Whakawhenua whare o waho

How this is calculated#

A shed, garage, sleep-out or workshop with its own board, fed by a submain, is an outbuilding. AS/NZS 3000 gives it two ways to be earthed. The tool asks which outbuilding it is and which way you plan, then checks the plan and gives the sizes.

An individual outbuilding is one structure, separated by land from the building whose board supplies it. It may take a protective earthing conductor run with the submain, connected at the main switchboard. Or it may be a separate MEN installation: the submain neutral becomes a PEN, the outbuilding board has its own MEN link, and an electrode is driven at the outbuilding. Combined outbuildings share a foundation, a metal roof or a metal frame. They take the earth with the submain only, so a separate MEN for them fails.

The sizes come from the same table as the earthing tool. With an earth in the submain, it is the protective earthing conductor for the submain's active size. With a separate MEN, the lead from the outbuilding board to its electrode is a main earthing conductor, sized from the submain. The PEN must be at least the neutral the submain needs and at least that earthing conductor, so the tool gives the larger. The electrode is one of the types the electrode table accepts, driven to the depth or laid to the length and cover for your country.

Worked examples#

• A shed on a 10 mm² submain, earthed through the submain. The protective earthing conductor is 4 mm². No electrode is needed at the shed.
• A shed on a 16 mm² submain as its own MEN. The PEN is at least 16 mm², the main earthing conductor 6 mm², and the rod goes at least 1.8 m deep.
• A sleep-out with a metal water pipe to the house. The PEN is at least 6 mm². The tool warns that the pipe can carry neutral current in parallel with the PEN.
• A farm shed in Australia on a strip electrode. The PEN is at least 25 mm² and the main earthing conductor 6 mm². The copper strip is at least 3 m long with 0.5 m cover.
• Two sheds on one slab as a separate MEN. The arrangement is Not allowed and fails: combined outbuildings take an earth with the submain only.

Limits#

The tool does not size the submain itself; use the cable size or total voltage drop tools for that. It does not check the earth fault-loop impedance at the outbuilding, which the loop impedance tool does. It does not decide whether a structure is an outbuilding: that is the definition and your site. Bonding of water pipes and other services follows the earthing tool.

See also: Earthing an outbuilding: earth or its own MEN, Shed earthing on a 16 mm² submain and Earthing and bonding.