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

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

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 subpanel using the grounded neutral wire to connect to the neutral bus. I would install a grounding rod and connect the grounding rod to a seperate grounding bus in the subpanel. I antcipate keeping the neutral bus and the grounding bus seperate in the subpanel and do not anticipte 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 '13 at 20:45
    
The existing wire is 8/3 w/o ground and the length is only 20 ft. –  Scooter Nov 11 '13 at 20:48

2 Answers 2

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 2011

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). In this case the grounded (neutral) conductor of the feeder should be bonded to the equipment grounding conductors, grounding electrode conductor, and enclosure for the first disconnecting means in the structure. So in this rare case, the grounded (neutral), and grounding buses may be bonded in the subpanel.

Exception: For existing premises wiring systems only, the grounded conductor run with the supply to the building or structure shall be permitted to be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded if all the requirements of (1), (2), and (3) are 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 '13 at 15:15

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 '13 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 '13 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 '13 at 4:14

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