83% rule

[Seekthetruth]

Does the 83% rule apply when a 320 continuous meter (double lug) is feeding two separate mains for the same residence?

 

[electrofelon]

Does the 83% rule apply when a 320 continuous meter (double lug) is feeding two separate mains for the same residence?

It applies to conductors serving the entire dwelling unit. So it would apply to the meter line side conductors (if under the NEC) but not the conductors between the meter and each panel.

 

[Seekthetruth]

Thank you

 

[LarryFine]

It applies to conductors serving the entire dwelling unit. So it would apply to the meter line side conductors (if under the NEC) but not the conductors between the meter and each panel.

We've been using 4/0 al for two-panel services just like one-panel services with no issues.

 

[wwhitney]

We've been using 4/0 al for two-panel services just like one-panel services with no issues.

4/0 Al has a 75C ampacity of 180A. So for a one-panel residential service, you'd have a single 200A feeder under the 83% rule.

For a two-panel residential service, you'll have two 180A feeders. Those can be protected at 200A under the next size up rule, but the NEC calculated load on each feeder needs to be 180A or less.

Cheers, Wayne

 

[winnie]

We've been using 4/0 al for two-panel services just like one-panel services with no issues.

IMHO this is a violation, but if both of the 200A panels are residential type loads it likely will never be a problem.

-Jon

 

[LarryFine]

IMHO this is a violation, but if both of the 200A panels are residential type loads it likely will never be a problem.

Yes, I was referring to residential services. When I added a second 200a panel for just a tankless water heater (4 x 40a), I ran 4/0 al SE along side the existing SER.

You're saying that adding the second feeder technically made the existing feeder too small.

 

[electrofelon]

IMHO this is a violation, but if both of the 200A panels are residential type loads it likely will never be a problem.

-Jon

The next size up rule makes it compliant, like WW said

 

[winnie]

Yes, I was forgetting the next size up rule.

When the second feeder was added, it invalidated the '83%' rule, but the 'next size up' rule may still apply. If both feeders calculate as less than 180A, then the install is compliant.

If the first feeder calculates as >180A but less than 200A, as a residence, then because there are 2 feeders it doesn't meet the requirements of the 83% rule, but IMHO is not likely to be a problem because residential load calcs are so conservative.

-Jon

 

[Greentagger]

Next size up rule and calculated load 180 A or less makes it compliant.
2/0 copper and different scenario. Only good for 175 A and the make a breaker that size.

 

[LarryFine]

The (already overloaded) original service's exterior main consistently tripped when they tried running all four elements, and they weren't happy with the performance with only two or three. (The guy did his own wiring with each conductor through its own KO!)

This is the job where the inspector cited me for having no neutral, called the head inspector (whom I know), stood by his decision, then the head inspector called him back and said I was right. (The POCO did not increase the underground service conductors.)