Can't quite figure this out.....?????

[220/221]

I'm an ace mechanic but I often have trouble with the technical end of electricity.

I've seen this happen a few times as I'm sure we all have.

Let's say that you have two 15 amp branch circuits, both installed on the same leg, and someone inadvertantly ties them together downstream.

Will it take 30 amps to trip this circuit?



I did a panel change last week and discovered the two circuits in question. I found where they were tied together and split them up.

Now, the microwave, which was on one of the 15's, trips the breaker. The micro/vent ended up on a 15A lighting circuit as they often do when someone swaps out the stock vent hood.

I went back and added a micro circuit to correct the problem but was wondering, since it didn't trip before, whould it have taken 30 amps to trip one or both breakers?

 

[StephenSDH]

Current prefers the path of least resistance. So one of the breakers would be likely to draw more current then the other, and trip sooner. After the first one trips the second breaker would have the full load and trip shortly after. If the lengths and gauge is the same for both, then it would be simular to 30A over current protection.

You did a good thing by separating the circuits, because the breakers are not protecting the wire or devices. Sounds like you found out why it was done "inadvertently".

 

[RICK NAPIER]

It would take 30 amps to blow the circuit if your load was in the exact resistance center of the circuit. Since current travels proportionally to resistance it will take more than 15 amps to blow the circuit but less than 30 amps. Or to put it another way the amperage draw on each circuit won't be equal but will draw more than 15 amps total at the load.

I typed to slow

 

[Dennis Alwon]

I guess I am confused -- it would appear to me that if one leg was over 15 amps for a certain period of time. Then One circuit is going to blow. The other to follow. I don't see why it would take 30 amps.

 

[RICK NAPIER]

These two breakers are acting as uneven parallel conductors. So for example you could have a 22 amp load with 12 amps on one breaker and 10 amps on the secound breaker.

 

[Dennis Alwon]

These two breakers are acting as uneven parallel conductors. So for example you could have a 22 amp load with 12 amps on one breaker and 10 amps on the secound breaker.

Yes, of course--duh -- I missed that for some reason. Thank you.

 

[One-eyed Jack]

I guess I am confused -- it would appear to me that if one leg was over 15 amps for a certain period of time. Then One circuit is going to blow. The other to follow. I don't see why it would take 30 amps.

I agree with Rick that if the load was in the exact center of the resistance of the circuit it would take near 30 to trip the first one. After the first one lets go the other will only be a second or two behind. In your example the one with the most will let go first,no matter what one is going to go first.

 

[ceb58]

I'm an ace mechanic but I often have trouble with the technical end of electricity.

I've seen this happen a few times as I'm sure we all have.

Let's say that you have two 15 amp branch circuits, both installed on the same leg, and someone inadvertantly ties them together downstream.

Will it take 30 amps to trip this circuit?



I did a panel change last week and discovered the two circuits in question. I found where they were tied together and split them up.

Now, the microwave, which was on one of the 15's, trips the breaker. The micro/vent ended up on a 15A lighting circuit as they often do when someone swaps out the stock vent hood.

I went back and added a micro circuit to correct the problem but was wondering, since it didn't trip before, whould it have taken 30 amps to trip one or both breakers?

I ran into a similar situation about a year ago. The house had an addition/kitchen remodel. They pulled a 15amp circ. and a 20 amp home run into a DG switch box in the kitchen next to the sink. By luck the two landed on the same phase. I guess the 20 was for a DW that never got installed. Any way they tied the two together, fed a light in the kitchen and fed the master bath lights with it. The carpenters were remodeling the bath when one of them cut the feed wire to the switch with a saw saw. They could not figure why it tripped two breakers. After about an hour of tracing the wires I found the two tied together. So there was around 35amp of protection for the lighting circ.

 

[growler]

I once worked a fire job where someone had tied two 15 Amp circuits togather and then replaced the 15 Amp fuses with 30 Amp fuses. We never found out how much it would take to blow the fuses because they didn't blow, instead all the wiring for those ciruits just sort of melted ( at least all the insulation did ).


I would guess that since a 15 Amp breaker doesn't actually trip at 15 Amps and you can draw a bit more current for a short peroids of time and depending on the type and age of the breaker that you could draw a good deal more than 30 amps for a short peroid of time and not have either breaker trip. It wouldn't surprise me if you couldn't load this circuit up to 35-40 amps for at least 5 minutes or long enough for the insulation to get nice and hot.

 

[Volta]

...Will it take 30 amps to trip this circuit?...

Yup. As others have mentioned, there may be a slight difference in resistance from leg to leg, but that will hardly matter. That could hold 30 A for a looong time.

 

[e57]

Since each breaker will pull over 15 each for a long period of time - it may collectively hold well over 30 - like closer to 40-50 for for a while before tripping one then the other. It won't trip right at 15 ea, unless it hit 15 each real fast. If it snuck up to 15 each over time it will hold it longer. = collectively with two 15's it would not be very fast - one load would come on - then the next - each circuit would only be roughly 50% of the total.

Since long steady high loads at ~95+% seem to damage CB components over time - the day it actually trips on one side - it might not trip the other... JMSO

 

[K8MHZ]

Are they GE breakers?

:roll:

If so, it shouldn't matter how you wire them, they are tough enough to handle any amount of current without tripping.

 

[K8MHZ]

Since long steady high loads at ~95+% seem to damage CB components over time - the day it actually trips on one side - it might not trip the other... JMSO

Do you have any documentation of this?

I don't agree. My experience has been that damage to CB's is the result of repeated tripping and re-setting, not constant use within their design parameters.



In the perfect world a 15 amp breaker would only trip with 16 amps of current after three continuous hours.

They are designed (unless they are instantaneous trip) to carry a whole bunch of amps for a short period of time.

 

[K8MHZ]

It won't trip right at 15 ea, unless it hit 15 each real fast.

I don't agree.

It won't EVER trip right at 15 if it is operating within it's design parameters.

It won't trip for three hours at anything over 15 if it is operating at it's design parameters.

I have seen 15 amp CH breakers carry over 115 amps for a couple seconds and not trip. I shut the circuit off rather than observe any ensuing mayhem.

 

[e57]

Do you have any documentation of this?

I don't agree. My experience has been that damage to CB's is the result of repeated tripping and re-setting, not constant use within their design parameters.

I don't agree.

It won't EVER trip right at 15 if it is operating within it's design parameters.

It won't trip for three hours at anything over 15 if it is operating at it's design parameters.

Documentation no - experience yes...

Sure a CB will take in rush for a while, it will even take well over its rating for a while - but IMO over the years - the breakers I find that won't trip are the ones cooking at their rating for 24/7 for a few years - it's not much different than tripping and resetting IMO. The bi-metal deforms slightly and you may or may not get it to trip right afterwards. Either before its rating or way above its rating - bear in mind we're talking about 2 parallel breakers here....

And yes - put a very high load at 100% of the rating on - with 100% load right away, and you might be re-setting before you take your hand away... It heats up with the inrush current - then doesn't have enough time to dissipate that heat - then continues to heat with the actual load - trips. Be it rare and a mistake IMO to install a circuit sized for 100% rated load - but I have be fools errand once or twice. i.e. 20A load - 20A breaker. Or troubleshooting a failed (open) breaker only to find out that's way it failed - put a new one in and it won't hold...

I have seen 15 amp CH breakers carry over 115 amps for a couple seconds and not trip. I shut the circuit off rather than observe any ensuing mayhem.

Wise move - it don't take long for a situation like that to increase to a few thousand amps after the insulation melts off.

 

[LarryFine]

Simply, each breaker will trip when it sees the same current for the same duration that would trip it if each was the only one. The division of current is what matters, and will depend on relative impedance of each pathway.

It seems that each part of a circular path will still be protected by one breaker, and only a portion that both paths feed could be overloaded; but then, it would still have to be by a load that is too large for a 15a circuit.

 

[growler]

Let's say that you have two 15 amp branch circuits, both installed on the same leg, and someone inadvertantly ties them together downstream.

Something to think about. Could this even happen with the new Arc Fault Combo breakers or would they sense an imbalance between hot an return and trip out letting you know there is a problem?

 

[realolman]

Simply, each breaker will trip when it sees the same current for the same duration that would trip it if each was the only one. The division of current is what matters, and will depend on relative impedance of each pathway.

It seems that each part of a circular path will still be protected by one breaker, and only a portion that both paths feed could be overloaded; but then, it would still have to be by a load that is too large for a 15a circuit.

It seems to me that the circuit will allow twice as much to any part of the line side of the two circuits as it would not being tied together....

I think , disregarding breaker time / trip characteristics, two 15 a breakers would allow 30 amps to any point of the line side of either branch circuit.

 

[iwire]

I'm an ace mechanic but I often have trouble with the technical end of electricity.

I've seen this happen a few times as I'm sure we all have.

Let's say that you have two 15 amp branch circuits, both installed on the same leg, and someone inadvertantly ties them together downstream.

Will it take 30 amps to trip this circuit?

Yes, or close to it without a doubt.

 

[growler]

It seems that each part of a circular path will still be protected by one breaker, and only a portion that both paths feed could be overloaded; but then, it would still have to be by a load that is too large for a 15a circuit.

I think , disregarding breaker time / trip characteristics, two 15 a breakers would allow 30 amps to any point of the line side of either branch circuit.

realolman has a point. You really would have a parallel feed at any point of the circuit. AC current isn't really directional. We like to think of it as hot and return because that makes things simple but that's not really correct. It's not going to matter very much where they are tied togather.


I'm really glad this subject got posted because I have never given it much thought before now ( stuff happens ). The more I think about it the more I realize what a dangerous situation this really can create. Not just to the building wire but to every cord and appliance thats plugged in with no protection that's even close to it's rating. I wouldn't even think about pluging a lamp cord into a 30A circuit but that's what's happening.