Wiring Methods and Grounding Work

Insulator wiring supports exposed wires on insulators, so it cannot be used in concealed places that cannot be inspected. Metal conduit, synthetic resin conduit and cable wiring can be used in every type of location if their conditions are met.

Metal raceway (senpi) wiring may be used only in dry exposed places and dry concealed places that can be inspected, so it cannot be used in damp places. Metal conduit, cable and Type 2 flexible metal conduit wiring can be used in damp or wet places.

Lighting duct wiring (300 V or less) may be installed in dry exposed places and dry concealed places that can be inspected. Damp and wet places are not dry, so they are not allowed, and concealed places that cannot be inspected are not allowed either.

The space above a ceiling with an inspection opening is hidden but can be inspected. The inside of a wall without an opening and the inside of a concrete floor cannot be seen without breaking the building, so they are concealed places that cannot be inspected. The surface of a wall is an exposed place.

In places with flammable gas only metal conduit wiring and cable wiring are allowed. Synthetic resin conduit is allowed (with conditions) for combustible dust and hazardous materials, but not for flammable gas. Metal raceway and insulator wiring are not allowed in places with flammable gas, combustible dust or hazardous materials.

In places with combustible dust, synthetic resin conduit wiring (excluding pipes under 2 mm thick and CD pipe), metal conduit wiring and cable wiring are allowed. Rigid PVC conduit 2 mm or thicker meets the condition. CD pipe is excluded, and insulator and metal raceway wiring are not allowed.

In places that make or store hazardous materials such as petroleum, synthetic resin conduit (excluding pipes under 2 mm thick and CD pipe), metal conduit and cable wiring are allowed. CD pipe is excluded. Metal conduit, rigid PVC conduit 2 mm or thicker and cable wiring with the cable in conduit or other protection are allowed (Article 177).

A floor duct is embedded in the floor and is used at 300 V or less in dry concealed places that cannot be inspected. It cannot be used in damp or wet places, it is not a ceiling method, and it cannot be used above 300 V.

Cable wiring, like metal conduit, synthetic resin conduit and Type 2 flexible metal conduit wiring, can be used in every location if its conditions are met. Metal duct and lighting duct wiring are limited to dry exposed places and dry concealed places that can be inspected. Insulator wiring cannot be used in concealed places that cannot be inspected.

In metal conduit wiring, wires must not be joined inside the conduit; joints are made inside boxes, so a joint in an outlet box is correct. Insulated wire (except OW) that is stranded, or solid wire of 3.2 mm diameter or less, may be used, so stranded PVC wire and 1.6 mm solid wire are both fine.

Under Article 159 of the Interpretation, metal conduit embedded in concrete must be at least 1.2 mm thick. Elsewhere 1.0 mm is the basic minimum, and 0.5 mm is allowed only for conduit without joints, 4 m or shorter, in dry exposed places. There is no 2.0 mm rule for metal conduit (it is easily confused with the thickness of synthetic resin conduit, 2 mm or more as a rule).

Grounding may be omitted (1) for conduit 4 m or shorter in a dry place, or (2) for 150 V or less to earth with conduit 8 m or shorter in a dry place (or with simple contact protection). 100 V, 6 m, dry fits (2). 200 V, 6 m fits neither. 10 m exceeds 8 m. Even 3 m does not fit (1) in a damp place.

Conduit wiring uses insulated wire, but outdoor PVC insulated wire (OW) is excluded. Stranded IV, HIV and 2.0 mm solid IV (3.2 mm or less) may be used.

Under Article 158, supports for synthetic resin conduit must be 1.5 m or less apart. 2.0 m is the figure for cables along the underside or side of building members, and 1.0 m is for cabtyre cable, so they are easy to confuse. There is no 3.0 m rule.

Without adhesive the insertion depth must be at least 1.2 times the outer diameter: 1.2 x 26 = 31.2 mm. 20.8 mm (0.8 x 26) is the value when adhesive is used. 26.0 mm (1.0 times) is not enough. 52.0 mm (2 times) is more than needed and is not the minimum.

CD pipe is not self-extinguishing and burns easily, so it must be embedded directly in concrete or placed inside a dedicated non-combustible or self-extinguishing flame-retardant pipe or duct. It cannot be left exposed indoors, above ceilings or on outdoor walls.

Under Article 160, flexible metal conduit at 300 V or less needs Class D grounding, but it may be omitted if the conduit is 4 m or shorter. 8 m is the figure for metal conduit when conditions such as 150 V or less to earth are met. There is no 10 m rule, and at 300 V or less grounding is still required in principle.

Conduit wiring uses stranded wire or solid wire of 3.2 mm diameter or less (4 mm for aluminium). 1.6 mm and 2.6 mm are common sizes in homes, not the upper limit. 5.0 mm exceeds the limit.

Under Article 164, cable fixed along the underside or side of building members must be supported at 2 m or less. 1 m is for cabtyre cable and 1.5 m is for synthetic resin conduit. 6 m is allowed only for vertical runs in places with contact protection.

Under Article 164, cabtyre cable must be supported at 1 m or less. 2 m is for ordinary cable, 1.5 m for synthetic resin conduit, and 6 m for vertical cable runs in places with contact protection.

Article 36 of the Interpretation requires a ground-fault breaking device on circuits supplying low-voltage equipment over 60 V with a metal frame, but it may be omitted when the equipment is installed in a dry place. A wet place is not a reason for omission (even for equipment of 150 V or less to earth, omission applies only outside wet places). Omission based on earth resistance needs 3 Ω or less, so 50 Ω is not enough. A molded-case circuit breaker breaks overcurrent and cannot replace a ground-fault breaking device.

Under Article 164, where cable may be under pressure from heavy objects or strong mechanical impact, suitable protection must be provided, such as metal conduit. More supports, tape or a thicker cable do not protect against crushing force and cannot replace protection.

Article 12 of the Interpretation requires that a joint must not reduce the wire's tensile strength by 20% or more, so at least 80% of the strength must remain. 10%, 30% and 50% are not the standard figures.

Article 12 requires joints to use a sleeve or other device or brazing, and to be fully covered with insulation at least as effective as the wire's. Just twisting without a device is not allowed. The joint's resistance must not increase and its insulation must not be weaker.

Article 12 allows connectors with insulating effect, and as a rule cables are joined to each other with a junction box or other device. Using push-in connectors inside a junction box is appropriate. The joint must not reduce tensile strength by 20% or more (at least 80% must remain), so 70% is not acceptable. Crimping over the sheath does not connect the cores, so the joint would not keep the resistance from increasing. The joint must be fully covered with insulation at least as effective as the wire's, so leaving the cores visible is not acceptable.

Using two or more sleeves for one joint is a defect. Cores sticking out above the top are a defect at 5 mm or more, so 3 mm is not. Bare core below the bottom is a defect at 10 mm or more, so 5 mm is not. Seeing all core tips from above is the correct state.

Under the criteria, if the core cannot be seen from the side at the tip, the wire is not pushed in far enough (a defect). Seeing the core means it is fully inserted. Bare core visible where the wire enters is a defect, for example from stripping too much. With the insulation on, no electrical connection is made.

Under Article 167, insulator wiring must be at least 10 cm from weak-current wires. 30 cm is the figure when the wire is bare. There are no 3 cm or 100 cm rules.

Under Article 17, Class D grounding must be 100 Ω or less. 10 Ω is the value for Class C. 500 Ω applies only when a device cuts the circuit within 0.5 seconds, which is not the case here. There is no 50 Ω rule.

Under Article 17, with a device that cuts the circuit within 0.5 seconds, Class D (and also Class C) grounding may be up to 500 Ω. 100 Ω is the value without such a device. There are no 200 Ω or 1000 Ω rules.

Under Article 17, Class C grounding must be 10 Ω or less. 100 Ω is the Class D value and 500 Ω applies when a device cuts the circuit within 0.5 seconds. There is no 50 Ω rule.

Under Article 17, grounding conductors for Class C and D are annealed copper wire of 1.6 mm diameter or more (or corrosion-resistant metal wire with tensile strength of 0.39 kN or more). 1.0 mm and 1.2 mm are too thin. 2.6 mm can be used but is not the minimum.

Under Article 29, low-voltage equipment at 300 V or less needs Class D and above 300 V needs Class C. Three-phase 400 V exceeds 300 V, so Class C is required. Class A is for high and extra-high voltage equipment, and Class B is for the low-voltage side of transformers.

Omission using an earth leakage breaker (15 mA or less, 0.1 s or less, current-operated) is allowed only in places other than wet places, so it cannot be used in a wet place. Equipment of 150 V or less to earth in a dry place, double-insulated equipment, and equipment handled on a dry wooden floor may omit grounding under Article 29.

Article 29 requires a current-operated breaker covered by the Electrical Appliance and Material Safety Act with rated sensitivity of 15 mA or less and operating time of 0.1 s or less. 30 mA and 100 mA are too large. 0.5 s is easily confused with the condition for relaxing grounding resistance to 500 Ω and is not the condition for omission.