We could continue to speculate until the cows come home (or choose your own adage).
120% rule clarification
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ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
How so? The fault is not actually in the conductor but orthogonal to it. At no point within the conductor itself in this scenario is the current more than 350A, and the conductor is rated to be less than 90 degrees C at 400A. Why do we care if the temperature of the conductor is lower at points removed from the fault as long as it stays below the maximum rated temperature for its insulation?
The reason that we rate conductors to stay below 90 degrees is not that 90+ degrees in and of itself will cause a fire, it's because 90+ degrees may compromise the insulation, which can lead to a fault, which can cause a fire. You have stipulated that the fault has already occurred (for whatever reason); once that happens, I do not see how oversized wiring will reduce the temperature of the fault. For a phase to phase fault at 480V to be sustained at 700A, the resistance of the fault would be about 0.68 ohms and would dissipate over 300,000 Watts. I would expect the lifetime of such a fault to be measured in microseconds before becoming a dead short and shutting the circuit down from both ends. I don't see where upsizing the conductors would make any difference at all.
The only way the conductor does not carry 700A (or less) of fault current at the point of the fault (if it is a single point fault rather than a distributed fault) is if the conductor is cut by an interposing conductive material. If the conductor is not cut, then the entire 700A (or less) exits the side of the conductor at a single point. Call the conductor to either side of the fault a separate conductor. Now "T" cadweld those two 400A conductors to a third 400A conductor. All 700A of fault current is carried by the third 400A conductor. What would be the temperature of that third conductor?
jaggedben
Senior Member
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- Northern California
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- Solar and Energy Storage Installer
As I see it, the 'conductor' here would really be the resistive fault path that you are postulating. The temperature of our 400A wire conductor at the point of the fault would depend in part on the heat transfer between the wire conductor and the resistive fault path and other surrounding materials. Hard to say what that would be without knowing the specific heat of those materials. (Hard for me to say regardless, not knowing really anything about heat transfer.)
You could probably only determine the answer with a lab test anyway. Using NEC tables is just not appropriate, given safety factors and probably a few other things.
ggunn is really right, I think. Once the insulation is compromised, all bets are off anyway. The NEC ampacity ratings are designed to prevent this fault, not prevent it from doing further damage.
Here's another point: the 120% rule only applies to a load side connection. This same 400A conductor would be allowed by the code if it were, say, a service entrance conductor that was tapped somewhere for the solar. In that case your resistive fault, which could also be fed by both the utility and the solar, could be as big as allowed by the sum of the circuit breakers on the solar and the utility transformer. Which would be way more than 120% of its ampacity. And not only that, but the full fault current could flow through some length of the conductor, not just at a single exit point.
In other words, your resistive fault condition would be just as bad (worse actually) on an allowed line-side tap than on this disallowed example of ggunn's. So whatever the reason that the CMP picked 120% (and I wonder sometimes if they pulled it out of a hat), it wasn't due to any consistent logic concerning a fault of this type.
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ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
The temperature of the third conductor would be not be dependent on the size/rating of the conductor(s) from which it sprang. In the primary conductor the only direction you could have a resultant current of 700A is out from the conductor at the point of your cadweld because the currents from the two supplying OCPD's are in opposite directions. There is no cross section of that conductor which will have 700A passing through it. Kirchoff's Law.
I have not disputed that the current through the fault would get very hot, I have only submitted that upsizing the conductor won't make it get any less so.
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Now we are getting much closer to my true beliefs on the matter :happyyes: so I'm not going to continue fueling the discussion as devil's advocate.
A larger conductor would be a lower temperature because of lower impedance. It would also act as a better heat sink.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Let me get this straight. If this were a supply side tap, I would not be allowed to put OCP on the disco for the conductors between the tap and the inverter breakers? That sounds like what Smart $ was saying earlier in this thread, and it makes no sense to me. Why would the code dictate that conductors be unprotected? I must be misunderstanding something, but if not, please cite the section of the code that addresses this. It's not that I don't believe you guys, but this is something I need to understand.
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
No, I didn't say that. I think you might be overthinking it.
It's just that the code allows a supply side connection through a separate set of service entrance conductors (705.12(A) and 230.40 Ex. 5). And this means that the regular service entrance conductors (those that supply the loads) could conduct an overcurrent from both the solar and the utility if a fault occurred on the line side of the regular service OCP.
My point was basically that if resistive faults of the magnitude that Smart$ is positing were a common problem, that would point to general problem with the way distribution networks are built. We'd have resistive faults starting fires on the line side of service OCPDs all the time (with or without solar). I take the general lack of such problems as evidence that high current resistive faults really aren't that much of a concern.
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jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Now we are getting much closer to my true beliefs on the matter :happyyes: so I'm not going to continue fueling the discussion as devil's advocate.
Okay. That's pretty cryptic. I guess that's your way of saying one of two things:
1) You've just been string us along for nothin'. In which case I please gullible. :ashamed1:
2) You were wrong about the danger of a resistive fault.
(See, I can use emoticons too. :lol
Could be either or both... I'll neither confirm nor deny...
1) You've just been string us along for nothin'. In which case I please gullible. :ashamed1:
2) You were wrong about the danger of a resistive fault.
...
In actuality, the major problem is I cannot submit any bona-fide empirical evidence. As you stated, the only way to obtain that information [safely] is "in the lab" with controlled testing. I don't have access to anything close, nor have I been able to find any published and substantially noteworthy documentation.
As to the danger of a resistive fault and frequency of such, I can only recall being involved in one instance in my electrical career... but it didn't involve a fault supplied by two power production sources connected to a single conductor. It involved a typical under-slab 20A circuit that used metal conduit as the EGC. An ungrounded conductor got pinched between a 4sq box and surface cover. The under-slab conduit had degraded to the point the concrete slab and perhaps earth became the fault current return path(s). The resistance of the fault return path(s) was high enough for the breaker to not trip. The story I was told was that the VCT on the slab was glowing red hot (which I assume to be an exaggeration). The fire department was called and they proceeded to find and shut down the effecting circuit. I was called in to fix it.
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