Loop Impedance in US (NEC) based designs

[cherryg]

Has there been any publication for loop impedance calculations under NEC environment, identical to IEC or BS 7671? The table 250.122 in NEC provides the minimum EGC size for particular breaker. Is this size linked with loop impedance? Over and above voltage drop, do we need to establish loop impedance criteria and determine limiting cable lengths?

 

[dpc]

The ground wire sizes in the NEC are intended to provide adequate thermal capacity to allow fault current to flow long enough to trip the PHASE overcurrent protection upstream for a line-to-ground fault. NEC requires that if you increase the phase conductor size due to voltage drop or other considerations, then the ground wire (EGC) must also be increased in size.

 

[bacon4life]

There have been a few past threads about loop impedance.

My understanding from

https://www.eng-tips.com/viewthread.cfm?qid=481354

was that the NEC has no direct requirement for such calculations. Instead of explicit loop impedance calculations, the NEC has generic rules about minimum EGC sizing under various cases. While the generic rules usually provide adequate capacity, there are cases of NEC compliant installations that have overly high loop impedance. This thread had several misunderstandings between posters, so hopefully folks will be gentle if I incorrectly summarized the thread.

 

[davidbeach]

"Loop impedance" is an IEC thing that has no place in the NEC realm. IEC stuff and IEC requirements work well in an IEC environment while ANSI (including NEC) stuff and ANSI requirements work well in an ANSI environment. But never the twain shall meet. You have to pick one and take the whole package; you can not build a complete package by taking bits and pieces from one and different pieces from the other and trying to make a hybrid. Won't work.

The "problem" is that there are a variety of underlying, but often not explicitly stated, assumptions upon which the practices are built. For instance, for single family residential installations, in the NEC world the service transformer will have a 240V single phase low-side with a solidly grounded mid-point tap. This produces services with two phase conductors, 240V between, and 120V between each and the neutral from the tap. A large service transformer will serve as many as 10 houses, each on its own radial connection.

On the other hand, as I understand it, in the IEC world the service transformers are considerably larger, are not solidly grounded and supply 3-phase services to many 10s of houses, on loops rather than radially, and cover considerably larger areas.

Other than the fundamental physics involved nothing is the same, nothing is compatible. They each work, but no portions of either are compatible with the other.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Allings Clark]

Hi David, this is simply untrue and a very dangerous assumption:

"Loop impedance" is an IEC thing that has no place in the NEC realm

Please see 110.10, 240.4 and 250.4 (A) (5):

(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material
likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

Table 250.122 makes a note of this.


Circuit impedance is required to be taken into consideration in order to allow an over current protective device to open before the thermal withstand of conductors carrying fault current is exceeded.

 

[davidbeach]

Then your understanding of “loop impedance” is rather different from what I’ve gathered over the years here. The NEC certainly implies requirements around path impedance. But loop impedance is a test for a circuit that you just won’t see in NEC-land.

Like I said, pick a system and stick with it. NEC rules, typical North American practices and designs work well together as do typical IEC rules and installations.

But you can’t mix and match. Pick one, stick with it. Everybody who works with either considers the other weird.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Allings Clark]

"Loop impedance" and "path impedance" are one in the same.


You're thinking of "earth fault loop impedance testing" which differs from calculating the series impedance of a circuit during design.

 

[davidbeach]

Ok. Have at it.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[sparkyusa]

Interesting thread peeps, may i ask a simple Q?

How long, how many seconds is good or bad to open any given OCPD?

~S~

 

[davidbeach]

That last question is really unanswerable. Or, perhaps, the answer is "it depends." Guessing from the use of OCPD that you're in the low voltage world, so 5 cycles being very slow clearing for 500kV faults may not be relevant.

When you're dealing with low-voltage (less than 1000V) you're dealing with both fault current clearing and overload tripping. The nature of the equipment makes for fast clearing of hard faults and very slow clearing of slight overloads. High resistance faults fall somewhere in between. Maintaining selectivity, particularly at the top end of you system can result in slow clearing without tools such as zone selective interlocking.

There aren't necessarily any right answers, but more money can always result in faster clearing until you reach the point that all faults are selectively clearly instantaneously.

Slower clearing means higher arc flash hazard levels, what can you live with?

I’ll see your silver lining and raise you two black clouds. - Protection Operations