Coordination Study - Time Current Graph for 3 pole breaker with one or two pole breaker

[budhiman]

Hello Everyone,

I have a question on TCC when completing a coordination study. Is it correct to show coordination of a 3 pole breaker with a single or two pole breaker. For example a panelboard has a main of 225A and the branch breakers are all single pole 20A. On the TCC is it correct to show how the main coordinates with 20A/1P breaker. If not Why?

 

[mgtrp]

Yes, you need to coordinate the three-pole breaker with the single-pole breaker (and can show this on a TCC). The three pole breaker trips all three poles for an overcurrent on a single phase, it does not require all three phases to have an overcurrent. Therefore, if there is a fault downstream of one of the single-pole breakers, one phase of the three-pole breaker will also see this same current and could trip if not coordinated properly.