When wiring an off-grid cabin, a marine vessel, or a backup DC system, you frequently need to select between two independent power sources—like a primary solar battery bank and a backup generator battery bank—without ever paralleling them. The solution is a Double Pole, Double Throw (DPDT) center-off toggle switch. An on off on switch wiring diagram for this application routes both the positive and negative conductors through the switch, providing a physical air-gap disconnect in the center position. Below is the exact terminal mapping, node-by-node trace, and verification procedure for wiring a 35A DPDT ON-OFF-ON switch (such as the Blue Sea Systems 9001e) for a 24V DC dual-source transfer application.

Terminal Mapping and Schematic Symbols

Before cutting any wire, you must understand the physical layout of the six terminals on the back of a DPDT switch. In an ON-OFF-ON configuration, the switch has three physical positions: UP (Source A connected), CENTER (All disconnected), and DOWN (Source B connected). The table below maps the physical terminals to their schematic symbols and operational states.

Terminal Number Physical Location Schematic Symbol Meaning State: UP (Position 1) State: DOWN (Position 3)
1 Top Left Source A Input (Pole 1 Throw 1) Closed (Connected to 2) Open
2 Middle Left Common Output (Pole 1 Wiper) Receives from 1 Receives from 3
3 Bottom Left Source B Input (Pole 1 Throw 2) Open Closed (Connected to 2)
4 Top Right Source A Input (Pole 2 Throw 1) Closed (Connected to 5) Open
5 Middle Right Common Output (Pole 2 Wiper) Receives from 4 Receives from 6
6 Bottom Right Source B Input (Pole 2 Throw 2) Open Closed (Connected to 5)

Decoding the Diagram Symbols

In a standard on off on switch wiring diagram, the DPDT switch is represented by two parallel "bowtie" or "wiper" symbols. The center dots of the bowties are the wipers (Terminals 2 and 5). The outer dots are the throws (1, 3, 4, and 6). A dashed mechanical line connects the two wipers, indicating they are actuated by the same physical toggle lever. When the diagram shows the wipers pointing to the top nodes, it represents the UP position; pointing to the bottom nodes represents the DOWN position. The center-off state is implied when the wipers are drawn resting in the open space between the throws.

Bench Tip: Never assume terminal numbering is universal across brands. While Carling and Blue Sea typically use the 1-2-3 (left) and 4-5-6 (right) layout, some cheap imported switches use a circular 1-through-6 pattern. Always verify with a multimeter in continuity mode before crimping.

Node-by-Node Circuit Trace: Source to Load

For this walkthrough, we are wiring a 24V DC Shurflo water pump (draws 10A max) that can be powered by either a Solar Battery Bank (Source A) or a Generator Battery Bank (Source B). We are using 8 AWG marine-grade tinned copper wire with adhesive-lined heat shrink crimps to prevent corrosion and voltage drop. According to Blue Sea Systems circuit protection guidelines, both the positive and negative conductors must be switched in a floating DC system to prevent stray current corrosion, which is why we use a DPDT switch rather than an SPDT.

The Positive (Hot) Trace

  1. Node 1 (Solar Source): 8 AWG Red wire leaves the Solar Bank positive busbar, passes through a 40A Class T fuse, and terminates on Terminal 1.
  2. Node 2 (Generator Source): 8 AWG Red wire leaves the Generator Bank positive busbar, passes through a 40A Class T fuse, and terminates on Terminal 3.
  3. Node 3 (Switch Wiper): The internal wiper for Pole 1 is Terminal 2. An 8 AWG Red wire leaves Terminal 2 and routes to the load.
  4. Node 4 (Load Positive): The Red wire from Terminal 2 terminates at the positive input terminal of the water pump.

The Negative (Return) Trace

  1. Node 5 (Solar Return): 8 AWG Black wire leaves the Solar Bank negative busbar and terminates on Terminal 4.
  2. Node 6 (Generator Return): 8 AWG Black wire leaves the Generator Bank negative busbar and terminates on Terminal 6.
  3. Node 7 (Switch Wiper): The internal wiper for Pole 2 is Terminal 5. An 8 AWG Black wire leaves Terminal 5 and routes to the load.
  4. Node 8 (Load Negative): The Black wire from Terminal 5 terminates at the negative input terminal of the water pump.
Safety & Code Caveat: Polarity and Ground Path
Do not confuse the DC Negative return (current-carrying conductor) with the Equipment Grounding Conductor (EGC). The Black wires on Terminals 4, 5, and 6 are current-carrying return paths. The water pump's metal chassis must still be bonded to the main DC ground busbar using a separate Green (or bare) EGC wire. This ensures that if a positive wire chafes against the pump casing, the fault current has a low-impedance path back to the source to trip the breaker, rather than energizing the chassis. Refer to NFPA 70 (NEC) Article 690 for specific off-grid DC grounding and bonding requirements in your jurisdiction.

Multimeter Verification and Voltage Drop Testing

Once the physical wiring is complete and all crimps are heat-shrunk, you must verify the circuit before applying full load. Grab your digital multimeter (DMM) and follow this exact decision path.

Step 1: Dead-Front Continuity Check (Power OFF)

Ensure both battery bank disconnects are OFF. Set your DMM to Continuity (beep) mode.

  • Test UP Position: Place probes on Terminal 1 and Terminal 2. You should hear a beep (< 1 ohm). Move probes to Terminal 4 and Terminal 5. Beep. Verify Terminals 3 and 6 show OL (Open Line).
  • Test CENTER Position: Place probes across any input-to-output combination (e.g., 1 to 2, 3 to 2). All should read OL. This confirms your physical air-gap disconnect.
  • Test DOWN Position: Place probes on Terminal 3 and Terminal 2. Beep. Move probes to Terminal 6 and Terminal 5. Beep. Verify Terminals 1 and 4 show OL.

Step 2: Live Voltage Verification (No Load)

Turn ON both battery bank disconnects. Set your DMM to DC Volts. Leave the switch in the CENTER (OFF) position.

  • Measure across Terminal 1 (+) and Terminal 4 (-). You should read nominal solar bank voltage (e.g., 25.6V for a resting 24V LiFePO4 bank).
  • Measure across Terminal 3 (+) and Terminal 6 (-). You should read nominal generator bank voltage.
  • Measure across Terminal 2 (+) and Terminal 5 (-). You must read 0.00V. If you read voltage here, your switch wipers are failing to disconnect, or you have a short downstream.

Step 3: Voltage Drop Testing (Under Load)

This is the step most DIYers skip, leading to melted terminals and starved motors. Flip the switch to the UP position to engage the solar bank. Turn on the water pump so it draws its full 10A load.

  1. Set your DMM to DC Millivolts (mV) or low-range Volts.
  2. Place the Red probe on the Solar Bank positive busbar (upstream of the fuse) and the Black probe directly on the switch's Terminal 1 lug.
  3. Place the Red probe on Terminal 2 and the Black probe on the pump's positive input terminal.
  4. Sum the two readings. According to ABYC and NEC best practices, the total voltage drop on the positive side should not exceed 3% of the system voltage (0.72V on a 24V system). If your drop is higher, you have a bad crimp, undersized wire, or a corroded terminal lug.
  5. Repeat the exact same process on the negative side (Terminals 4, 5, and the negative busbars).

By strictly following this on off on switch wiring diagram trace and verification protocol, you ensure that your dual-source DC system operates safely, prevents cross-banking, and delivers full voltage to your loads without thermal degradation at the switch terminals.