Middle of Run Double Outlet Wiring Diagram: Complete Trace & Terminal Guide

Wiring a duplex receptacle in the middle of a circuit run means the device must act as both a termination point for incoming power and a junction for downstream loads. Unlike an end-of-run outlet where you only connect one cable, a middle of run double outlet wiring diagram requires you to manage two separate sets of conductors (LINE and LOAD) on a single yoke. The direct answer for standard 120V residential wiring is to connect the incoming black and white wires to one set of brass and silver screws, the outgoing black and white wires to the remaining brass and silver screws, and splice all bare ground wires together with a pigtail to the green grounding screw.

Reading the Schematic Symbols

Before tracing the physical wires, you need to understand what the standard electrical symbols in a middle of run double outlet wiring diagram actually mean. Misinterpreting a schematic symbol is the most common reason DIYers wire a pass-through circuit incorrectly.

• Two Parallel Lines (| |): Represents the duplex receptacle itself. In a schematic, this symbol sits on the branch circuit line.
• Solid Black Dot (•): Indicates a mechanical and electrical splice (a wire nut connection). If two lines cross without a dot, they do not connect.
• Circle with a Cross (⊗): Often used in detailed wiring diagrams to represent a wire connector or wire nut joining multiple conductors.
• Diagonal Hash Marks (///): Denotes the number of conductors inside a single cable sheath. Three hash marks on a line mean a 2-wire cable with a ground (Black, White, Bare).
• Arrow Pointing Left/Right: Indicates the direction of power flow. Left-pointing or incoming arrows denote the LINE (source); right-pointing or outgoing arrows denote the LOAD (downstream).

Node-by-Node Trace: Source to Load

Let's trace the current path from the breaker panel, through the middle-of-run receptacle, and out to the next device. This trace assumes a standard 15A or 20A branch circuit using 14 AWG or 12 AWG NM-B (Romex) cable.

1. The Source (Panel to LINE Cable): Power leaves the 15A/20A single-pole breaker on the hot bus bar, traveling via the black conductor of the incoming (LINE) NM-B cable. The white neutral connects to the neutral bus bar, and the bare ground connects to the grounding bus bar.
2. Entering the Box: The LINE cable enters the single-gang box through a cable clamp. You strip back 3/4 inch of insulation from the black and white conductors.
3. Hot Path (LINE): The incoming black (hot) wire loops clockwise around the top brass screw on the receptacle and is tightened to 14 in-lbs of torque.
4. Hot Path (LOAD): The outgoing black wire of the LOAD cable loops clockwise around the bottom brass screw. The brass fin (break-off tab) on the side of the yoke remains intact, electrically bonding the top and bottom brass screws.
5. Neutral Path (LINE): The incoming white (neutral) wire connects to the top silver screw.
6. Neutral Path (LOAD): The outgoing white wire connects to the bottom silver screw. The silver fin tab remains intact.
7. The Ground Path: The bare copper wires from both the LINE and LOAD cables are twisted together with a 6-inch bare copper pigtail. A wire nut secures the splice. The free end of the pigtail loops clockwise around the green grounding screw on the receptacle. If using a metal box, a second pigtail must bond the metal box to this ground splice.
8. Exiting to Downstream: The LOAD cable exits the box, carrying the pass-through current to the next outlet, switch, or fixture in the daisy chain.

Terminal and Pin Mapping Table

Physical devices like the ubiquitous Leviton 5320-W 15A duplex receptacle have specific terminal layouts. Use this spec-sheet table to ensure every conductor lands on the correct metal contact. Never use the push-in 'backstab' holes for middle-of-run pass-through wiring; they are prone to high-resistance arcing failures under continuous downstream loads.

Verifying Connections with a Multimeter

Once the wires are landed and the device is pushed back into the box, do not blindly flip the breaker. Use a digital multimeter (DMM) to verify the integrity of your middle of run double outlet wiring diagram.

1. Continuity Check (Power OFF): Set your DMM to the continuity (beep) setting. Place one probe on the receptacle's green ground screw and the other on the bare ground wire in the LOAD cable. You should read less than 1 ohm, confirming the ground path is continuous to the next box.
2. Short Circuit Check (Power OFF): With the DMM still on continuity, test between the black (hot) and white (neutral) wires of the LOAD cable. The meter should read 'OL' (Open Line). If it beeps, you have a dead short downstream—do not energize.
3. Voltage Verification (Power ON): Turn the breaker on. Set the DMM to AC Voltage (V~). Measure from the top brass screw to the silver screw. You should read between 114V and 126V (nominal 120V).
4. Polarity and Ground Check (Power ON): Measure from the brass screw to the green ground screw. It must read the same 114V-126V. Finally, measure from the silver (neutral) screw to the green ground screw. This should read less than 2V. If neutral-to-ground reads 120V, your hot and neutral are reversed.

Frequently Asked Questions

Can I use the push-in backwire holes for a middle of run double outlet?

No. The push-in 'backstab' terminals on standard 15A receptacles rely on a small internal spring clip that makes minimal contact with the wire. When used in a middle-of-run configuration, the downstream load current must pass through this tiny clip, which frequently leads to high-resistance connections, melting, and voltage drop. Always use the side-wire terminal screws or, if your receptacle supports it, the screw-and-clamp backwire plates that physically trap the wire under a metal plate.

Do I need to pigtail the neutrals on a middle of run receptacle?

For a standard 2-wire (one hot, one neutral) 120V branch circuit, the NEC allows you to use the receptacle's silver screws to pass the neutral through to the next device. However, if your circuit is a Multiwire Branch Circuit (MWBC)—meaning it shares a single neutral between two different hot phases (L1 and L2)—NEC 300.13(B) strictly requires you to pigtail the neutral. If you break the neutral connection by removing the receptacle on an MWBC, the downstream devices will be subjected to 240V, instantly destroying electronics and creating a fire hazard.

What happens if I swap the line and load wires on a standard duplex outlet?

On a standard, non-GFCI/AFCI duplex receptacle, swapping the LINE (incoming) and LOAD (outgoing) cables on the brass and silver screws makes zero functional difference. The internal brass and silver fin tabs bridge the top and bottom screws, so power will flow to both the receptacle slots and the downstream devices regardless of which physical screw gets the incoming power. However, if you are wiring a GFCI or AFCI receptacle in the middle of a run, swapping LINE and LOAD will cause the device to trip immediately or fail to protect downstream outlets, as these devices contain internal sensing circuitry that strictly monitors directional current flow.

Why does my downstream outlet read 105V when the middle-of-run outlet reads 120V?

This indicates severe voltage drop, almost always caused by a high-resistance connection at the middle-of-run receptacle. The most common culprit is a loose side-wire screw, a wire looped counter-clockwise (which pushes the wire out as the screw tightens), or a damaged backstab connection. Turn off the breaker, pull the receptacle out, and check that the bare copper is not pinched under the brass or silver screws, and that the wire loops are tight and clockwise. Use a torque screwdriver set to the manufacturer's spec (usually 14 in-lbs) to ensure a gas-tight mechanical bond.