How to Change a Dryer or Range Cord from Three-Wire to Four-Wire
Why this matters
A customer moves an electric dryer or range into a house with a four-slot receptacle, and the old three-prong cord does not fit. Fitting the new cord is ten minutes of work. Getting the bond right is the part that matters: on a three-wire connection the cabinet is deliberately tied to neutral, and on a four-wire connection that tie must come off and the cabinet goes to the separate ground conductor instead. Leave the strap in place on a four-wire cord and the ground wire carries normal neutral current all the time, in parallel with the neutral, and the cabinet the customer touches rides on the neutral instead of on a clean ground. The code forbids that parallel path, and it hides a loose neutral until something else fails. One continuity reading proves the conversion before the appliance goes back against the wall.
Power off, proved dead
Turn off the appliance's double-pole breaker and unplug the cord. Prove the receptacle dead, L1 to L2, each L to neutral and each L to ground, and the appliance terminal block dead the same way, with a meter checked on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024). Both legs get checked because a double-pole breaker with a broken internal tie can leave one leg live. Every reading in this card is taken dead; the receptacle's own live check before the appliance is plugged back in belongs to '240 Volt Appliance Receptacles and What Each Reads'.
Pull the appliance out with the floor protected. On a range, engage the anti-tip bracket again when it goes back; on a dryer, mind the vent connection, which is the subject of its own card.
What the code allows, in substance
NEC 250.140, as adopted by your jurisdiction, requires the frames of ranges, wall ovens, cooktops and clothes dryers to be connected to an equipment grounding conductor. Its exception lets an existing branch circuit, where no equipment grounding conductor is present in the outlet or junction box, keep the old method, the frame tied to the grounded (neutral) conductor through a three-wire connection, only where all of its conditions hold. In substance, check the current edition for the exact wording:
- the supply is a 120/240 V single-phase three-wire circuit (or 208Y/120 V derived from a three-phase four-wire system);
- the neutral is no smaller than 10 AWG copper or 8 AWG aluminum;
- the neutral is insulated, or it is uninsulated as part of Type SE service-entrance cable with the branch circuit starting at the service equipment;
- grounding contacts of receptacles furnished as part of the equipment are bonded to it.
What that means at the back of the appliance: a three-wire receptacle on an existing circuit can stay, and the appliance is connected with a three-wire cord and the bonding strap in place. A four-wire receptacle means the circuit carries a separate ground, and the appliance gets a four-wire cord with the strap removed. Never mix them: a three-wire cord on an adapter into a four-wire receptacle, or a four-wire cord with the ground conductor cut off, throws away the protection the circuit provides.
Two limits on your side of the job:
- The receptacle is not yours to change. Replacing a three-wire receptacle with a four-wire one is a circuit change and needs a real ground conductor run to it. That is electrician's work where your state or locality licenses it, not something solved with a new receptacle and a jumper inside the box.
- Manufactured homes. Dryer and range makers' instructions require a four-wire connection in a manufactured or mobile home, and NEC Article 550, as adopted, governs those installations. The three-wire exception does not apply there.
The terminal block, both ways
appliance terminal block, rear access cover off
THREE-WIRE CORD (existing circuit, strap ON)
L1 N L2
[ o ] [ o ] [ o ] cord: L1, N, L2
|
BONDING STRAP or strap or green/white
GROUND WIRE wire ties neutral
| to the cabinet
CABINET GROUND SCREW (o)
FOUR-WIRE CORD (strap OFF)
L1 N L2
[ o ] [ o ] [ o ] cord: L1, white N, L2
strap removed, or wire
CABINET GROUND SCREW (o) moved off ground screw
|
green cord conductor
Makers do the three-wire bond in two ways. Some use a flat metal strap from the center neutral terminal to the cabinet. Others use a short green, or green-and-white, wire from the neutral terminal to the cabinet ground screw, and the instructions for a four-wire conversion say to take that wire's end off the ground screw and land it on the neutral terminal with the cord's white conductor, or to remove it, depending on the design. Follow the label on the back of the appliance or the installation instructions for that model; they are what the listing is based on.
The conversion
- Remove the access cover and the old cord. Note which conductor lands where before you loosen anything. Back out the three terminal nuts or screws and the strain relief clamp, and pull the old cord out.
- Remove or relocate the bond. Take off the bonding strap and keep it taped inside the access cover, so the next tech or a future move back to three-wire can reuse it. Or, on a ground-wire design, relocate or remove the wire as the instructions direct. Do not leave a strap hanging on the neutral terminal with its other end loose; it can swing onto a line terminal.
- Fit a listed strain relief. A cord kit for this rating usually includes one; some appliances use a clamp built into the cabinet. It goes in the cord entry hole and clamps on the cord's outer jacket, not on individual conductors. Its job is to keep a pull on the cord from reaching the terminals. A cord with no strain relief, or a strain relief clamped onto bare conductors, is how terminals work loose and burn.
- Use the right cord. Four-wire, rated for the receptacle: a dryer typically a 30 A cord to a 30 A receptacle, a range a 40 A or 50 A cord to match its receptacle, with ring terminals on every conductor. The cord's ampere rating and the receptacle's must match. A bare wire wrapped under a terminal screw is not a termination.
- Land the conductors. Line conductors (black and red, or the two outer conductors) on L1 and L2, in either order on a single-phase 240 V appliance. White on the center neutral terminal. Green on the cabinet ground screw. Tighten to the maker's torque if the label gives one; otherwise firmly, and check each ring will not turn under the nut. Loose terminals at a dryer block are the classic burnt-block call.
- Clamp the strain relief. Snug on the jacket, with the cord running straight out of the cabinet.
- Read it before the cover goes on. The finish test below.
A worked case
An electric dryer moved from an older house into a newer one. The new laundry has a four-slot 30 A receptacle; the dryer has a three-prong cord. The customer has a four-wire 30 A cord kit.
Breaker off, cord unplugged. Receptacle read with a proved meter: all six pairs near 0 V. Terminal block dead.
The old cord comes off. This dryer bonds with a short green-and-white wire from the center terminal to the cabinet screw. The label on the back says, for four-wire connection, take that wire's end off the cabinet screw and land it under the center terminal. That is done, so the green-and-white wire now runs only to the neutral and nothing ties the neutral to the cabinet, and the strain relief from the kit goes into the entry hole.
The new cord lands: black on L1, red on L2, white on the center terminal with the relocated green-and-white wire, green alone under the ground screw. Each ring checked tight. Strain relief clamped on the jacket.
Readings, cord unplugged and the dryer's power switch or timer off:
- Neutral prong to cabinet bare metal: OL.
- Ground prong to cabinet: 0.2 ohms.
- L1 prong to cabinet: OL. L2 prong to cabinet: OL.
- Neutral prong to ground prong: OL.
The neutral is now isolated from the cabinet and the cabinet goes back to the panel only through the green conductor.
Then the cover goes on, the dryer goes back against the wall with the vent reconnected, and the breaker goes on. The receptacle's live readings come first, before the plug goes in, per the receptacle card: about 240 V L1 to L2, about 120 V each L to neutral and to ground, and near 0 V neutral to ground. Then a run on heat for a few minutes.
What would have changed the call. If neutral to cabinet had read near zero after the conversion, the bond is still in place somewhere, a strap, a second jumper, or the green-and-white wire still on the cabinet screw, and the dryer does not get plugged in until it reads OL. If the new laundry had a three-slot receptacle, a three-wire cord goes on with the bond left in place, and neutral to cabinet then reads near zero, which is correct for that connection. If the house were a manufactured home with a three-slot receptacle, the dryer does not go on a three-wire cord at all, and the customer is referred to an electrician for a four-wire circuit.
The finish test
With the cord unplugged and before the access cover goes on:
- Neutral to cabinet: OL on a four-wire connection. This is the reading that proves the bond came off.
- Ground prong to cabinet: near zero ohms.
- Each line prong to cabinet: OL.
- Neutral prong to ground prong: OL.
Write on the ticket: receptacle type, cord rating, bond method and what was done with it, the four readings, and the receptacle's live readings before the plug went in.
References
- NFPA 70, National Electrical Code, Section 250.140 (frames of ranges and clothes dryers), as adopted by your jurisdiction; Article 550 for manufactured homes, as adopted
- NFPA 70E, Standard for Electrical Safety in the Workplace, Article 120, current edition (verifying absence of voltage)
- See related: 240 Volt Appliance Receptacles and What Each Reads; Appliance Electrical Safety Check Before Service
- See related: How to Connect a Dryer to the Wall Vent with a Transition Duct
- See related: How to Upgrade a Three-Wire Dryer Circuit to Four-Wire
- Manufacturer installation instructions and back-panel label for the model: bond method, cord rating and terminal torque