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L, P posted on 10-11-2010 1:58 PM

I am being asked to install a 3-pole 3-wire NEMA 10-30P cord on a new electric clothes dryer. The dryer nameplate power requirement is 120/208 volts or 120/240 volts 1-phase. This is a common ordinary dryer that can be purchased at any appliance store.

This is an existing installation, the electrical panelboard and dryer receptacles are already installed. The panelboard power supply is 240/120 volt 3-Phase 4-Wire center tap grounded Delta. Two of the phases are 120 volts to ground and neural and the third phase or “High-Leg” is 208 volts to ground or neutral.

The receptacles are NEMA 10-30R 125/250 volt. All the receptacles have a neutral connected to the “W” terminal and a 120-volt phase and a 208-volt HIGH-Leg phase connected to the other terminals. The receptacle wiring is not consistent; one receptacle has the High-Leg  connected to the “Y” terminal and the 120 volt phase connected to the ”X” terminal and another receptacle has the High-Leg connected to the “X” terminal and the 120 volt phase “Y” terminal.

The standard Line-to-Neutral voltage for an electric clothes dryer is 120 volts. I am being asked to connect the dryer cord in the following manner to correspond to the internal voltage of components of the dryer; the 208-volt High-Leg to the terminal of the dryer that is connected to the heating elements which is a Line-to-Line load and the 120-volt phase to the terminal of the dryer that is connected to the dryer controller which is a Line-to-Neutral Load. Since the dryer receptacles are all wired differently this dryer will only operate in the corresponding receptacle with the same polarity.

Receptacles are manufactured differently in order to prevent equipment designed for one voltage to be inadvertently connected to another to prevent damage.I never seen a High-Leg connected to a NEMA 125/250 volt 10-30R, it has been my understanding the “/” symbol indicates 125 volts to Ground and 250 volts Line to Line. If I had wired these dryer receptacles, I would have wired them 120/240 volt configuration without the high-leg.

Nowhere in the National Electrical Code or within NEMA can information be found to confirm that the a High-Leg can be used on a NEMA 10-30R or NEMA 14-30R.

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