OCDP Size And Extreme Circuit Length

[wwhitney]

For those locations on the NEC 2023, short circuits are defined by voltage not by amount of current.

Right, my last question wasn't about whether the OP scenario is a short circuit or not, but it was about what NEC section says that a protective device does have to clear a short circuit. Your original point still holds, a non-overcurrent short circuit will not damage the conductors.

Cheers, Wayne

 

[tortuga]

So to be pedantic, what NEC section requires protecting against a non-overcurrent short circuit?

110.7?

 

[wwhitney]

110.7?

That reads as a one-time requirement that needs to be true at the time the wiring installation is completed. Delete the word "completed" and your interpretation works.

Cheers, Wayne

 

[jim dungar]

110.7?

No.
That section is for the installer.

There is no single section that applies. In general section 240 requires conductors to have overcurrent and short circuit protection as well as ground fault and arc fault protection in some circumstances. In some sections like 430, short circuit protection is specifically mentioned.

Section 100, as of 2023, has a definition of short circuit that addresses the voltage between the conductors without mentioning the amount of current.

 

[tortuga]

That reads as a one-time requirement that needs to be true at the time the wiring installation is completed. Delete the word "completed" and your interpretation works.

At what point would the installation no longer be completed after its been completed?

 

[wwhitney]

At what point would the installation no longer be completed after its been completed?

Obviously once it changes, e.g. develops a short circuit. : - )

Cheers, Wayne

 

[jaggedben]

So to be pedantic, what NEC section requires protecting against a non-overcurrent short circuit?

Thanks,
Wayne

What about 110.10? Particularly it's references to "circuit impedance" and "total impedance".

 

[wwhitney]

What about 110.10? Particularly it's references to "circuit impedance" and "total impedance".

Right that was my original question earlier, the language in question in 2017 NEC 110.10 says that the circuit characteristics need to be designed "to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit." So clearly for the typical case that an ongoing fault would result in damage to the electrical equipment, the fault needs to be cleared by the circuit protective device.

But if the fault is not an overcurrent and so will not damage the equipment even if left uncleared, does that language imply that the circuit protective device must still clear it? Or is only required to act if it would otherwise damage the equipment?

Cheers, Wayne

 

[tortuga]

So if its a hazard ? Wait did we establish this is a hazard?
The purpose of the NEC is the practical safeguarding of persons and property from hazards arising from the use of electricity, not a design manual.
The NEC should address it if its a hazard, and I'd say that a installation is complete when it gets a final inspection or certificate of occupancy then 110.7 applies. If you pull a permit to work on something say to modify the installation then 110.7 does not apply.

 

[jim dungar]

What about 110.10? Particularly it's references to "circuit impedance" and "total impedance".

Section 110.10 is about having enough impedance that the amount of fault current does not damaged the conductor or equipment. It says nothing about protection when the amount of fault current is lower than the rating of the SCCR.

In the case of the OP, 110.10 was satisfied as the fault current was only 15A max.

 

[jaggedben]

...

But if the fault is not an overcurrent and so will not damage the equipment even if left uncleared, does that language imply that the circuit protective device must still clear it? Or is only required to act if it would otherwise damage the equipment?

Section 110.10 is about having enough impedance that the amount of fault current does not damaged the conductor or equipment. It says nothing about protection when the amount of fault current is lower than the rating of the SCCR.

In the case of the OP, 110.10 was satisfied as the fault current was only 15A max.

I'm not convinced. 110.10 is pretty broad and to my eyes it's written to be comprehensive, and it contains no language limiting its scope to 'overcurrent' or current above SCCR.

If a piece of equipment is designed in such a manner that a typical fault within the equipment itself is ordinarily expected to trip the max overcurrent device that the equipment may be connected to, but this will not happen because of conductor impedance, then I'd argue that 110.10 requires the conductor impedence to be 'selected and coordinated' so that the 'total impedance' still allows the overcurrent device to 'clear a fault ... without extensive damage to the electrical equipment of the circuit.' A 1A load may be extensively damaged by 15A fault current where that same fault would be expected to trip the overcurrent device at lower conductor impedance.

 

[jim dungar]

I'm not convinced. 110.10 is pretty broad and to my eyes it's written to be comprehensive, and it contains no language limiting its scope to 'overcurrent' or current above SCCR.

In the OP, the equipment, which is just the #12AWG conductors, never sees a current in excess of 15A. This is below the rating of the conductor so 110.10 is automatically met and no further protection is required.

Branch circuit OCPD are not intended to protected end use equipment, which is the function of supplemental protective devices.