First off, if the cable is not installed in a raceway, and is direct burial, then you could not just add an equipment grounding conductor and bury it. The conductor would, of course, need to be rated for direct burial. If it is not underground or direct burial and not in a raceway, this would not comply to the requirements for cables in the NEC [I do not recall the article, but it's been a subject here and there in several forums over the past year, plus as I remember, it allows for only branch circuits that are being "upgraded" which have existing 2 wire non-metallic sheathed cable to have the eq. grounding conductor run externally].
However, if you have the cable running through conduit that is continuous from the main service panel to the sub panel, you could run an equipment grounding conductor through that conduit with the cable, provided you have ample room
It's kind of a revolving situation to clarify it in the NEC, but as long as they all occupy the same RACEWAY, it should be compliant.
** Don, Bennie and Rick: - let me know how you guys would feel on this interpretation!!
To me, it seems that it would have been less expensive to use #2 THHN / THWN cu, than to install the SE cable, however I am biased by the fact that I rarely touch Romex or any non-metallic sheathed cable - so I am not sure if there is a cost difference or not.
Anyhow, try to stay away from using the methods described in 250-24 [deriving a new grounding electrode system and bonding the grounded conductor to it in remote buildings from a common AC service], as it should only be done for certain circumstances. I know that this statement will generate lots of flames, but it should not be done in liew of running an equipment grounding conductor.
If there is any metallic conduits or other types of piping that interconnects the two buildings, along with any concrete that contains REBAR that interconnects the two buildings, you may not use 250-24.
Personally, I would only use this method on very remote buildings, like 1,000 feet away remote! That is where this method becomes an advantage.
So to sum it up, install a ground bond if you used conduit as described above. Otherwise, replace the cable with one that has an insulated grounding conductor [4 wire]. You could [more like should
] consult the Building Department / Electrical Inspector for that city/county and find out what they have to say about the whole thing.
Do stay away from direct burial of a single conductor for a ground bond. This is a very critical conductor! more critical than the grounding electrode system! You can have major hazards [no relation to Toy Story
] if the bond is insufficiently continuous and there is a ground fault off the sub panel.
If the cable is not run underground, maybe the AHJ will allow you to just simply add an equipment grounding conductor - attached to the cable's exterior - instead of replacing the entire cable. This would be the first logical step to take after getting suggestions from this forum, plus be the final determination for the installation. I have a hard time answering code related questions like this one, in which the installation's compliance is an issue rather than the installation it's self. This is when the Inspectors are called into the picture!
It would sure make things like this easier if the person at the Home Improvement Centers would explain to the customer when not to use a 3 wire cable, but that makes too much sense I guess
I am not sure if the owner purchased this cable themselves or if the cable was something convenient.
There seems to be an ever increasing number of questions relating to this type of installation - mostly an underground, or direct burial sub feed - where the person wants to use 3 wire cable. I usually do not reply to these threads, in hopes that another member can better tackle the question, but your's got my interest. Also I want to see what the Inspectors in the group have to say about the situation and apply it to future postings.
Comments are welcome [AKA - let the flames begin
Hope this was helpful.