rtu amperage spike

[andinator]

To make this brief (more info upon request) My question is on a three ph. 20A 480V breaker how much of a spike could the breaker take at start-up? The guy I work with(23 years exp.) contends that at 20.1A that sucker is tripping. I say (7 years resi., 8 mos. comm.) that while it won't take a thousand amp spike at start up it will take over 20A.

 

[raider1]

An inverse time breaker trips on a curve. The larger the current flow the quicker the breaker will trip. Typically a 20 amp breaker won't trip at all with a 20.1 amp load. I have seen 20 amp inverse time breakers that have held at 21 to 22 amps for 8 to 10 hours.

Chris

 

[andinator]

I agree and have seen this. I guess my question is what when a motor starts there is an inrush of current right at start up. What kind of inrush amperage, just for that split second, can the breaker handle. Thanks for the help.

 

[iwire]

What kind of inrush amperage, just for that split second, can the breaker handle. Thanks for the help.

That really depends on the particular breaker and the ambient temperature it is located in.

A 20 amp breaker might handle quite a few 100 amps for a short time.

I had an amprobe on a 30 amp circuit that was very long and had a short at the far end, the 30 amp breaker held 175 amps for about 5 seconds, it might had gone longer but a switch in the circuit opened instead.

 

[bob]

The guy I work with(23 years exp.) contends that at 20.1A that sucker is tripping. I say (7 years resi., 8 mos. comm.) that while it won't take a thousand amp spike at start up it will take over 20A.

The curve only goes to 10,000 seconds but you are closer that you buddy.

 

[andinator]

I was fairly sure I was right. On a funny note, we were working on this rtu at an elem. school in dunwoody GA this afternoon. I'm up on the roof and all of a sudden every rtu on the roof turned off. It took me a second to realize that all Fulton county public schools are controlled in downtown ATL.:grin:


edit to add: I wasn't fairly sure I was right, I frikkin darn well new I was right.