I subscribe to "ECM", an Electrical Contractors' magazine. It has a
monthly feature on Electrical Code application. Generally, in something
like an existing electrical dryer installation with 3 wires; one being
the combined neutral/ ground; the installation would be grandfathered.
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.
Paul, W9AC
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