When ungrounded system is permitted by NEC

[unsaint34]

NEC article 250.21 & 22 exempt the system-ground requirement for the following applications (very crudely described);
* industrial furnaces
* control circuit of separately derives system
* recifier circuit for VFD of separately derived system
* electric crane
* healthcare facilities
* electrolytic cell facilities
* 30V or less lighting circuit of secondary
* underwater lighting circuit of secondary

But there has to be other approved ungrounded systems that don't fall into these situations. Is there another section of NEC for different situations where system can be ungrounded? Or does that fall into AHJ decisions?

 

[don_resqcapt19]

In general the requirement to have a grounded system is based on voltage. The rules in 250.20 tell you what systems must be grounded systems. For most other systems the choice to ground or not is a design issue. 250.22 tells you the systems that are not permitted to be grounded systems.

 

[raider1]

I agree with Don,

250.20 Gives you the requirements for when a system must be grounded. If the system is not required to be grounded as specified in 250.20 it is permitted to be ungrounded. 250.22 gives you the requirements for systems not permitted to be grounded.

Chris

 

[jaggedben]

Solar systems can be ungrounded if they meet 690.35. Most DC systems are not required to be grounded.

 

[unsaint34]

I see. thank you all.

 

[unsaint34]

I agree with Don,

250.20 Gives you the requirements for when a system must be grounded. If the system is not required to be grounded as specified in 250.20 it is permitted to be ungrounded. 250.22 gives you the requirements for systems not permitted to be grounded.

Chris

I gave it more thought. I can definitely see your explanation. But 250.21 lists systems that are permitted not to be grounded. So, why does NEC need to specify those when other systems not mentioned in 250.21 are also permitted not to be grounded?

 

[raider1]

I gave it more thought. I can definitely see your explanation. But 250.21 lists systems that are permitted not to be grounded. So, why does NEC need to specify those when other systems not mentioned in 250.21 are also permitted not to be grounded?

I agree, 250.21 is really not needed as (A)(4) of that section says "Other systems that are not required to be grounded in accordance with the requirements of 250.20(B)".

This may be a candidate for a code change proposal.

Chris