83 percent rule for single family dwelling

[rapidelec]

I'm a little bit confused on where to apply the 83 percent rule. In the 2020 code book, 310.12 (B) talks about applying the 83 percent rule to feeders in dwelling units. Does this mean, that if I am adding a 100 amp sub panel to a 200 amp service main panel, that I can run #4 THHN (which is 85 amps in the 75 degree column) and put it on a 100 amp breaker?

 

[tom baker]

No. The rule only applies to the total connected load. Once you run to a feeder the diversity no long applies that allows the 83 % rule

 

[jaggedben]

No. The rule only applies to the total connected load. Once you run to a feeder the diversity no long applies that allows the 83 % rule

I would just say the rule doesn't apply because that's what the rule says.

I'm not very convinced there is any coherent reasoning behind it.

(Here's a common example that illustrates my point: A existing service consists of a 100A service disconnect feeding a subpanel inside the house with 4awg cu. This has always been fine at 100A because it's mostly 15-20A breanch circuits, plus an oven (or dryer). Owner wants to upgrade the outside disconnect to a 200A panel in order to add EV charger, heat pump, and electrify the dryer (or oven). Does the new subpanel breaker have to be downsized to 90? Yes. Why? Because the rule says so, no other logical reason.)

 

[tom baker]

The logical reason is diversity. The CMP understands the reason and this rule has been in the NEC for over 50 years
But make a pubic input for the 2026 NEC. Often a public input allows the CMP to explain the reasoning.

 

[electrofelon]

The logical reason is diversity. The CMP understands the reason and this rule has been in the NEC for over 50 years
But make a pubic input for the 2026 NEC. Often a public input allows the CMP to explain the reasoning.

Usually all they will do is say "proposer has not provided evidence that.....". Even though the rule trying to be changed has no evidence or substantiation or no one can remember what it was. They love throwing that out there to deny things yet half the things in that book have no substantiation or are not even based on electrical theory....

 

[AC\DC]

In his example the diversity has not changed.
The load on the 100 amp is still the same existing feeder is still the same

 

[AC\DC]

Usually all they will do is say "proposer has not provided evidence that.....". Even though the rule trying to be changed has no evidence or substantiation or no one can remember what it was. They love throwing that out there to deny things yet half the things in that book have no substantiation or are based on electrical theory....

Just come up with something you can market and sell then a new rule
Will be passed to benifit you.
Sounds like the government

 

[jaggedben]

In his example the diversity has not changed.
The load on the 100 amp is still the same existing feeder is still the same

Exactly. The load has not changed at all. Thank you.

By the diversity logic, 310.12 should simply apply to all feeders of the applicable sizes in the covered occupancies.

 

[letgomywago]

90 amp breaker is cheaper than a 100 amp so I don't see any issue in the end. There's no way that the load will come out to need the 100 amp breaker in the end anyway.

 

[AC\DC]

Still a dumb rule when nothing has changed, beside a definition

 

[mtnelect]

Usually all they will do is say "proposer has not provided evidence that.....". Even though the rule trying to be changed has no evidence or substantiation or no one can remember what it was. They love throwing that out there to deny things yet half the things in that book have no substantiation or are not even based on electrical theory....

According to Don, the CMP now use the word "Resolved" to turn down a proposal.

 

[Tulsa Electrician]

If you look at .83% rule
Divide 75c by 90c
75/90= .83333
Just for fun.
Run the numbers carry load at 75c and allow for the round up rule and 240.6 on the 90c.

Remember just for fun
lets use 175 amp rated service.
175*.83= 145.25

CU @75 1/0 @ 150
CU@90 1/0@ 170 up to 175

AL @ 75 3/0@ 155
Al @ 90 3/0 @ 175

Try another one, 225
225*.83= 186.75
3/0 CU
4/0 Al

Another
400*.83=332
400 mcm CU
600 Al

 

[mtnelect]

According to don, my two PI will have the same fate ... "Resolved".

 

[don_resqcapt19]

Still a dumb rule when nothing has changed, beside a definition

So submit a PI to make a change or add an exception in the 2026 code for an installation like the one in this post. You have until 5p, eastern on 9/7.

 

[AC\DC]

Maybe I will if I can’t complain I guess if I don’t try and change it.