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USA (California) home with 240V 150A service.

I have a contractor working on the house siding. He accidentally drilled though NM-B/Romex on a 15A circuit. (The repair of this is not my question and I will make sure that any splices will be in accessible junction boxes.) The the 15A breaker for this circuit is in a sub-panel, which is fed from a 240V 60A breaker in the main box. This, the breakers are effectively in series.

The interesting thing is that the 60A breaker tripped but, as far as I could tell, the 15A breaker did not. (I tried resetting the 15A breaker before I discovered the 60A was tripped; the 15A appeared not to be tripped but I know it isn't always obvious.)

My question is should I be concerned that the 15A breaker did not trip before the 60A breaker did? I realize that breakers should trip nearly instantly on a short circuit. Is it normal that the 60A could beat the 15A? (The 150A main did not trip.)

BTW, the sub was installed a few years ago by a licensed electrician, with permit and inspection. I'm certain the 60A breaker was new. The 15A breaker may have been moved from the main panel but if so, I'm pretty sure was installed a few years earlier in a remodel.

  • Think about the direction of current flow... The 60 ampere breaker actually experienced the overcurrent situation longer; even if just by nanoseconds, than the 15 ampere breaker. Actually, I wouldn't have been surprised if the 150 ampere breaker tripped. Since it experienced the overcurrent the longest. However, it all depends on the breaker, as they all have slightly different trip characteristics. – Tester101 Sep 25 '14 at 10:24
  • @Tester101 - I'm not sure what you mean. The breakers are effective in series: 150A feeds stuff plus the 60A which feeds stuff plus the 15A which was shorted. It seems like they all see the short for the same time. Also, I'm assuming that the additional draws on each breaker is fairly insignificant compared to the short. – DoxyLover Sep 25 '14 at 18:44
  • Think of the overcurrent like a tidal wave. It has to pass point a (the 150 amp breaker), to get to point b (the 60 Amp breaker), and point c (the 15 amp breaker). – Tester101 Sep 25 '14 at 18:54
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My question is should I be concerned that the 15A breaker did not trip before the 60A breaker did?

No, not a tall. During bolted fault conditions (short circuit) it is not the smallest breaker that always trips. during a sort circuit amperages can be ten thousand or more for fractions of a second, and sometimes a larger breaker reacts faster than a smaller one.

I've seen more than once a 100A breaker trip due to a 15A circuit short.

Again, great screen name DL. :)

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While non-intuitive, this situation does not necessarily mean anything is amiss.

First of all, a drill bit going through a wire pair doesn't necessarily create a short circuit. It is more likely to be an alternating heavy load with no or low load on each bit rotation.

Breakers are not required to trip on instantaneous high load unless the current is very high (5000+ amps). A 14 gauge wire can withstand a few milliseconds of 100 amp flow before it heats appreciably. Even at 30 amp flow, a 15 amp breaker is probably willing to let several 60 Hz cycles go by before it trips.

I surmise that the 60 amp breaker is likely "smarter" than the 15 amp breaker in that it reacted to large current pulses whereas the 15 amp breaker is less intelligent and reacts to average current.

For this explanation to be validated, compare the breakers: are they the same technology? Do they have intelligence in one but not the other? Is one rated for motor starting and the other not? Do they have additional specifications for maximum instantaneous current?

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When you consider that standard breakers primarily trip on heat, it's not really all that confusing. As current most flow from the 60A breaker to the 15A breaker, the 60A breaker is going to see an increase in heat first, thus, be the first to trip when its tolerance is reached.

If the 60A breaker was a 200A breaker instead, that tolerance would be much higher, thus the 15A breaker would trip instead.

Now, 'smarter' breakers (GFCI (ground fault), AFCI (arc fault)) are designed specifically to trip before heat alone would cause the condition, thus they'll be more likely to trip before a standard breaker. Similarly, GFCI protected receptacles will themselves trip (usually) before the branch circuit breaker.

Or, put more simply, the heat that was caused by shorting the conductors in the non-metallic conductor was felt first in the primary panel, then in the secondary panel, and ultimately in the drill bit that your contractor probably had to replace :)

This is relatively common in sub panels less than 100 - 150 amps; a very common call for a service electrician is "my daughter's hair dryer is taking out the whole garage" - where you arrive to find the bathroom receptacle had been re-fed from the garage sub panel in the process of other renovations.

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This (i.e. your feeder breaker tripping before the subpanel breaker) happens in residential subpanels because they lack the nice feature known as selective coordination. In large commercial and industrial installations, an engineer will sit down and work out a series of calculations based on available fault currents, and tables provided by the circuit breaker maker so that the breaker closest to the fault trips before any feeder or service breakers do.

In more detail, for those who wish to know: A typical modern residential/commercial (molded case) circuit breaker has two trip 'regions' in its time-to-trip vs. current curve. The first region is the instantaneous trip region, where the magnetic trip mechanism of the circuit breaker kicks in right away to trip on a bolted fault (dead short). The second region is the thermal, or inverse-time region, where the breaker trips after a short period of time based on the amount of heat the fault overcurrent can generate in an internal element.

However, the trip process, even in a magnetic trip, is not instantaneous: the breaker contacts produce an arc as they come apart, which has a dynamic impedance (or resistance) that changes as the arc is struck, stretched, and then finally extinguished by the breaker's chutes and blowout devices. This, in turn, decreases the short-circuit current seen by upstream breakers for a short period of time, keeping them from tripping as well in a properly coordinated setup.

However, ensuring proper coordination requires compatible breakers in both panels (i.e. same manufacturer and series), and quite a bit of extra design effort compared to simply putting in what's required by the Code, so it is generally not done outside of setups where continuity of power is vital. If you want all the gory details, this article is a good primer.

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