Figure 4-5. Systems of 50 to 1,000 V AC that operate grounded are required to have the grounded conductor

DOE-HDBK-1092-98

4. 120/240-V, three-phase, four-wire, delta. (Midpoint of one phase used as a

grounded circuit conductor.)

The following systems are not required to be solidly grounded:

1. 240-V, three-phase, three-wire, delta

2. 480-V, three-phase, three-wire

3. 600-V, three-phase, three-wire.

These electrical systems do not supply phase to neutral loads. They supply only phase-to-phase

loads.

1φ, Wire

1φ, 3 Wire

120 V

120/240 V

(Neutral)

Grounded conductor

Transformer

secondary

3φ, 4 W

120/208 V

277/480 V

120/240 V

and

1φ, 208 V

(Neutral)

Grounded conductor

corner ground

3φ, 3 W

240 V

480 V

Grounded conductor

(Not a neutral)

NEC Section 250-5 (b)

OSHA Section 29 CFR 1910.304 (f) (1) (iv)

Figure 4-5. Systems of 50 to 1,000 V AC that operate grounded are required to have the grounded conductor

connected to earth ground at the supplying transformer and service equipment.

4-8