The reference to that is NEC Article 250.140. Specifically, it
address the use of an external grounding conductor to the frame of a
4-wire dryer on legacy 3-wire service when several conditions are
met:
(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or
208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system;
(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG
aluminum; (3) The grounded conductor is insulated, or the grounded
conductor is uninsulated and part of a Type SE service-entrance cable
and the branch circuit originates at the service equipment; and (4)
Grounding contacts of receptacles furnished as part of the equipment
are bonded to the equipment.
The 1996 NEC revision had much to do with this change. However,
nowhere in the NEC does it state that a new dryer connection must be
4-wire. But since many dryers need 4-wire connectivity, the NEC
indirectly compels the use of a 4-wire branch and receptacle because
as of 1996, the neutral and ground can no longer be shared at the
dryer.
Some imported Korean laundry dryers do not require a neutral since
they only utilize the line-to-line 240VAC circuit. Hypothetically,
under new construction when using one of these new dryers only 3-wire
branch and receptacle service is required ( L1-L2-G, no neutral).
That's what the NEC says -- or rather doesn't say. Local codes will
vary.
The days of "big iron" motors and blowers in appliances running off
of 60 Hz are coming to a close. My own dryer uses switch-mode
direct-drive motors (one for the drive, the other for the exhaust
blower) running at close to 100 kHz. Of course in the name of lower
production costs and energy conservation, we then have to contend
with the RFI/EMI issues, especially from direct-drive washers.