MCB Rating 125%????

[AWinston]

If you have a panelboard with loads connected. Each branch circuit breaker is then sized to be 120% higher than each load. Is it necessary to size the main circuit breaker for the panelboard to 120% of the total load?

 

[infinity]

The calculated load is used to determine the size of the panelboard. That calculation should already include the 125% factor for continuous loads.

 

[AWinston]

The calculated load is used to determine the size of the panelboard. That calculation should already include the 125% factor for continuous loads.

Sorry, I mislabeled the title of the thread. It should read 120%. I'm referring to keeping the load within 80% of the circuit breaker rating.

 

[topgone]

Sorry, I mislabeled the title of the thread. It should read 120%. I'm referring to keeping the load within 80% of the circuit breaker rating.

No. Your subject title is correct if you wanted to keep your load at 80%. It's because 1/0.8 = 1.25.

 

[AWinston]

No. Your subject title is correct if you wanted to keep your load at 80%. It's because 1/0.8 = 1.25.

Gotcha. So just to reiterate, as long as you applied the 125% rule to your continuous loads, then the MCB sizing (sized with the panel) falls into place.

 

[topgone]

Gotcha. So just to reiterate, as long as you applied the 125% rule to your continuous loads, then the MCB sizing (sized with the panel) falls into place.

125% applied on the continuous loads is a requirement when you size your feeder wires. Your MCB size depends on the type of load you are supplying. You might have a motor load which require a relatively large starting current and using 125% to size your breaker may result into a breaker that trips on starting. For motor loads, please refer to Article 430.52 to size your breaker. The idea is to choose an MCB that trips to protect your loads but allow starting/inrush currents to flow.

 

[AWinston]

125% applied on the continuous loads is a requirement when you size your feeder wires. Your MCB size depends on the type of load you are supplying. You might have a motor load which require a relatively large starting current and using 125% to size your breaker may result into a breaker that trips on starting. For motor loads, please refer to Article 430.52 to size your breaker. The idea is to choose an MCB that trips to protect your loads but allow starting/inrush currents to flow.

Understood. Thank you very much.

 

[templdl]

If you have a panelboard with loads connected. Each branch circuit breaker is then sized to be 120% higher than each load. Is it necessary to size the main circuit breaker for the panelboard to 120% of the total load?

Whast is the breasker's protecting? Wire? Isn' the wire sized such that it carries 125% of the continuous load + 100% of the non continuous load? Isn't the OCPD sized to protect the wire's rated ampacity? If there isn't a standard tasted OCPD fo that ampacity then seen 't you allowed to select the next size OCPD rating? Wouldn't this mean that the OCPD will be applied at no more than 80% of its rating of its rating?
The question then becomes what comes first the chicken or the egg?
If the wire is sized correctly and the OCPD sized to protect the wire then what is the issue?

 

[MD84]

Whast is the breasker's protecting? Wire? Isn' the wire sized such that it carries 125% of the continuous load + 100% of the non continuous load? Isn't the OCPD sized to protect the wire's rated ampacity? If there isn't a standard tasted OCPD fo that ampacity then seen 't you allowed to select the next size OCPD rating? Wouldn't this mean that the OCPD will be applied at no more than 80% of its rating of its rating?
The question then becomes what comes first the chicken or the egg?
If the wire is sized correctly and the OCPD sized to protect the wire then what is the issue?

I think it's time for a sit-down with Siri.

 

[Fitzdrew516]

There's a lot of misconception in this industry as far as the 80% rated breaker vs 100% rated breaker myth. As the others have mentioned, if you have calculated loads with the proper demand factors then a standard breaker (what some would call an 80% breaker) will handle the calculated load.

Here is some good information. The link to the podcast in this link is great and I have sent it to others before and it seemed to help.

http://blog.schneider-electric.com/...g-confusion-80-vs-100-rated-circuit-breakers/