LRG Cable Sizing

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Electriman

Senior Member
Location
TX
I have a 400 A LRG (Low Resistance Ground) at the transformer, what is the appropriate size of cable to run between transformer secondary neutral and LRG and why?
 

Smart $

Esteemed Member
Location
Ohio
Normal grounding is designed and intended to be low resistance grounding. There is nothing in Code which suggests Low Resistance Grounding would mean anything else.

Do you perhaps mean High Resistance Ground? If so, see 250.36, specifically (B) and (G).
 

Electriman

Senior Member
Location
TX
Low resistance ground is usally recommended for MV(2.4 and 4.16 kv) power distribution. The resistance usually limit the current to 400A.
 

dkidd

Senior Member
Location
here
Occupation
PE
I have a 400 A LRG (Low Resistance Ground) at the transformer, what is the appropriate size of cable to run between transformer secondary neutral and LRG and why?

You still have to follow T250.66. The cable must be fully insulated.
 

dkidd

Senior Member
Location
here
Occupation
PE
Normal grounding is designed and intended to be low resistance grounding. There is nothing in Code which suggests Low Resistance Grounding would mean anything else.

Do you perhaps mean High Resistance Ground? If so, see 250.36, specifically (B) and (G).

High resistance grounding generally limits the fault current to 10 amps. Low resistance grounding usually limits the fault current to 100-600 amps. 400 amps is typical.
 

Smart $

Esteemed Member
Location
Ohio
High resistance grounding generally limits the fault current to 10 amps. Low resistance grounding usually limits the fault current to 100-600 amps. 400 amps is typical.
Fine... but the NEC does not call it low resistance grounding. Instead, it is simply termed Impedance Grounded Neutral Systems... and it is still high impedance grounding compared to directly connected grounding. See 250.187.. but it does not specifically call out for any particular size. I would use the LV requirements as a guideline.... essentially the neutral to impedance sized by impedance current rating.
 
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