Maximum Cable Length

[kevin458]

The IEC breakers have a limit on the length of cable that they can protect. This depends on the impedance of the cable plus the impedance of the ground path, called ground fault loop impedance (Zs). So in IEC the maximum cable length is determined by the values of Zs and Voltage drop. Do in NEC also the maximum cable length is determined by Voltage drop and Zs, or is it only determined by Voltage drop? Thanks

 

[Smart $]

The NEC does not impose a maximum on cable length in this context.

 

[Dennis Alwon]

The IEC breakers have a limit on the length of cable that they can protect. This depends on the impedance of the cable plus the impedance of the ground path, called ground fault loop impedance (Zs). So in IEC the maximum cable length is determined by the values of Zs and Voltage drop. Do in NEC also the maximum cable length is determined by Voltage drop and Zs, or is it only determined by Voltage drop? Thanks

I agree with Smart. In fact, the nec does not even require us to worry about voltage drop except on fire pumps. Not certain if there is something else but certainly only one or two things are a concern for the NEC with VD.

 

[GoldDigger]

Some GFCI manufacturers specify a maximum length of wire that can be protected, but that is based on tripping from capacitive leakage, not a failure to trip.
Under the NEC the fault current path will be a wire EGC, so the normal circuit wire sizing, plus the upsizing of the EGC when the circuit conductors are upsized for VD provides the same protection as the IEC calculation
Actually, since NEC does not directly regulate VD, you could have a situation with 90% VD under normal load which would not trip a breaker on a bolted fault, but as a practical matter nobody would install such a circuit!

 

[jim dungar]

The IEC breakers have a limit on the length of cable that they can protect.

Which IEC breakers have this limitation? Or are you describing application considerations for the Residual Current Devices?
I have sold and applied IEC breakers for several decades now, and have never seen this issue.

 

[GoldDigger]

Which IEC breakers have this limitation? Or are you describing application considerations for the Residual Current Devices?
I have sold and applied IEC breakers for several decades now, and have never seen this issue.

Possibly it is in the electrical code for some IEC countries rather than in the breaker specs. The idea of a test calculation sounds a lot like their EC philosophy.

 

[kwired]

Some GFCI manufacturers specify a maximum length of wire that can be protected, but that is based on tripping from capacitive leakage, not a failure to trip.
Under the NEC the fault current path will be a wire EGC, so the normal circuit wire sizing, plus the upsizing of the EGC when the circuit conductors are upsized for VD provides the same protection as the IEC calculation
Actually, since NEC does not directly regulate VD, you could have a situation with 90% VD under normal load which would not trip a breaker on a bolted fault, but as a practical matter nobody would install such a circuit!

Don't underestimate your fellow humans or they will surprise you every time

 

[jim dungar]

Possibly it is in the electrical code for some IEC countries rather than in the breaker specs. The idea of a test calculation sounds a lot like their EC philosophy.

In which case it is a code issue, not an equipment, breaker, one.

 

[AlabamaElectronMover]

The IEC breakers have a limit on the length of cable that they can protect. This depends on the impedance of the cable plus the impedance of the ground path, called ground fault loop impedance (Zs). So in IEC the maximum cable length is determined by the values of Zs and Voltage drop. Do in NEC also the maximum cable length is determined by Voltage drop and Zs, or is it only determined by Voltage drop? Thanks

The NEC does not REQUIRE a minimum voltage drop, as has been stated by others here, although it does recommend <3% for feeders and <5% for feeders + branch circuits. See NEC 2014: 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2

I believe the misunderstanding in your thought is this:

There is no actual maximum cable length restriction because the voltage drop calculation is determined by the impedance of the cable, as you have stated. That being said, there is no code that restricts upsizing a cable for a service. The larger the cable, the more copper cir mils, and thus a lower impedance, which produces a lower voltage drop. Basically, you can upsize the cable until the voltage drop is below the recommended %. The only limiting factor would be the connection points at the breaker terminals and the load terminals. If the distance from the source to the load is too far for <3%VD based on the terminals at source and load, then the source breaker spec and load spec could (not 'SHALL' because not required by code, but recommended for optimal performance and reliability) be revised to allow for a larger cable installation.

 

[petersonra]

It is not an absolute thing. The std has some cable lengths you can use without having to do the calculations. If your cable length exceeds what is in the chart you have to run the numbers.