Breaker rule

[The electron man]

Hey my fellow electricians ,question about the 80% breaker rule



Where dose this rule apply I'm seeing different answers and I'm a bit confused

 

[infinity]

The 80% applies to the circuit when there is a continuous load. For example if you had a 20 amp non-continous load you could run that on a 20 amp circuit. If the load were continuous like lighting in an office building then you can only load the circuit to 80% or 16 amps.

 

[HEYDOG]

Hey my fellow electricians ,question about the 80% breaker rule



Where dose this rule apply I'm seeing different answers and I'm a bit confused

When supplying a continuous load the breaker can’t be loaded beyond 80%. If you have a 100 amp breaker with a continuous load you can’t load it beyond 80% * 100 amps= 80 amps. If I you had an 80 amp continuous load you would need to size your overcurrent device at 80 * 1.25 = 100 amps. 1/.8 is where the 1.25 comes from. Unless the overcurrent device is rated for a continuous load. Your everyday breakers are not. You can look at 210.19(A)(1)(A) for branch circuits. A “Continous“ load is defined in article 100 as where the maximum current is expected to last for 3 hours or more. Good example would be a lighting load in a school that is going to be on all day long!

 

[jim dungar]

When supplying a continuous load the breaker can’t be loaded beyond 80%. If you have a 100 amp breaker with a continuous load you can’t load it beyond 80% * 100 amps= 80 amps. If I you had an 80 amp continuous load you would need to size your overcurrent device at 80 * 1.25 = 100 amps. 1/.8 is where the 1.25 comes from. Unless the overcurrent device is rated for a continuous load. Your everyday breakers are not. You can look at 210.19(A)(1)(A) for branch circuits. A “Continous“ load is defined in article 100 as where the maximum current is expected to last for 3 hours or more. Good example would be a lighting load in a school that is going to be on all day long!

Do not forget your conductors are also sized based on continuous or non-continuous loads. It should not be a surprise that breakers are built and Listed to provide conductor protection.

 

[The electron man]

Would this rule apply for a main breaker

When supplying a continuous load the breaker can’t be loaded beyond 80%. If you have a 100 amp breaker with a continuous load you can’t load it beyond 80% * 100 amps= 80 amps. If I you had an 80 amp continuous load you would need to size your overcurrent device at 80 * 1.25 = 100 amps. 1/.8 is where the 1.25 comes from. Unless the overcurrent device is rated for a continuous load. Your everyday breakers are not. You can look at 210.19(A)(1)(A) for branch circuits. A “Continous“ load is defined in article 100 as where the maximum current is expected to last for 3 hours or more. Good example would be a lighting load in a school that is going to be on all day long!

So on a continuous load the ocpd and the conductors need to be rated at 1.25 ?

 

[The electron man]

The 80% applies to the circuit when there is a continuous load. For example if you had a 20 amp non-continous load you could run that on a 20 amp circuit. If the load were continuous like lighting in an office building then you can only load the circuit to 80% or 16 amps.

Dose this only apply to the branch circuits or a main breaker too , like a main breaker supplying the house or a building

 

[Joethemechanic]

Dose this only apply to the branch circuits or a main breaker too , like a main breaker supplying the house or a building

Something would have to be seriously wrong with your calculations if the service and the main breaker were continuously loaded more than 80%

 

[HEYDOG]

Would this rule apply for a main breaker


So on a continuous load the ocpd and the conductors need to be rated at 1.25 ?

Yes!

 

[infinity]

Dose this only apply to the branch circuits or a main breaker too , like a main breaker supplying the house or a building

Feeder or branch circuits.

215.3 Feeder Overcurrent Protection.
Feeders shall be protected against overcurrent in accordance with Part I of Article 240. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the
overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

210.20(A) Continuous and Noncontinuous Loads.
Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of
the continuous load. Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device
shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

 

[The electron man]

Something would have to be seriously wrong with your calculations if the service and the main breaker were continuously loaded more than 80%

Feeder or branch circuits.

Feeders

 

[The electron man]

Something would have to be seriously wrong with your calculations if the service and the main breaker were continuously loaded more than 80%

We were doing a Job once and the home owner wanted to add some things we checked the main breaker and at the time it was loaded to 83% , it was an old home

 

[jim dungar]

We were doing a Job once and the home owner wanted to add some things we checked the main breaker and at the time it was loaded to 83% , it was an old home

That is okay, as long as that condition did not last longer than 3 hours.

 

[The electron man]

That is okay, as long as that condition did not last longer than 3 hours.

How would I know I'm not there all day checking ?

 

[retirede]

How would I know I'm not there all day checking ?

That’s why you’re supposed to do an NEC load calculation.

 

[jim dungar]

How would I know I'm not there all day checking ?

Water heaters, air conditioning, baseboard heat, and ovens all have thermostats, so they do not draw full power continuously once they reach their setpoint they cycle on and off which is how many people are still living with 60A and 100A services.