240 Volt Appliance Receptacles and What Each Reads

Why this matters

An electric dryer that tumbles but will not heat, or a range whose surface elements work and whose oven does not, is a parts order waiting to happen. Before any heater, thermostat or board gets condemned, the receptacle has to prove it is delivering both legs. A lost leg is common, it is not the appliance's fault, and the tech who skips the receptacle reads a dryer's heat circuit as open at the element, orders an element, and comes back to the same no-heat call. The readings take two minutes and they are the same set of numbers every time. This card is those numbers, what a good circuit gives, and the patterns a bad one leaves.

Identifying the receptacle face itself (which slot is which, and what each configuration is rated for) belongs to the Electrical shelf card 'NEMA Receptacle Faces for 240 Volt and RV Circuits'. The two this card logs are the dryer receptacles you meet most: the four-wire 14-30 (two hots, neutral, ground) and the older three-wire 10-30 (two hots and neutral, no ground pin).

Safety for live readings at a receptacle

Reading voltage at a receptacle is energized work. Use a meter rated for the category (CAT III is the common choice for fixed premises wiring) with shrouded probes, wear the protective equipment your shop's NFPA 70E program sets for 240 V testing, and keep one hand's fingers behind the probe guards. Unplug the appliance first so you are reading the supply, not the supply plus whatever the appliance feeds back. Prove the meter on a known live source before and after the readings, because a meter with a blown fuse or a bad lead reads a believable zero, and a believable zero on one leg is exactly the finding this card is looking for.

Set the meter to AC volts. Where it has a low-impedance (LoZ) setting, use it for the dead-leg check: a high-impedance meter can read a ghost voltage on a disconnected conductor that sits beside a live one in the same cable, and a ghost reading of 30 or 60 V on a leg that is actually open sends the diagnosis sideways.

The appliance tech reads the receptacle. Opening the panel, changing a breaker, or repairing branch wiring is electrician's work where your state or locality licenses it, and a fault upstream of the receptacle routes there.

What a good circuit reads

Name the contacts the way the receptacle face does: X and Y are the two hot legs (L1 and L2), W is the neutral, G is the ground.

On a 14-30, six readings, every one of them taken.

• X to Y: about 240 V. Both legs present and on opposite sides of the service.
• X to W and Y to W: about 120 V each, and within a volt or two of each other.
• X to G and Y to G: about 120 V each, matching the readings to neutral.
• W to G: near zero, typically well under a volt with nothing plugged in. Neutral and ground are bonded at the service and nowhere else, so with no current on the neutral there is nothing to separate them. Under load a volt or two can appear on a long run, because the neutral now carries current and the ground does not; several volts with nothing plugged in is a different story, covered below.

On a 10-30, three readings, because there is no G.

• X to Y: about 240 V.
• X to W and Y to W: about 120 V each, balanced.

The ground readings the 10-30 lacks are not a fault on an existing installation. NEC 250.140, as adopted in your jurisdiction, permits an existing three-wire branch circuit to stay in service with the dryer or range frame bonded to the neutral, provided the conditions of its exception are met, while new branch circuits are four-wire. Changing the cord to match is covered in 'How to Change a Dryer or Range Cord from Three-Wire to Four-Wire'.

"About" means a healthy residential supply. Utility service voltage is held to ranges set by ANSI C84.1, about 5 percent either side of nominal for the service (Range A), so readings of 114 to 126 V and 228 to 252 V are ordinary. What matters more than the exact figure is that the two legs agree with each other and that X to Y is close to the sum of the two leg readings.

The meter log: two calls, two receptacles

CALL A  14-30, electric dryer, tumbles, no heat
dryer unplugged, LoZ setting, meter proven before
  X - Y     1 V     (expect about 240)
  X - W   121 V
  Y - W     0 V     (expect about 120)
  X - G   121 V
  Y - G     0 V     (expect about 120)
  W - G   0.2 V
meter proven after: OK
finding: Y leg dead at the receptacle

CALL B  10-30, electric dryer, slow drying
dryer unplugged, meter proven before
  X - Y   208 V     (240 expected nominal)
  X - W   120 V
  Y - W   120 V
meter proven after: OK
finding: both legs present, 208 V service

Call A, read pattern by pattern. X reads 121 V to both neutral and ground, so X is live and the neutral and ground are sound (W to G 0.2 V). Y reads 0 V to everything. X to Y reads 1 V, not 240 V and not 121 V, because with the dryer unplugged and a low-impedance meter across X and Y, the dead Y conductor is simply pulled to X's potential through the meter. The circuit is delivering one leg. On this dryer's diagram the motor and timer are wired from the X leg to neutral, which is why it tumbles; its heater is wired across both legs, which is why it will not heat. Had the motor hung on Y, the same fault would have presented as a dead dryer, and the receptacle readings would have been identical. Nothing inside the dryer has been proven bad.

The tech checks the breaker: a double-pole 30 A breaker, handle in the on position, not tripped. A common-trip double-pole breaker trips both poles together, so one dead pole with the handle on points at an internal breaker fault or a loose conductor at the breaker or the receptacle terminals. The tech does not open the panel. He records the six readings, tells the customer the dryer is not the fault, and routes the call to a licensed electrician. When the electrician finds a burned Y terminal at the breaker and replaces breaker and terminal, the return-visit readings are the finish test: X to Y 241 V, both legs to neutral 120 and 121 V, both legs to ground matching, W to G 0.1 V, and then a dryer run with the heater drawing current on a clamp meter.

Call B, read pattern by pattern. Both legs are present and balanced, but X to Y reads 208 V. This is an apartment building on a 120/208 V three-phase service, where any two legs are 208 V apart. It is not a fault and no electrician can fix it at the receptacle. A heater built for 240 V on 208 V makes less heat: power goes with voltage squared, and 208 squared over 240 squared is 43,264 over 57,600, about 0.75, so the element runs at roughly three quarters of its rated wattage. A dryer with a 240 V element dries noticeably slower, which is the complaint. Some dryers and ranges are rated for both voltages or offer a 208 V element; the rating plate and the tech sheet say which. The fix, where one exists, is the right element or a model rated for 208 V, and on the dryer it is also worth adding up the exhaust run, because a slow heater and a long vent compound each other.

The patterns, and what each one means

• One leg zero to neutral and ground, X to Y near zero or ghost. Lost leg upstream: breaker, a loose lug, a burned receptacle terminal. Routes to an electrician.
• X to Y reads about 120 V instead of 240 V. One leg is open and is being pulled to neutral by a 120 V load connected to it (this happens when an appliance is still plugged in, or at a range whose clock and lights hang on the dead leg). Unplug the appliance and read again; the dead leg then reads zero to neutral, the first pattern above.
• Both legs read 120 V to neutral but X to Y reads near zero. With the appliance connected and running, the dead leg is being backfed through a heater from the live leg, so it looks live to neutral. With nothing connected, both hots are landed on the same leg of the service, a wiring error at the panel. Never accept two good leg-to-neutral readings without the X to Y reading.
• Leg-to-neutral readings unequal under load, for example 95 V and 145 V, on a circuit that read balanced with nothing connected. Open or high-resistance neutral: the 120 V loads are now in series across 240 V. This one can damage the appliance's electronics and motors and can put voltage on a frame bonded to neutral on a three-wire circuit. Leave the appliance unplugged and route it.
• W to G reads several volts with nothing plugged in. Current is flowing on the neutral from elsewhere, or the neutral is poor. Note it and route it; it is not an appliance fault.
• Readings fine with the appliance unplugged, appliance still dead. The supply is proven. Now the cord, the terminal block and the appliance's own circuit are in play, which is where the other cards on this shelf start.

What changes the reading

• A range receptacle (14-50 or the older 10-50) reads the same pattern as its dryer counterpart; the face and the breaker size differ.
• A hard-wired appliance has no receptacle. The same readings are taken at the appliance terminal block with its cover off, which puts your hands closer to energized parts, so the PPE and the before-and-after meter proof matter more.
• Generator or transfer-switch circuits can carry only one leg in some configurations. If the house is on backup power, ask before you diagnose.

References

• NEC Section 250.140 (as adopted by your jurisdiction, check the edition), frames of ranges and clothes dryers
• NFPA 70E, current edition, for energized voltage testing and the shop's PPE program
• ANSI C84.1, which sets utility service-voltage ranges
• See related: NEMA Receptacle Faces for 240 Volt and RV Circuits
• See related: How to Walk an Electric Dryer Heat Circuit with a Meter