Electrical – install a Subpanel (in a seperate building) using a 3 wire connection (2 hots and 1 grounded neutral)

electricalsubpanelwiring

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)?

Best Answer

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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