I am seeking the knowledge of when did the NEC require sub panels to have separate neutrals buss and ground buss (and the panel bonded to the ground only). I would think this has been a requirement since 1993 but can not confirm. Please advise JoeMed@cableone.net
It has been required to segregate the neutral from the ground downstream from the service over-current device since the two both existed, or since Ben Franklin was flying kites, whichever came first.
I'll forgive you, though, as the NEC has refined its' language regarding grounding quite a bit recently. It was not unusual for three experts come up with three mutually exclusive readings of the code. The code itself has been somewhat contradictory and confusing- put it down to poor literacy on the part of engineers trying to be lawyers!
Nor will I dispute that many, many places have "ancient" panels wired wrong. That doesn't make it right.
#50130 - 03/23/0510:18 PMRe: History of Sub Panels- Separate Neutral and Groungs
In older Code issues it was in Section 250-23. The oldest Code I have, 1931 has it in section 903. "The grounding connection shall be made at the transformer, generator,...on the supply side of the first switch controlling the system." Seventy five years and they still get it wrong. Alan - Inspector.
Alan-- If it was easy, anyone could do it.
#50132 - 03/24/0511:54 AMRe: History of Sub Panels- Separate Neutral and Groungs
Here's a quote from the 1911 NEC. Pg 25, Section 15 "Grounding Low-Potential Circuits". (FYI low potential was defined as 550 volts or less). Sub title "Alternating-Current Secondary Systems" b,3."The ground connection must be at the transformers or on the individual service as provided in sections c to g, and when transformers feed systems with a neutral wire, the neutral wire must also be grounded at least every 500 feet." Notice the requirement to ground the neutral every 500 feet.
#50134 - 03/28/0503:24 AMRe: History of Sub Panels- Separate Neutral and Groungs
A ground installed every so many feet is very similar to the PME (Protective Multiple Earthing) system used in Britain. While originally employed only in certain rural areas, it's become much more common in recent years, and in fact most LV distribution networks are now PME equipped (all in my local area, so I have been told).
The multiple grounds are only on the distribution network though. Once past the meter on a service, N and G are kept strictly separate.