Alternating current systems of less than 50 volts shall be grounded as required in NEC Section 2505(a). Systems of 50 to 1,000 V should be solidly grounded as required by NEC Section 250-5. Systems supplying phase-to-neutral loads shall also be solidly grounded (See Figure 4-5). The following electrical systems are required to be solidly grounded:

1. 120/240-V, single-phase, three-wire

2. 208Y/120-V, three-phase, four-wire,

3. 480Y/277-V, three-phase, four-wire,

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.

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.