My 50amp breaker feed at the main panel trips before the subpanel on a 30amp 240v circuit. Shouldn't the 30amp breaker trip first at the subpanel? They're different brands: Square D QOs at the subpanel and standard GE at the main. Is the breaker at the subpanel broken?
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an alternative browser.
You should upgrade or use an alternative browser.
Main panel breaker trips before subpanel
- Thread starter bigmaq
- Start date
Lightning rod
Well-known member
Breakers (and fuses) have two ratings
The continuous load rating (whats printed on the handle) and the interrupting current rating
which is printed on a label on the side of the breaker.
The higher the breaker rating, the higher the interrupting rating typically.
So if you overload the circuit above the breaker rating but below the interrupting capacity
of the main breaker , you will trip the lower circuit
If the "short is higher than the circuit breaker interrupting rating, the main breaker will trip
to protect the circuit and the the actual lower interrupting rated breaker from damage
So if you have a dead short , it may trip the main before the circuit.
Or a sub-panel breaker before the breaker on your power bar.
The continuous load rating (whats printed on the handle) and the interrupting current rating
which is printed on a label on the side of the breaker.
The higher the breaker rating, the higher the interrupting rating typically.
So if you overload the circuit above the breaker rating but below the interrupting capacity
of the main breaker , you will trip the lower circuit
If the "short is higher than the circuit breaker interrupting rating, the main breaker will trip
to protect the circuit and the the actual lower interrupting rated breaker from damage
So if you have a dead short , it may trip the main before the circuit.
Or a sub-panel breaker before the breaker on your power bar.
Interrupting rating means it can trip with that many amps flowing and it won't blow up.Breakers (and fuses) have two ratings
The continuous load rating (whats printed on the handle) and the interrupting current rating
which is printed on a label on the side of the breaker.
The higher the breaker rating, the higher the interrupting rating typically.
So if you overload the circuit above the breaker rating but below the interrupting capacity
of the main breaker , you will trip the lower circuit
If the "short is higher than the circuit breaker interrupting rating, the main breaker will trip
to protect the circuit and the the actual lower interrupting rated breaker from damage
So if you have a dead short , it may trip the main before the circuit.
Or a sub-panel breaker before the breaker on your power bar.
The things we are concerned about are thermal trip, and magnetic trip
wyliesdiesels
Well-known member
that rating has nothing to do with the subject at handOk, it's tripping on short (miswired my motor). As long as I know I'm protected somehow. I just hate making that trip inside to reset. I just looked at the interrupting rating: 10K amps?!?
Innovate1
Well-known member
As Bert noted that's the current it can interrupt without blowing up. When a direct short happens the breaker takes a short amount of time to open (even though it is so fast it may seem immediate). During the very brief time the current may be much higher than the rating of the breaker. This can be hundreds of amps. That also explains why the 50A is opening - the rating of both breakers is exceeded. Depending on the details of the breaker (you should be able to look up the time vs. current trip curves if you are interested) the 50A may open quickly enough in the case of a short or massive overcurrent to keep the 30 from opening too.Ok, it's tripping on short (miswired my motor). As long as I know I'm protected somehow. I just hate making that trip inside to reset. I just looked at the interrupting rating: 10K amps?!?
mike93lx
ALLIANCE MEMBER
High is good. Molten bits are bad.Ok, it's tripping on short (miswired my motor). As long as I know I'm protected somehow. I just hate making that trip inside to reset. I just looked at the interrupting rating: 10K amps?!?
That's what it can handle and still trip. You want that to be high
alfredeneuman
Well-known member
A fault can be on the order of thousands of amp depending on the Kva rating and impedance of the transformer feeding it.During the very brief time the current may be much higher than the rating of the breaker. This can be hundreds of amps.
Innovate1
Well-known member
True. Many hundreds. Probably not many thousands at least in residential settings.A fault can be on the order of thousands of amp depending on the Kva rating and impedance of the transformer feeding it.
alfredeneuman
Well-known member
There's a good reason behind the 10K rating of most residential breakers.True. Many hundreds. Probably not many thousands at least in residential settings.
The calculation is complicated, but there are calculators specifically designed for the use.
Example: A 25KVA, with an aluminum #2 service drop 75' long will have a 3695A short circuit available at the end of the drop. All of these are typical of a 200 Amp residential service.
Last edited: