replacing a fused subpanel
 

The more I research, the more I know/ don't know.
The subpanel in question is about 40 feet from the newly updated service & panel and is upstairs.
It feeds lights & receptacles and none of the circuits are 3 wire grounded. The sub is fed with service entrance cable thru the attic and has a red, black and 3rd wire, all insulated.
My first thought was to shut off the main/sub breaker and copy the current wiring. Red & black to the load terminals and the 3rd wire to the neutral, removing the ground strap.
Research states to be code, I should run a new 4 wire cable so as to attatch the 4th, ground wire to the box ground.

I had planned to do this in the future but did want to replace the fused box with breakers this year. Question is, can I replace box using the current 3 wire cable and still be safe?
Thanks KB

The ground wire does not have to be in the same sheath as the current conductors. A separate wire can be installed for this as long as is of the same guage as the current conductors.
It doesn't even need to be insulated. Bare copper is acceptable.

You definately need to install the ground, even though the circuits that originate at the subpanel don't have grounds. Be sure to properly isolate the ground/neutrals in the subpanel.

I don't believe this is allowed for this situation. All circuit conductors including the equipment ground must be contained in the same cable or raceway unless specifically allowed. See 300.5 I then 250.102(C) and 250.130(C).

Question is, can I replace box using the current 3 wire cable and still be safe?

No .....3 wire connections require bonding of the neutral to the metal enclosure creating objectionable neutral current on the metal. This can only be done at the service equipment disconnecting means. There are a few exceptions to this rule in 2005 but were eliminated in the 2008 code cycle. NEC 250.24A5 does not allow connections or grounding of the grounded conductor (Neutral) on the load side of the service disconnecting means.

In my area a separate ground may be used to ground an up-grade panel. It may be run to the main or a suitable ground such as an incoming water main or ground rods.

What on earth code allows that? Earthing/grounding ALWAYS occur at the main, NEVER from a sub-panel.

There are situations where branch circuit equipment grounds under NEC 250.130(C) can be ran back to the service disconnecting means or any accessible point on the grounding electrode conductors or to the metal cold water pipe if the connection is made within 5 feet of where the pipe enters the house. This is allowed only for wiring extensions to grounding type receptacles added to ungrounded branch circuits.
A feeder to a subpanel would not be allowed in the NEC to have the equipment ground in the feeder raceway or cable ran separately in this manner. Then again thats the NEC I'm not sure what Canada or some local codes may allow. This would be rather risky in my opinion to allow the effective fault path for a sub-panel to be connected in this a manner.
The other situation where this is allowed would be for paralleled service entrance conductors NEC 250.102(C)

That is insane, you obviously do not understand grounding...

I've been told that before...........
Of course, i'd be interested to know, on what basis, you would base your observation! :wink:

Not from a sub-panel! To a sub-panel!

NEC does not apply in my area!

If you would put your location on your profile that would help and we could have saved some typing....:)

Did you notice under the "Friends" tab it says you haven't made any yet......:laughing:

My friend, you've got to cite some sort of code section or something to substantiate statements like the one you made about grounding. If you do, people won't jump on you. If your area doesn't enforce the NEC that's ok, but the vast majority of the people on this site reside in areas that enforce it so people need to be careful with such statements.

YES! You are correct, and I have come to realize this. Although, I'm a newby to this site. I am a retired electrician, presently living in Ontario, Canada.
When I retired, I passed on my copy of the Ontario Electrical Safety Code, to another fellow!
A copy of our code may viewed at our municipal library and if anyone is interested, I could go there, and look up the revelent, references.
It never occurred to me, that other jurisdictions would , for what ever reason, have different methods.
In this case, I wonder what would be the logic of disallowing a separate ground cable to be run for this panel up-grade. Especially, if it required damaging the building, to enable the up-grade.
Grounding has only one purpose and that is to 'trip' the circuit protection, should a fault occur that would allow a voltage potential to be crossed to any electrical device chassis.