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Can I use a 3 wire connection (2 hots and 1 grounded neutral) to install a sub panel in a separate structure if I install a separate ground rod to the grounding bus in sub panel?

I have 8/3 w/o ground running from the main panel for an existing 240 connection in a separate shed. I plan on replacing the existing 30 amp double breaker in the main panel with a 50 amp double breaker and I want install a sub panel that would allow me to have a 240 and a 120 outlet running off a 30 amp double breaker and a 20 amp breaker in the sub panel.

If possible and appropriate, I would like to use the existing 3 wire connection (8/3 w/o ground) to make the connection from the main to the a sub panel using the grounded neutral wire to connect to the neutral bus. I would install a grounding rod and connect the grounding rod to a separate grounding bus in the sub panel. I anticipate keeping the neutral bus and the grounding bus separate in the sub panel and do not anticipate bonding these two buses.

Will this work or do I need to replace the 3 wire connection (8/3 w/o ground) with a 4 wire connection (8/3 w/ground)?

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    What gauge and length is the existing wire on the 30 amp breaker? You can't just increase the breaker size without the appropriate wiring.
    – BMitch ♦
    Nov 11, 2013 at 20:45
  • The existing wire is 8/3 w/o ground and the length is only 20 ft.
    – Scooter
    Nov 11, 2013 at 20:48

6 Answers 6

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You can use a 3 wire feeder to supply a separate building, if...

  • The installation was in compliance with a previous edition of National Electrical Code (existing premises wiring).
  • An equipment grounding conductor is not run with the supply to the structure.
  • There are no continuous metallic paths bonded to the grounding system in each structure (bonded water, or gas piping, other conduit, etc.).
  • Ground-fault protection of equipment has not been installed on the supply side of the feeders.

National Electrical Code 2014

Article 250 Grounding and Bonding

II. System Grounding

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).

(B) Grounded Systems.

(1) Supplied by a Feeder or Branch Circuit. An equipment grounding conductor as described in 250.118 shall be run with the supply conductors and be connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

Exception No 1: For installations made in compliance with previous editions of this Code that permitted such connection, the grounded conductor run with the supply to the building or structure shall be permitted to serve as the ground-fault return path if all of the following requirements continue to be met:

(1) An equipment grounding conductor is not run with the supply to the building or structure.

(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.

(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).

If the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:

(1) That required by 220.61

(2) That required by 250.122

Changing from a 30A breaker to a 50A breaker can only be done, if you also change the wires to 6 AWG. In which case you'll have to follow current codes, and install 6/3 with ground. Breakers (and fuses) are always sized to protect the wire connected to them, so you can't change the breaker size without also changing the wire size (unless you're going down e.g. 50A to 30A).

However, depending on what you're doing, you may not have to change the breaker at all. If the planned circuits in the structure are not going to be fully loaded, you may well be able to supply the subpanel with a 30A breaker. Just because the subpanel has 50 amperes worth of overcurrent protection, does not mean the supply breaker has to be 50A. Whether or not you actually need a 50A breaker on the supply, depends entirely on what the subpanel will be powering.

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  • Thanks to Tester 101 and @BMithch. I appreciate the insight. The subpanel will never be powering both circuits at the same time, so it looks like I won't need to replace the existign 30 amp breaker in the main. I will verify that all the other conditions Tester 101 outlines for using a 3 wire feed to the subpanel exist and if they do, I'll proceed by bonding the grounded (neutral) bus to the grounding bus in the subpanel and add a grounding rod and grounding connection to the grounding bus in the subpanel. Sounds like you are in effect creating an other "main panel" in the subpanel.
    – Scooter
    Nov 12, 2013 at 15:15
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I'm visiting from the future.

In the future, we change NEC to allow retrofitting of ground wires on almost any circuit. This will happen in NEC 2014, which due to a number of other insane requirements (hot tip: buy stock in companies that make GFCI devices) many states will really drag their feet to adopt, so you may not see it adopted until 2018 or so.

There's no such thing as a "grounded neutral wire", that's a wish-burger such as "driver-friendly alcohol". Neutral is never grounded except right at the N-G equipotential bond inside the main panel. 3-wire subpanel connections are throwbacks to the age before any grounding at all. They're just attaching local grounds to the neutral using the same logic used to justify the same on dryers and ranges: "it's too expensive to force a fix, so choose the least bad option". The practice is now barred, which tells you what they really think.

Ground rods alone are not sufficient because during a bolted ground fault (hot-ground short), the ground needs to be able to return hundreds of amps to assure a rapid breaker trip. Including dirt in the grounding path limits you to about 4 amps at best. It's not going to happen.

A solitary ground wire can be buried. Not sure on the burial depth, but if you bury Rigid conduit, that only needs 6" of cover, and the pipe itself is the ground path, so you're done. Expensive stuff, but you can trench it with a garden trowel, which means no worries of destroying other utilities.

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I don't believe you can run 50 amps over 8awg, most of the charts I'm seeing limit you to 40 amps. You have to upgrade the wire to 6awg to get all the way to 50 amps. Since you have to upgrade the wiring, you should also get a 6/3 line with a ground. If you're burying the wire, make sure to get one rated for burying and consider conduit, sand, and warning tape. Make sure the ground and neutral are not bonded in the sub-panel, only in the main panel.

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  • Ok if I limit the breaker to 40 amp, can I wire the subpanel the way I'm suggestign (using a new ground rod for the grounding bus) or is a ground wire needed form the main to the subpanel?
    – Scooter
    Nov 11, 2013 at 21:12
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    If I remember correctly, you can only use a 3 wire feeder if there are no other possible conductive paths between the structure (water pipe, gas pipe, other conduit, etc.). In this case, the separate structure would need its own ground rod(s), and the grounding and grounded buses would be bonded at the subpanel. I'd have to check the code to be positive, but I'm fairly sure the buses should be bonded. NEC 2008 250.32(B). If you run new cable between the structures, a grounding conductor is required.
    – Tester101
    Nov 11, 2013 at 22:19
  • @Tester101 quit commenting on my answer and just post your own. You know your answer will be better than mine anyway. :)
    – BMitch ♦
    Nov 12, 2013 at 4:14
  • The allowance to use a "3-wire" feeder was removed a while ago, I believe after the 2005 NEC.
    – Speedy Petey
    Apr 7, 2015 at 1:53
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8/3 would be too small for a 50amp circuit. If your running above ground to a sub panel it should be 3 wire and a ground rod at location of sub panel ,bonded with neutral as if it was a main service. If your running under ground then it should be 4 wire. Seperate neutral and ground bars in sub panel. No ground rod. Confirm the conduit isn't metallic and bonded. Confirm the two buildings are not grounded to water pipes.

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You need four wires no 6 for 50 amps to 60. You might get away using three sixes and a eight if Your running 6 THNN copper. THNN 6 copper will carry the current up to about 180 feet. 8 will not and is rated about 45 amps at a short run. And a ground Rod with a fifth wire by building with a ground rod to panel box on new building 8 feet in ground. But the code enforcement wants to see all equal wire size if not you can mix a 8 in on neutral.

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You can use 8awg on 50amps. Refer to NEC 310.15 (B) (16)) Personally I use 6awg, but it can be done

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    not in this instance you can't. NM cable is limited to the 60 deg C column which would limit it to 40A. #8cu conductors in conduit would be allowed on a 50A breaker for this.
    – Speedy Petey
    Apr 7, 2015 at 1:52

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