Grounding requirements

[mporterf]

I am working in an industrial facility where a new service panel has been set along with sub-panels and HVAC equipment. I have been asked to verify that the new service panel and all equipment has been properly grounded. They also want to know the acceptable ohm reading allowed by code for test sheets they will be creating. I think for lightning protection it is 25ohms but does the NEC have an acceptable limit?

 

[Dennis Alwon]

The NEC does not require a minimum cohm reading. Art. 250 states that if you drive one rod and get 25 ohms then you don't need another rod. If the reading is higher then 25ohms then another rod is needed however that does not mean 25 ohms is required. You can still have 2 rods and only have 80 ohms and you would be compliant.

Engineers often require a certain level not the NEC

 

[don_resqcapt19]

The equipment grounding needs to have a low enough impedance to get into the instantaneous trip range of the OCPD. The NEC does not have a specific standard, but if you are testing the EGCs, I would expect to see the resistance just a bit higher than the resistance of the EGC itself. For example the DC resistance of 1,000' of #12 copper is less than 2 ohms. The EGC would not normally be anywhere near 1,000 feet long. If it was 100 feet I would be looking for a DC resistance reading between the equipment and the Main Bonding Jumper of less than 0.25 ohms.