worst case I replace the 2-2-4 feeder with some 1/0-1/0-2 I have laying around.
 
tp_spark said:
I think I'm probably fine but just had the nagging worry that i'm sitting on an edge case since this is a cabin that sits powered off aside from the fridge. Worst case I replace the 2-2-4 feeder with some 1/0-1/0-2 I have laying around.
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The load will probably never approach the number that came out of the NEC load calculation but there is nothing wrong with using larger than required conductors.
 
At some point I posted a reply to another thread that explained the history and issues fully to the best of my knowledge, but I failed to bookmark it and the search function on this site isn't up to the task. Here are the main points.

310.15(B)(7) used to have a table that applied only to certain 75C and 90C wire types but also appeared to exempt such installations from normal derating requirements. The table referred to the 'rating of the service or feeder' rather than ampacity. The table was equivalent to the results of the 83% calculation if no other deratings were required. The re-written rule made the derating requirements consistent with other applications while still producing the same results as the table when derating was not required. I strongly believe they did it wrong: they should have just revised the table to be like other tables and refer to ampacity instead of the rating of the service.

As far as why a smaller wire size would be allowed in the first place, originally 310.15(B)(7) only applied to 240V installations, quite likely on the theory that a 3-wire 240V installation has only two current carrying conductors while a 3-wire 208V circuit has three. Either that, or load diversity was considered a justification. Or some combo of both. Apparently this reasoning was at least partially forgotten a few years ago when the 208V feeder and services were included.
 
It seems like it'd be better practice to fix the service calculation method to better reflect reality, than to make a double standard for sizing the service conductors. A breaker is supposed to be the weak link that trips, before wires and terminations overheat. The same physical principles would apply regardless of whether a circuit is a service or a feeder.

An adversarial user could load the service conductors to 90% the service rating (against all odds), which would exceed the ampacity of the service conductors, and it would go unnoticed by the service main breaker.
 
Carultch said:
It seems like it'd be better practice to fix the service calculation method to better reflect reality, than to make a double standard for sizing the service conductors. A breaker is supposed to be the weak link that trips, before wires and terminations overheat. The same physical principles would apply regardless of whether a circuit is a service or a feeder.

An adversarial user could load the service conductors to 90% the service rating (against all odds), which would exceed the ampacity of the service conductors, and it would go unnoticed by the service main breaker.
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What probability of 90% the service rating?

nec use probability methods better
 
Carultch said:
It seems like it'd be better practice to fix the service calculation method to better reflect reality, than to make a double standard for sizing the service conductors. A breaker is supposed to be the weak link that trips, before wires and terminations overheat. The same physical principles would apply regardless of whether a circuit is a service or a feeder.

An adversarial user could load the service conductors to 90% the service rating (against all odds), which would exceed the ampacity of the service conductors, and it would go unnoticed by the service main breaker.
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With the advent of EV chargers it’s much more possible to get loads approaching the full main breaker rating.

These are pretty much the only really high wattage loads that could run continuously for much greater than 3 hours.

I realize AC is a load that can run for extended periods if time but with the higher Seer numbers the amp draw is considerably less than years back.
 
jim dungar said:
3 wire 120/208V feeders and services have been common in multi-family construction for at least the 50 years I have been in the industry.
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Yes that's what we commonly install in apartment buildings. The NEC only recognized the 83% rule for these 1Ø, 120/208 volt feeders in 2017 code. Prior to that the 83% rule only applied to 120/240 systems which I think was jaggedben's point.
 
tp_spark said:
Interesting, thats kind of the sentiment I was getting from reading this forums and other forums on the same topic.

I think I'm probably fine but just had the nagging worry that i'm sitting on an edge case since this is a cabin that sits powered off aside from the fridge. Worst case I replace the 2-2-4 feeder with some 1/0-1/0-2 I have laying around.
Click to expand...
A little real-world food for thought....

About 4 years ago I put together a 400 amp service on a house for which my load calculation was 256 amps known and 30-60 amps unknown. The power company dragged a #2 service drop from a 100 amp (25kva) transformer which fed 3 other houses.

I just last week replaced a 150 amp service on a lake house to go from overhead to underground. That house is fed from a transformer which has 9 other houses on it. These are full-time residences. Ten in total. It's undoubtedly a 200 amp (50kva) transformer because pole-to-pole is 1/0 or 2/0

But in each case, that's only 25 amps per house on average. And it's done all the time
 
James L said:
The power company dragged a #2 service drop from a 100 amp (25kva) transformer which fed 3 other houses.
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The 25 kva on the pole in front of my house is feeding two larger homes each with a 200 amp service. My neighbors have a 48 amp EV charger and with 4 central AC units running during the 100° days this summer and their EV charging running at the same time we didn't have a problem.
 
infinity said:
The 25 kva on the pole in front of my house is feeding two larger homes each with a 200 amp service. My neighbors have a 48 amp EV charger and with 4 central AC units running during the 100° days this summer and their EV charging running at the same time we didn't have a problem.
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It takes some getting used to, when our brains are trained to think in terms of all these numbers, but our eyes are telling us something completely different
 
James L said:
A little real-world food for thought....

About 4 years ago I put together a 400 amp service on a house for which my load calculation was 256 amps known and 30-60 amps unknown. The power company dragged a #2 service drop from a 100 amp (25kva) transformer which fed 3 other houses.

I just last week replaced a 150 amp service on a lake house to go from overhead to underground. That house is fed from a transformer which has 9 other houses on it. These are full-time residences. Ten in total. It's undoubtedly a 200 amp (50kva) transformer because pole-to-pole is 1/0 or 2/0

But in each case, that's only 25 amps per house on average. And it's done all the time
Click to expand...
Funny you say this because Im in engineering for the local poco and I've seen the same type of setups you described. I dont know why I didn't think of the parallel.
 
James L said:
It takes some getting used to, when our brains are trained to think in terms of all these numbers, but our eyes are telling us something completely different
Click to expand...
James L said:
A little real-world food for thought....

About 4 years ago I put together a 400 amp service on a house for which my load calculation was 256 amps known and 30-60 amps unknown. The power company dragged a #2 service drop from a 100 amp (25kva) transformer which fed 3 other houses.

I just last week replaced a 150 amp service on a lake house to go from overhead to underground. That house is fed from a transformer which has 9 other houses on it. These are full-time residences. Ten in total. It's undoubtedly a 200 amp (50kva) transformer because pole-to-pole is 1/0 or 2/0

But in each case, that's only 25 amps per house on average. And it's done all the time
Click to expand...
Which leads one to believe how little 320/400 amp residential services are really required.

Yes, the load calc tells us we need to install it but the real data tells a different story.

I have never in close to 30 years experienced a tripped 200 amp main breaker because of an overload.
 
tp_spark said:
Funny you say this because Im in engineering for the local poco and I've seen the same type of setups you described. I dont know why I didn't think of the parallel.
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There are many locations in town ( especially poco feeds from an alley for instance) that will have 10 or more residences fed from one 25kva pole mounted xfmr.

And I know that alot of those xfmrs are as much as 50+ years old.
 
Deltaforce said:
What probability of 90% the service rating?

nec use probability methods better
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I'm aware it is unlikely, and probability isn't the point. The point is, that if the improbable event does happen, you want the breaker to catch it, before it becomes a serious problem. I can understand probability and statistical models of user behavior, being used for sizing the service in general as a nuanced approach to "adding up" total power, but I don't see this as a reason to justify the double standard we have for allowing 166A worth of conductors on a 200A service. It seems like it would be better practice to modify the load calculation, so that the main breaker and service conductors are consistent with each other, and a 175A service with 175A worth of conductors, would be installed in the same situation. I get that people generally standardize on nice-round numbers for simplicity, so 175A services are rare, except when there's a strategic reason for them.

You'd have to go out of your way to deliberately turn on more loads than even the most careless user would turn on at the same time, to load your service to 90% its rating. I could also see it happening, from renovating an existing service to replace gas appliances with electric appliances. Since a lot of Watt-intensive loads are time-flexible, you can plan around this issue with whole service metering that pauses these loads any time there are high load conditions of other loads.
 
Carultch said:
An adversarial user could load the service conductors to 90% the service rating (against all odds), which would exceed the ampacity of the service conductors, and it would go unnoticed by the service main breaker.
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And which would not be a real world hazard. Not only the load calculations, but also the conductor ampacities in Table 310.16 are very conservative.
 
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