Meter panel combo and bonding questions

[bradler]

I am trying to bid a new project upgrading a residential shop to a 200 amp service. The existing meter base panel combo has a 100amp disconnect and is to be replaced with a 200amp meter main disconnect with feed through lugs to the shop. There is currently a three conductor service going to a pump house and another three conductor service going to the shop building with neutrals and ground terminated on the same bar. The existing panel in the shop has the bonding jumper installed. Does this not create the danger that if this panel loses its ground/neutral since the branch circuits have un-separated grounds/neutrals, that all grounded surfaces in the shop will be energized?


Perhaps three conductor services are allowed here (outside of normal city limits/inspector I usually work with) but why was this not originally wired with four conductors? As the meter main combo will be the first point of disconnect should the neutral bond not be required to happen here with four conductors/separate neutral and ground going to the shop? As we will be installing new wire from the meter/panel already it would be easy to run four conductor feeders to the shop and the new panel will not be bonded.

My big dilemma is then what to do with the three conductor service to the well house. Upgrading this to a four conductor feeder and re working the panel could more than double my bid. The guy that I sub my excavation too wants nothing to do with trenching to the well house because its a mine field of water lines, cable, high voltage, and septic lines plus the run is over 200feet. Maybe the inspector will let me leave the 3 conductor service to the well house and just reconnect it to the new meter/main?

 

[ActionDave]

There is nothing electrically unsafe about a three conductor service or feed from a meter main to a building with the neutral bonded at the building. Before 2005 that was the way we all did it. It's not like somebody is going to run a water line or phone out to the meter. There is an exception to 250.32(B) for existing installations as long as there are no other metallic paths between the two structures.

 

[jaggedben]

... Does this not create the danger that if this panel loses its ground/neutral since the branch circuits have un-separated grounds/neutrals, that all grounded surfaces in the shop will be energized?

That could happen but you'll be alerted to it quickly because electrical things will stop working. (This can also happen if you lose a neutral on a 3-wire service, and it does happen sometimes, and yet nobody is going to change the number of wires utilities bring to a service.) If you have a 3 wire feed and no neutral/ground bond, then the danger is that grounded parts become energized without tripping a breaker and everything continues operating normally until someone gets shocked. If you have separated grounds and neutrals and you lose the EGC you end up with a similarly unsafe situation. So with an existing 3-wire feed it's much safer to have the neutral-to-ground bond, and there could even be an argument that the 3-wire feed is safer. I think the reason for separating neutrals and grounds in NEC covered areas is as much to avoid objectionable current as to keep grounded parts from developing a shock hazard. (Not that the two are entirely unrelated.)

Maybe the inspector will let me leave the 3 conductor service to the well house and just reconnect it to the new meter/main?

The inspector really ought to permit it if you show him 250.32(B) Exception, as Dave mentioned, and you can show that the well house installation was inspected and approved at the time it was installed. I would separate this part out in your bid and put in a clause in that clearly absolves you of responsibility if the inspector doesn't play along with that.

 

[bradler]

Quick update. The inspector did allow me to reconnect the existing three conductor service to the well house per 250.32(B) as there are no changes to the original permitted installation. The new panel in the shop will require four conductor feeders to meet current code and will be treated as a typical sub panel with unbonded neutral and separate ground.

One other question did come up. The inspector stated that the EGC to the shop panel should be #4 copper or #2 alum. The shop panel is to be supplied by feed through lugs in the meter/main with a 200amp breaker. Why would the EGC not be sized by the feeder overcurrent protection device per 250.122 which allows #4 Alum?

Cheers

 

[texie]

Quick update. The inspector did allow me to reconnect the existing three conductor service to the well house per 250.32(B) as there are no changes to the original permitted installation. The new panel in the shop will require four conductor feeders to meet current code and will be treated as a typical sub panel with unbonded neutral and separate ground.

One other question did come up. The inspector stated that the EGC to the shop panel should be #4 copper or #2 alum. The shop panel is to be supplied by feed through lugs in the meter/main with a 200amp breaker. Why would the EGC not be sized by the feeder overcurrent protection device per 250.122 which allows #4 Alum?

Cheers

Based on your description the inspector is wrong.

 

[kwired]

Based on your description the inspector is wrong.

I agree.