Timeline for Which breaker should trip with a ground fault with a subpanel?
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| Jun 16, 2020 at 10:07 | history | edited | CommunityBot |
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| Feb 2, 2016 at 12:25 | vote | accept | ETrussell | ||
| Feb 1, 2016 at 19:26 | comment | added | ArchonOSX | You are right. I see that now upon re-reading that section. I was steered wrong by an inspector I had a couple years ago that said it applied to feeders. Which it does, but only if they supply the entire premisis load as you quoted. So, that does not apply to a remote building fed from a main panel. Thanks for the correction. | |
| Feb 1, 2016 at 17:23 | comment | added | Tester101 | @ArchonOSX I don't think you can feed the second panel with #4. The old table 310.15(B)(7) only applied to service conductors, so wouldn't apply to branch feeders. The new wording allows 83% of rating, but still only applies to service conductors and feeders that supply "the entire load associated with a one-family dwelling". | |
| Feb 1, 2016 at 15:30 | comment | added | ArchonOSX | That looks good. I would agree that is a fair example. You are actually being generous with the wire since this is a dwelling unit the 200 amp feed would require 2/0 and the 100 amp would be #4 so that would have a little higher resistance than your example. The Square D Thermal-Magnetic curve I am looking at shows the instantaneous trip at 9 times the trip rating of the breaker so if you exceed that for any two breakers in series than either of them could trip. So, if the garage was fed with a 40 amp and #8 then a ground fault of 360 amps would be likely to cause a coodination problem. | |
| Feb 1, 2016 at 14:33 | comment | added | TFK | @Tester101 In a tree diagram of the electrical system, where each part down the line has a smaller breaker (main as trunk, sub as limb, circuit as branch), wouldn't the nearest breaker (before the fault) always trip first as it'd have the smallest breaker - so the fastest trip? (We'll assume here than the inst. trip settings are all coordinated) | |
| Feb 1, 2016 at 13:35 | history | edited | Tester101 | CC BY-SA 3.0 |
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| Jan 31, 2016 at 21:28 | history | edited | Tester101 | CC BY-SA 3.0 |
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| Jan 31, 2016 at 20:55 | comment | added | TFK | @EmmettTrussell No, it is just like a short. A ground wire is basically a spare neutral in a sense that if anything connects to it, it'll also run the current back to the source. A short would be between say the hot and the neutral. A ground fault is just another term for a short, but between the hot and some conductive surface. A ground should have almost the same path and resistance as the neutral and so they would flip basically at the same time. | |
| Jan 31, 2016 at 19:48 | comment | added | ETrussell | Thank you, that makes sense. I was under the impression that the breaker handled a ground fault like a short. So a ground fault would not trip the breaker as fast as a short, I assume? | |
| Jan 31, 2016 at 17:37 | history | edited | Tester101 | CC BY-SA 3.0 |
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| Jan 31, 2016 at 17:17 | history | answered | Tester101 | CC BY-SA 3.0 |