Does a 240 volt heater need a neutral?

Only if it has a 120 volt component such as a control transformer, blower, or control board. A straight resistance heater needs two hots and a ground.

What size wire for a 30 amp heater circuit?

10 AWG copper, limited to 30 amperes by NEC 240.4(D)(7), with a 10 AWG copper equipment grounding conductor from Table 250.122.

Why is a heater circuit sized at 125 percent?

NEC 424.3(B) requires fixed electric space heating branch circuit conductors and overcurrent devices to be at least 125 percent of the total load.

Do I need a neutral for my 30 amp disconnect supplying a heater?

Look at the heater nameplate and the wiring diagram on the inside of the cover. If the unit is a straight 240 volt or 480 volt resistance heater with a line voltage thermostat and no 120 volt components, you need two ungrounded conductors and an equipment grounding conductor. No neutral. If the unit has a 120 volt control transformer that is fed line to neutral, a 120 volt blower motor, a 120 volt control board, or a 120 volt receptacle in the cabinet, then you need the neutral.

What the NEC says

Section 250.24(C) requires a grounded conductor to be run to each service disconnecting means, but that is a service rule, not a feeder or branch circuit rule. On the load side, Section 215.2 and 210.19 only require the conductors the load actually needs. There is no code section forcing a neutral to equipment that does not use one.

Section 250.134 requires the equipment grounding conductor, and Table 250.122 gives 10 AWG copper for a 30 ampere overcurrent device. That conductor is never optional. Section 250.142(B) prohibits using the grounded conductor for grounding non current carrying metal parts of equipment on the load side of the service disconnect, so you cannot substitute a neutral for a ground.

Sizing the 30 ampere circuit

Fixed electric space heating is a continuous load. Section 424.3(B) requires the branch circuit conductors and overcurrent protective devices supplying fixed electric space heating equipment to be sized at not less than 125 percent of the total heating load. A 5,760 watt heater at 240 volts draws 24 amperes, and 24 times 1.25 is 30 amperes, which is exactly why 30 ampere circuits are so common on baseboard and unit heaters. The conductor is 10 AWG copper, limited to 30 amperes by 240.4(D)(7).

The disconnect itself

Section 422.31(B) requires that for a permanently connected appliance rated over 300 volt-amperes, the branch circuit switch or circuit breaker be permitted to serve as the disconnecting means where it is within sight from the appliance or capable of being locked in the open position. Section 424.19 requires the disconnecting means for fixed electric space heating equipment to disconnect the heater, the motor controller, and any supplementary overcurrent protective devices from all ungrounded conductors. A two pole disconnect satisfies that for a 240 volt heater.

If the disconnect is a fused type, watch the fuse rating against the conductor. A 30 ampere fused disconnect with 30 ampere fuses on 10 AWG copper is fine. If the heater nameplate calls for a smaller maximum overcurrent device, honor the nameplate under 110.3(B).

What to do with an unused neutral terminal

Many disconnects ship with an insulated neutral kit. If you do not need it, leave it in the box unused or omit it. Do not land the equipment grounding conductor on an insulated neutral bar. Bond the enclosure with the green screw or a listed lug per 250.8.

Field takeaway

No 120 volt load in the heater means no neutral. Two hots, one 10 AWG copper equipment grounding conductor, 30 ampere two pole overcurrent device, conductors at 125 percent of the heating load. Confirm against the unit wiring diagram before you close the wall.