Breakers of the same size in series

[mbrooke]

I once had a fault in a 30 amp circuit, my helper at the time thought tightening the clamp on the romex connector meant turn the screws until they don't turn anymore I guess:blink:. Not only did my helper need clean underpants, it tripped the 30 amp breaker as well as the 200 amp main breaker in the panel.

That will happen if the short circuit current exceeds the magnetic trip rating of both the main and branch breaker. Typical molded case breaker trip at about 10x the handle rating instantaneously; so a 30 amp breaker starts at 300amps and a 200 amp breaker about 2,000. Some main breakers even start below 10x. So an 8000 amp utility source can easily trip both if the fault and cable impedance is low enough as 8,000 or even 2,500 is still higher than 2,000.

 

[kwired]

That will happen if the short circuit current exceeds the magnetic trip rating of both the main and branch breaker. Typical molded case breaker trip at about 10x the handle rating instantaneously; so a 30 amp breaker starts at 300amps and a 200 amp breaker about 2,000. Some main breakers even start below 10x. So an 8000 amp utility source can easily trip both if the fault and cable impedance is low enough as 8,000 or even 2,500 is still higher than 2,000.

:thumbsup:

Absolutely. Overload conditions, the 200 amp main would not have tripped. Short circuits and ground faults, anything in the current path will open if it's instantaneous trip level is exceeded. Had the fault in my example happened on the load end of the circuit in question - the resistance of the branch circuit conductors may have reduced the fault current enough that the main doesn't trip. But in this case the fault was right at the fitting where the circuit leaves the panel - very short distance of smaller conductor and not nearly as much resistance results in higher fault current.

 

[mbrooke]

:thumbsup:

Absolutely. Overload conditions, the 200 amp main would not have tripped. Short circuits and ground faults, anything in the current path will open if it's instantaneous trip level is exceeded. Had the fault in my example happened on the load end of the circuit in question - the resistance of the branch circuit conductors may have reduced the fault current enough that the main doesn't trip. But in this case the fault was right at the fitting where the circuit leaves the panel - very short distance of smaller conductor and not nearly as much resistance results in higher fault current.

Yup- thats how it works. Even POCOs have to worry about it. Often fuses in series on 12.47kv taps close to the substation will not coordinate due to the high fault current. 100K, 65K and 30K fuses will all blow at the same time for squirrel or failed transformer. On the other hand on long feeders a fault at the far end (say 20 miles away) may not get picked up the feeder breaker requiring more sensitive over current devices out on the line.

 

[jaylectricity]

I love being accused of "overthinking" and "underthinking" the same issue in the same thread by different people.

In Goldilocks terms, it sounds like I'm thinking "just right".

 

[mbrooke]

I love being accused of "overthinking" and "underthinking" the same issue in the same thread by different people.

In Goldilocks terms, it sounds like I'm thinking "just right".

I'd say your just learning :thumbsup:

 

[dkidd]

I'd say your just learning :thumbsup:

you're

 

[mbrooke]

you're

I never passed spellin'!

 

[kwired]

I never passed spellin'!

If you were from down south it should have been "yer".

 

[mbrooke]

If you were from down south it should have been "yer".

Yesir indeed!

 

[JFletcher]

I once had a fault in a 30 amp circuit, my helper at the time thought tightening the clamp on the romex connector meant turn the screws until they don't turn anymore I guess:blink:. Not only did my helper need clean underpants, it tripped the 30 amp breaker as well as the 200 amp main breaker in the panel.

Ha. Best I have is watching an industrial electrician create a phase to ground fault in a 480V cabinet for a 1/2HP shaft drive motor... that breaker never tripped, the incinerator building's 2000A main went first. I saw the arc flash, building goes dark, e-lights come on, hear the ID fan winding down... fun stuff.

Last time I blew up something was cutting out some NM for a kitchen demo. Just got there, was told the circuit was dead by the boss. I didnt doublecheck... put my Kleins into the cable and squeeze...loud pop, bright flash, blown Kleins. That wasnt the first or last pair I blew up working with him, tho I havent blown up anything else since I moved on working for him. cuz, yanno, verifying power is off and securing a breaker takes a whole two minutes or so... two minutes is a small price to pay vs $20 paperweight pliers or looking like a j/a in front of the HO.

Dual breakers for one circuit, not my favorite, tho no NEC issue. Jraef's points are valid. I run across dual GFCI protection much more often, even once saw numerous GFCI receps for a kitchen fed line/load off one another, coming from a 20A GFCI breaker (HO wired). If ground fault protection (x1) works, GFCI protection x4 must be 4x as good, right?

 

[kwired]

Ha. Best I have is watching an industrial electrician create a phase to ground fault in a 480V cabinet for a 1/2HP shaft drive motor... that breaker never tripped, the incinerator building's 2000A main went first. I saw the arc flash, building goes dark, e-lights come on, hear the ID fan winding down... fun stuff.

Last time I blew up something was cutting out some NM for a kitchen demo. Just got there, was told the circuit was dead by the boss. I didnt doublecheck... put my Kleins into the cable and squeeze...loud pop, bright flash, blown Kleins. That wasnt the first or last pair I blew up working with him, tho I havent blown up anything else since I moved on working for him. cuz, yanno, verifying power is off and securing a breaker takes a whole two minutes or so... two minutes is a small price to pay vs $20 paperweight pliers or looking like a j/a in front of the HO.

Dual breakers for one circuit, not my favorite, tho no NEC issue. Jraef's points are valid. I run across dual GFCI protection much more often, even once saw numerous GFCI receps for a kitchen fed line/load off one another, coming from a 20A GFCI breaker (HO wired). If ground fault protection (x1) works, GFCI protection x4 must be 4x as good, right?

2000A main probably had GFP and that is what initiated the trip. I've done that before myself, and was probably lucky nothing serious happened. Was drilling a hole in back panel in a MCC to mount a control relay, timer, or something - was over 20 years ago don't remember that detail anymore. Drill bit went through and into either bus connector or conductors coming off the bus connector. Most of the flash was behind the panel I was drilling through was probably the lucky thing about this. I also had to find my way out of a dark room after that happened.

 

[mbrooke]

2000A main probably had GFP and that is what initiated the trip. I've done that before myself, and was probably lucky nothing serious happened. Was drilling a hole in back panel in a MCC to mount a control relay, timer, or something - was over 20 years ago don't remember that detail anymore. Drill bit went through and into either bus connector or conductors coming off the bus connector. Most of the flash was behind the panel I was drilling through was probably the lucky thing about this. I also had to find my way out of a dark room after that happened.

Probably- and everyone who has seen in the least least a few panel boards in the wild knows the dials never get turned up past the factory setting.

 

[JFletcher]

2000A main probably had GFP and that is what initiated the trip. I've done that before myself, and was probably lucky nothing serious happened. Was drilling a hole in back panel in a MCC to mount a control relay, timer, or something - was over 20 years ago don't remember that detail anymore. Drill bit went through and into either bus connector or conductors coming off the bus connector. Most of the flash was behind the panel I was drilling through was probably the lucky thing about this. I also had to find my way out of a dark room after that happened.

Yeah very lucky. I watched a YT video of a man killed when he tested a 4160V MCC with a meter rated to just 600V. As soon as he touched the leads to the energized fuses, it blew up in his face. The resulting flash also destroyed the cabinet. I think he lived about a day. iirc it happened in a plant in NC.

 

[Dinasaur]

Back to the post........ Two CB's protecting same load. In series. Years ago when installing a sub panel I was required by the building dept to place a 100 amp CB both in the feeder panel and in the house main, which was a sub panel. It took little thinking to realize that this was the best installation for safety and convenience. And ever since that one I have always placed the same size CB both in the main/feeder panel as is used for supply in the sub panel. If one needs to work on a sub panel he simply turns off the main CB and proceeds. Otherwise he may have to go up or down 50 feet and around the corner to turn the panel off to be worked on. Were there an emergency where the power needed to be cut on the sub panel immediately it may be too late to have to go all the way back to the feeder panel.

Routinely on spa installations two CB's of the same size are required, the one at the feeder and the other in the spa cutoff box, which is a sub panel. This also is for safety, the cutoff spa box must be located within sight of the spa (and ought to be within sight of the control panel of the spa) so no one can turn power back on while it is being worked on.

Otherwise, there are many different places in which two circuit breakers of the same size are in series, and for good reason.

 

[kwired]

Back to the post........ Two CB's protecting same load. In series. Years ago when installing a sub panel I was required by the building dept to place a 100 amp CB both in the feeder panel and in the house main, which was a sub panel. It took little thinking to realize that this was the best installation for safety and convenience. And ever since that one I have always placed the same size CB both in the main/feeder panel as is used for supply in the sub panel. If one needs to work on a sub panel he simply turns off the main CB and proceeds. Otherwise he may have to go up or down 50 feet and around the corner to turn the panel off to be worked on. Were there an emergency where the power needed to be cut on the sub panel immediately it may be too late to have to go all the way back to the feeder panel.

Routinely on spa installations two CB's of the same size are required, the one at the feeder and the other in the spa cutoff box, which is a sub panel. This also is for safety, the cutoff spa box must be located within sight of the spa (and ought to be within sight of the control panel of the spa) so no one can turn power back on while it is being worked on.

Otherwise, there are many different places in which two circuit breakers of the same size are in series, and for good reason.

In each of your examples overcurrent protection on the supply end and a switch with no overcurrent protection on the load end would have still met your goals as well as been code compliant. In the real world however it often is simple or cost effective to just use a device with overcurrent protection in those situations on the load end. Chances are fairly high in most cases that if you have a short circuit/ground fault beyond the load end device - it is tripping both, that may or may not be a problem depending on other conditions.