Mike Dolt Enterprises
Mike Dolt Enterprises
User: bpk, Location: Northern Minnesota
Asked on: Friday, April 29, 2011 at 1:52 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:47 am EDT
Question: I have a small 20 space 100a panel that has a backfed 60amp being used as the main. On this install or lots of others like this, can you get a breaker with a slower trip curve so when there is a dead short downstream it doesnt take out the whole panel. I know there is a lot of info and calculations that go into selective coordination for large systems but I've never heard about small ones like this. This panel is a QO with a 2 pole 60 amp backfed being used as the main.
Yes, it is possible to get a breaker with a slower trip curve for a small panel like the one you described, and it can be beneficial in preventing unintended outages.
Selective Coordination for Small Systems
Selective coordination is a technique used to ensure that faults in downstream circuits only trip the breakers protecting those circuits, without affecting the upstream breakers. In a small system like yours, selective coordination can help prevent unnecessary outages by allowing the 60-amp main breaker to remain operational during a fault in a downstream circuit.
Breaker Trip Curves
Circuit breakers have different trip curves that determine how quickly they trip when subjected to overcurrent. The most common trip curves for residential applications are: Type A
Fast trip, designed for light loads
Type B
Intermediate trip, designed for general-purpose loads
Type C
Slow trip, designed for motor loads (these are not typically used in residential applications)
Applying Selective Coordination
To achieve selective coordination in your panel, you would need to replace the 60-amp main breaker with a slower trip curve breaker, such as a Type B breaker. This will allow the main breaker to withstand the initial surge of current from a downstream fault, giving the downstream breaker time to trip before the main breaker does.
Important Considerations Confirm the short-circuit current
Determine the maximum short-circuit current (SCCR) for your system to ensure that the chosen breaker can withstand it without tripping prematurely.
Choose the correct breaker size
The downstream breakers should be sized to protect the circuits they serve, while also ensuring that the main breaker can handle the combined load.
Consider the panel rating
The panel must be rated for the total amperage of all the breakers installed.
Recommended Breaker
For a 100-amp panel with a 20-space configuration, a
QO2150B (2-pole, 60-amp, Type B) breaker from Square D would be a suitable choice for the main breaker.
Note: It is recommended to consult with a qualified electrician to properly size and install the breakers and confirm selective coordination. Flag for review