4-Prong to 3-Prong Wiring Diagram
This is a free printable 4 prong to 3 prong wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A 4-prong to 3-prong conversion diagram shows how to reconfigure a dryer terminal block when moving an appliance from a modern NEMA 14-30 outlet to a legacy NEMA 10-30 outlet, including bonding strap requirements and the safety implications.
When moving a dryer purchased after 1996 into a home that has an older 3-wire NEMA 10-30 outlet, the appliance's power cord must be changed from a 4-prong (NEMA 14-30P) to a 3-prong (NEMA 10-30P) configuration. This is a reversal of the direction that electrical codes are pushing — codes mandate 4-wire circuits for new work — but it is a legal and common field situation when the existing outlet cannot be economically upgraded.
A modern dryer with a 4-wire cord has its terminal block configured for separate neutral and ground connections: the white wire goes to the neutral terminal (centre), the green or bare wire goes to the ground terminal, and the two hots (black and red) go to the outer terminals. Crucially, there is a factory-fitted bonding strap or jumper between the neutral terminal and the appliance chassis ground that was removed at the factory before installing a 4-wire cord. For a 3-wire connection, this strap must be reinstalled.
The reason is identical to the original 3-wire design logic: without a separate ground wire, the only path for fault current to return to the panel is through the neutral. The bonding strap connects the chassis to the neutral terminal, so any fault from a live element to the chassis returns via neutral to the panel. Without the strap, the chassis is floating and a fault would leave the chassis at line voltage.
The reverse conversion — going from a 3-wire outlet to a 4-wire cord — is straightforward and safer: remove the bonding strap (or replace it with a separate green ground wire to the chassis ground lug if there is no lug) and run a 4-wire cord to a NEMA 14-30R outlet. The best solution in any home undergoing renovation or panel work is to upgrade the outlet to NEMA 14-30 and avoid this conversion entirely. This diagram is a reference for the conversion, not a recommendation.
How to wire 4 prong to 3 prong wiring diagram
- Isolate power and remove the old cord Unplug the dryer (if accessible) or turn off the dedicated 30 A double-pole breaker. Verify zero volts at the outlet. Remove the rear service panel of the dryer to access the terminal block. Note the position of all wire connections and photograph for reference before disconnecting.
- Locate the terminal block and bonding strap The terminal block has three or four screw terminals (L, N, L from left to right, with a separate ground lug on 4-wire models). Find the bonding strap or bonding wire — it is typically a metallic strap, sometimes with a rubber insulator over it, stored near or partially installed on the neutral terminal. Consult the appliance installation manual for its exact location.
- Reinstall the bonding strap Connect the bonding strap between the centre neutral terminal and the appliance chassis grounding point. Some models use a green screw on the chassis; others have a dedicated lug. The strap creates the combined neutral/ground path required for a 3-wire installation. Tighten to the terminal torque specification.
- Install the 3-wire cord Thread the new NEMA 10-30P 3-wire cord through the strain-relief fitting on the dryer chassis. Connect: black to the left hot terminal (L1), red to the right hot terminal (L2), white to the centre neutral terminal. Do not connect anything to the chassis ground lug — the bonding strap handles that connection. Tighten all terminals to spec.
- Secure the cord and replace the service panel Tighten the strain-relief clamp on the cord so the cord cannot be pulled free from inside. Replace the rear service panel. Plug the dryer into the NEMA 10-30R outlet and restore power. Test the dryer on a full cycle.
Specifications
| Source outlet (4-wire) | NEMA 14-30R: 30 A, 250 V, 4-wire (L1, L2, N, G) |
|---|---|
| Destination outlet (3-wire) | NEMA 10-30R: 30 A, 250 V, 3-wire (L1, L2, N/G combined) |
| Circuit breaker | 30 A double-pole |
| Supply voltage | 240 V split-phase (120 V L1-N, 120 V L2-N, 240 V L1-L2) |
| Bonding strap status for 3-wire | Must be installed — connects neutral terminal to chassis ground |
| Bonding strap status for 4-wire | Must be removed — separate green/bare ground wire used instead |
| NEC reference | NEC 250.140 — prohibits new 3-wire installations; permits conversion of appliance cord for existing grandfathered outlets |
| Cord length typical | 1.8 m (6 ft) to 2.4 m (8 ft) |
Safety warnings
- This conversion moves the appliance to a less safe 3-wire configuration. The recommended solution is always to upgrade the wall outlet to NEMA 14-30 with a proper 4-wire circuit. Only perform this conversion when a 4-wire outlet cannot be installed.
- Never omit the bonding strap when wiring for a 3-wire connection. A missing bonding strap leaves the appliance chassis ungrounded, which is a serious shock hazard if an internal fault develops.
- Always isolate the circuit at the breaker and verify zero volts before working on any appliance wiring. Dryer circuits operate at 240 V, which is sufficient to cause death.
- Do not install a 3-wire NEMA 10-30 outlet in a location that previously had a 4-wire NEMA 14-30 outlet — this is a downgrade that violates NEC 250.140 for new work.
- This conversion should be performed by or reviewed by a licensed electrician, particularly if the installer is not familiar with the appliance's terminal block layout and the location of the bonding strap.
Tools needed
- Insulated screwdrivers (flat and Phillips)
- Torque screwdriver
- CAT III voltage tester
- Pliers (for strain-relief tightening)
- Camera or phone (to photograph original wiring before removal)
Common mistakes
- Forgetting to reinstall the bonding strap — the most dangerous mistake; the chassis becomes ungrounded.
- Connecting the white (neutral) wire to a hot terminal or connecting a hot wire to the neutral terminal, causing an immediate trip or appliance damage on power-up.
- Not using a strain-relief fitting, allowing the cord to be pulled and terminals to loosen over time.
- Purchasing a 4-wire (NEMA 14-30P) cord instead of the required 3-wire (NEMA 10-30P) cord — they are not interchangeable.
- Assuming that because the dryer runs normally the wiring is correct — a missing bonding strap presents no immediate symptom until an internal fault occurs.
Troubleshooting
- Dryer heats on high heat only, or does not heat at all
- Cause: One hot conductor is not connected or is connected to the wrong terminal; only one 120 V leg is present Fix: Isolate circuit. Check that both hot terminals have conductors firmly connected. Measure voltage: should be approximately 240 V across L1 and L2 terminals, and approximately 120 V from each hot terminal to neutral.
- Dryer chassis gives a shock
- Cause: Bonding strap missing or an internal component fault is placing voltage on the chassis Fix: Immediately unplug the dryer. Inspect the terminal block for the bonding strap — if missing, reinstall it. If the strap is present, have a qualified appliance technician inspect the internal wiring for a fault from a live conductor to the chassis.
- Breaker trips immediately on plugging in the dryer
- Cause: Short circuit in the cord or an internal appliance fault; possibly hot-to-hot short from incorrect wiring at the terminal block Fix: Isolate and unplug. With the cord disconnected from the appliance, check resistance across L1 and L2 — should be open. Check from each hot to neutral — should be open or high resistance. Locate the fault in the cord or appliance.
Frequently asked questions
When converting from 4-wire to 3-wire, what must be done to the dryer terminal block?
The bonding strap (or jumper wire) between the neutral terminal and the appliance chassis must be reinstalled. This strap was removed when the factory installed a 4-wire cord. Without it, the chassis has no grounding path in a 3-wire installation.
Is a 4-prong to 3-prong dryer cord conversion legal?
The conversion of the appliance cord is generally permitted when connecting to an existing grandfathered 3-wire NEMA 10-30 outlet. However, running a new 3-wire circuit or replacing a 4-wire outlet with a 3-wire outlet is prohibited under NEC 250.140. Check with your local authority having jurisdiction.
What happens if I forget to reinstall the bonding strap?
The dryer chassis will have no grounding path. If an internal fault connects a live conductor to the chassis, the chassis rises to a dangerous voltage. A person touching the chassis while standing on a grounded surface could receive a serious or fatal electric shock.
What are the four wires in a NEMA 14-30 dryer cord?
Black (L1 hot), red (L2 hot), white (neutral), and green or bare (equipment ground). The NEMA 10-30 3-wire cord has only black (L1), red (L2), and white (neutral/ground combined) — there is no separate ground wire.
Is there a safer alternative to the 4-to-3 prong conversion?
Yes. The preferred solution is to upgrade the wall outlet from NEMA 10-30 to NEMA 14-30 by running new 4-wire cable from the panel. This keeps the appliance in a safer 4-wire configuration. In some cases, NEC 250.140 permits an alternative using a GFCI breaker with a 3-wire circuit — consult a licensed electrician.
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