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I'm planning to run 300 ft of direct burial, 12 gauge, 3-conductors, no ground cable. Buried underground. The cable will carry 240v split phase.

The main panel at the start of the run is already grounded.

Do I absolutely need to have a grounding wire INSIDE the cable?

Or can I just properly ground the sub-panel at the end of the run?? (grounding rod, grounding wire to the ground post of the sub-panel)

EDIT: the loads will be a few lights, a 12v battery charger and 2 electric space heaters, so all resistive loads...believe it or not there's already a "ghetto" 200 ft run of 14/2 indoor romex wire going there (from previous owners) but the lights and battery charger work fine even with the massive voltage drop..

EDIT2: yes this is mostly a cost consideration, however after a bit more research I will use a cable with a grounding wire inside (didnt buy the cable yet)

Thanks everyone

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  • What do you mean by "properly grounded"? Where are you even obtaining /3 no-ground cable fit for direct burial? Can you get /2?
    – Harper - Reinstate Monica
    Dec 8, 2017 at 22:00
  • edited question
    – cannotcompute
    Dec 8, 2017 at 22:14
  • At 300' 240 v your maximum load before the voltage drop exceeds code is 6 amps! If you bump the voltage up and back down with transformers 480-600v range it could be done with smaller wire ( 5kva transformers at each end.) At this point it would be a seperatly derived system and local ground rod would be ok.
    – Ed Beal
    Dec 8, 2017 at 22:34
  • OP have you already bought this cable?? @EdBeal would you happen to have the code citation for maximum allowed voltage drop? I have not been able to find it. It is possible the customer only needs 6A. Also you just said my answer in 2 sentences lol. Well done!
    – Harper - Reinstate Monica
    Dec 8, 2017 at 22:38
  • 1
    I think you're confusing ground and ground. Unless you run a properly sized grounding conductor between the panels, there's no way to "properly ground" the remote panel.
    – Tester101
    Dec 9, 2017 at 1:18

2 Answers 2

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The National Electrical Code, that is the controlling document is most locations of the US, requires a 4 wire connection between a main and sub panel now.

That is; the two hot legs, the neutral, and a grounding wire. The ground wire can be a separate wire from the other wires but should be run in the same trench with them.

You still have to establish a ground at the separate building using one of the Code approved methods for a grounding electrode. Such as the one you cite in your question.

There are many questions here on SE that address this issue.

Good luck!

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    For health and safety, the cable ground is more important than the ground rod. The ground wire returns currents that would otherwise kill you, whereas the ground rod is only used to return natural energy (static electricity and lightning, the first won't kill you, and the second, the ground rod won't save you). Ground rods can't return large amounts of current because dirt doesn't conduct electricity very well.
    – Harper - Reinstate Monica
    Dec 8, 2017 at 22:58
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I think there is something you are missing.

Sure if you measure the voltage from the Hot to the Neutral, you would see say a 20% voltage drop on a long line with a large load.

The thing is if you measure from the Hot to the Ground you would only see about a 10% drop. (assuming you are only loading one phase) This is because the load is pulling up the Neutral at the same time it is pulling down the Hot.

Would you expect that putting a grounding rod into dirt, which isn't as conductive as a wire, would be enough to do much to counteract that effect much?

If the Neutral has a loose connection then the voltage increase on it will be more than the voltage drop on the Hot. What if the Neutral becomes disconnected entirely at the breaker panel, now you might see 120V on it because the load is pulling it up.

The reason why the Neutral is earthed is to reduce the voltage differential from the ground you walk on to the entire electrical system in your house.

Just think of it this way, the Neutral is a current carrying wire and so has more in common with the Hots than with the Ground wire. If you think this way you are going to have a system that is more likely to be safe.

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