Equipment Grounding Conductor

[rlbowen]

If I have a low impedance grounding system and am running a parallel feed to an MCC underground. Can I size my equipment grounding conductor according to my limited current due to the low impedance grounding or must I stick to Table 250-122? The NEC does not reflect this and I am a little confused.

 

[rlbowen]

This should read as follows:


I have a quick question I thought you might be able to help with. If I have a low impedance grounding system and am running a parallel feed to an MCC underground. Can I size my equipment grounding conductor according to my limited current due to the low impedance grounding or must I stick to Table 250-122? The NEC does not reflect this and I am a little confused.

 

[dpc]

I don't think you get to take any credit for the resistance ground when sizing equipment grounding conductors. Resistor could fail, could be changed, removed, yada, yada, yada.

I've always sized by 250-122, even with high-resistance grounding.

 

[resqcapt19]

And if you are using parallel raceways or cables, 250.122(F) requires full sized EGC in each raceway or cable. That is the EGC in each raceway or cable must be sized per Table 250.122 based on the size of the OCPD for the parallel circuit.
Don

 

[ESOXmaniac]

Hi - I see two issues.

1. Is use of the term "low impedance", I assume you actually mean high impedance, i.e., where the fault current is limited to a maximum value via either an resistor or inductor inserted in the place of the bonding jumper..

2. However, it really doesn't matter, I would recommend that you use the values specified by table 250.122 as minimum EGC ratings. Especially when you consider what NFPA 70-2005 Article 110.10 says:

"110.10 Circuit Impedance and Other Characteristics
The overcurrent protective devices, the total impedance, the component 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 components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section."

F. ex. most engineers only consider a single fault to earth/ground. But what if we have a double fault with two different phases faulted to ground in separate loads on the load side of the high impedance system? Then the ECG's between the two faulted equipment's are going to carry full rated fault current, i.e., the fault current is not limited by the grounding impedance, and the double ground fault actually looks the same as a phase to phase fault. Not pretty, but it could happen.

IMHO- "Other characteristics of the circuit" does include the impedance of the ECG's. If you've undersized the ECG's, you are going to likely have a damaged wiring system and a very good chance of not tripping the overcurrent protective device(s).

As a side note: The values given in table 250.122 were originally developed from testing of 100' circuit's. That's why it say's minimum size.

"Table 250.122 Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment"

It certainly doesn't say correct size!If your circuit is longer than 100' you may want to seriously consider larger ECG's.

If you want a failsafe design, make the ECG the same size as the phase conductor(s), i.e., many consulting engineers specify a same size ECG because it can safely carry the same fault current as what the phase conductor delivers to the fault location.

Al

 

[davidbeach]

It could be low impedance grounded or it could be high impedance grounded. In general high impedance grounding ends at about 10A of ground current and low impedance grounding generally provides ground fault currents in the 100A to 400A range. I've also seen reference to grounding such as to limit ground faults to not exceed the 3-phase fault current.