OCDP Size And Extreme Circuit Length

[wwhitney]

Say I have a 2000' long 2-wire #12 Cu 120V branch circuit for a very small load, 1A. #12 Cu is 2 ohms/kft per Chapter 9 Table 9, so the round-trip resistance is 8 ohms, meaning the voltage drop is only 8V, which may be acceptable. Does the NEC permit protecting this circuit at 20A?

At 8 ohms and 120V, the AFC at the far end is at most 15A. So a fault there will never trip a 20A OCPD, you'll just indefinitely have a 2000' long 1800W heater. Is there an NEC section that requires the circuit to be designed to avoid this outcome?

Thanks,
Wayne

 

[Elect117]

It definitely permits it.

210.20 points you to 240.4 which has 240.4(D)(5).

There is the informational note at end of the first part of 240.4, but I can't remember if those are enforceable. Plus, I doubt a inspector would even be checking that.

 

[jim dungar]

At 8 ohms and 120V, the AFC at the far end is at most 15A. So a fault there will never trip a 20A OCPD, you'll just indefinitely have a 2000' long 1800W heater. Is there an NEC section that requires the circuit to be designed to avoid this outcome?

No.
The NEC is only concerned that the conductor OCPD does not exceed its ampacity.

 

[wwhitney]

There is the informational note at end of the first part of 240.4, but I can't remember if those are enforceable. Plus, I doubt a inspector would even be checking that.

Do you mean after 240.1? That points you towards 110.9 and 110.10.

2023 NEC 110.10 first sentence says "The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit."

That doesn't quite come out and say "The overcurrent protective device shall clear a fault," but I wonder if it is reasonable to interpret it that way? I.e. this slight variation of the sentence seems equivalent and would clearly require fault clearing:

"The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit."

Cheers, Wayne

 

[Elect117]

Maybe they changed it in the 2023?

I was talking about this one
"Informational Note: See ICEA P-32-382-2007 (R2013), Short Circuit Characteristics of Insulated Cables, for information on allowable short-circuit currents for insulated copper and aluminum conductors."

Not that it sounds like enforceable language.

 

[wwhitney]

Maybe they changed it in the 2023?

No, it's still there, I just didn't understand that "first part" meant first paragraph.

Cheers, Wayne

 

[winnie]

A fault at the far end of this 2000 foot circuit won't clear, but also won't damage the circuit; the current is limited to less than the ampacity of the conductor.

Is there an intermediate distance where the time current curve of the breaker intersects with the damage curve of the conductor?

-Jonathan

 

[wwhitney]

A fault at the far end of this 2000 foot circuit won't clear, but also won't damage the circuit; the current is limited to less than the ampacity of the conductor.

Right, so 240.4 is satisified, the conductor is protected against overcurrent in accordance with its ampacity.

The question now is whether 110.10 has the meaning that the circuit needs to be designed so that the breaker will clear the fault. Or whether not clearing the fault still satisfies 110.10.

Cheers, Wayne

 

[jim dungar]

A 15A load on a 20A conductor is not considered an overnight current nor a short circuit by any portion of the NEC.

 

[jim dungar]

Is there an intermediate distance where the time current curve of the breaker intersects with the damage curve of the conductor?

A properly selected UL OCPD will always protect a conductor sized by the NEC, regardless of the conductor damage curve.

 

[wwhitney]

A 15A load on a 20A conductor is not considered an overload current nor a short circuit by any portion of the NEC.

Short circuit is not defined in the NEC, but I would think that if you directly connect an ungrounded circuit conductor to a grounded circuit conductor, that should be considered a short circuit, even if it only results in a 15A current on a 20A conductor.

Ground fault is defined in the NEC, and if the 15A fault current is due to an ungrounded circuit conductor faulting to the EGC, that is definitely a ground fault, even when the conductors have 20A ampacity.

What do you think about 110.10?

Cheers, Wayne

 

[jim dungar]

Short circuit is not defined in the NEC, but I would think that if you directly connect an ungrounded circuit conductor to a grounded circuit conductor, that should be considered a short circuit, even if it only results in a 15A current on a 20A conductor.

Ground fault is defined in the NEC, and if the 15A fault current is due to an ungrounded circuit conductor faulting to the EGC, that is definitely a ground fault, even when the conductors have 20A ampacity.

What do you think about 110.10?

Cheers, Wayne

If the connection results in less than the rated current it is not a short circuit. Sketch out a circuit diagram showing the conductor impedance. This is one reason the NEC has tried to protect against arcing faults, and why some people want to see glowing connection protection.

Ground faults are not necessarily short circuits, which is why some NEC sections require additional protection functions beyond a plain OCPD.

110.10 only deals with protecting conductors against currents in excess of 'rated'.

 

[wwhitney]

If the connection results in less than the rated current it is not a short circuit.

I guess we have different definitions of "short circuit." To me, a short circuit is a direct connection between two circuit conductors at different nominal voltages, whether or not it results in overcurrent.

110.10 only deals with protecting conductors against currents in excess of 'rated'.

It certainly says that the OCPD needs to be able to operate before the conductors get damaged. The question is whether it also means that the OCPD must operate when there is a fault. I guess it almost but not quite says that, so I'm unclear on whether it should be read as implying that.

Cheers, Wayne

 

[jim dungar]

The question is whether it also means that the OCPD must operate when there is a fault.

If there is no overcurrent how can you say there is a fault?

I once built a breaker test device that used several hundred feet of conductor, wound around an air coil. The intent was to create enough impedance to force a 15A breaker out of its short circuit region.To make sure I did not inadvertently trip the branch breaker I made sure my test set was plugged into a 100' #14AWG extension cord. The resistance of the conductors ensured that the impedance of the circuit kept NEC 110.10 from being violated while allowing the long and transitional time performance of the test breakers to be compared.

 

[wwhitney]

If there is no overcurrent how can you say there is a fault?

It's a question of intended operation. If the 2000' of #12 copper wire was installed as a big heater, e.g. ice melting under a driveway, then sure it's not a fault. While if it's not the intention to short the two circuit conductors together at the end of the circuit, but instead to power some utilization equipment, then I would call such a short a fault.

Cheers, Wayne

 

[jaggedben]

Say I have a 2000' long 2-wire #12 Cu 120V branch circuit for a very small load, 1A. #12 Cu is 2 ohms/kft per Chapter 9 Table 9, so the round-trip resistance is 8 ohms, meaning the voltage drop is only 8V, which may be acceptable. Does the NEC permit protecting this circuit at 20A?

At 8 ohms and 120V, the AFC at the far end is at most 15A. So a fault there will never trip a 20A OCPD, you'll just indefinitely have a 2000' long 1800W heater. Is there an NEC section that requires the circuit to be designed to avoid this outcome?

My first thought upon reading this was "That's what they think 250.122(B) addresses, but it doesn't." Upon further thought maybe that's not such an accurate take, but I'm still interested in how your thinking could lead us away from the stupidity of that section.

Something like a new rule in 240 that says something along the lines of: The OCPD rating shall not exceed [the Ohm's law solution for amps with inputs of NEC voltage to ground and the Chapter 9 table value*conductor length for resistance]. And then 250.122(B) would refer to that rule for when upsizing the EGC is required.

Or maybe 250.122(B) just does the reference to Ohm's law directly, without referring to a 240 rule. But you get the general idea.

If you can figure out a way to write out the part in brackets so that regular electricians can understand.

 

[jim dungar]

. While if it's not the intention to short the two circuit conductors together at the end of the circuit, but instead to power some utilization equipment, then I would call such a short a fault.

You can call it a fault.
Just not one in which the NEC requires a Listed OCPD to operate as there is no danger in starting a fire from conductors that are operating beyond their NEC rating.
As I mentioned earlier, this is similar to a high resistance fault causing 'glowing' connections.
No overcurrent means no protection by standard OCPDs.

 

[tortuga]

Say I have a 2000' long 2-wire #12 Cu 120V branch circuit for a very small load, 1A. #12 Cu is 2 ohms/kft per Chapter 9 Table 9, so the round-trip resistance is 8 ohms, meaning the voltage drop is only 8V, which may be acceptable. Does the NEC permit protecting this circuit at 20A?

At 8 ohms and 120V, the AFC at the far end is at most 15A. So a fault there will never trip a 20A OCPD, you'll just indefinitely have a 2000' long 1800W heater. Is there an NEC section that requires the circuit to be designed to avoid this outcome?

We have an old thread on this topic, I think it was on a electrocution due to a voltage gradient happened from what you describe.

 

[ptonsparky]

We have an old thread on this topic, I think it was on a electrocution due to a voltage gradient happened from what you describe.

We had a member here that pressed this point regularly. At the time, I did some VD and fault current calculations for the conductors I was using on two corner pivots with one well. It wasn't likely to blow a fuse for sometime. More likely to have the control circuit shut things down after misalignment.

 

[Dsg319]

We had a member here that pressed this point regularly. At the time, I did some VD and fault current calculations for the conductors I was using on two corner pivots with one well. It wasn't likely to blow a fuse for sometime. More likely to have the control circuit shut things down after misalignment.

Mbrooke? I havnt heard from him in some time.