A 208V single-phase circuit utilizes two ungrounded conductors (L1 and L2) derived from a 120/208V three-phase Wye transformer. This configuration delivers 208V line-to-line and 120V line-to-ground. For a standard 6kW commercial hardwired unit heater on this circuit, the direct answer for your installation is a 2-pole 40A breaker and 8 AWG THHN copper wire with a 10 AWG equipment grounding conductor.
Below is a complete, decision-forward walkthrough of the wiring diagram, tracing the path from the panel to the heating elements, mapping the physical terminals, and providing the exact math to size your components.
Decoding the Diagram Symbols and 208V Power Source
Before tracing the wires, you must understand the schematic symbols specific to commercial HVAC and heating diagrams. A 208V single-phase system is not a standalone phase; it is a line-to-line extraction from a three-phase Wye source. Because the voltage vectors are 120 degrees apart, the line-to-line voltage is 208V (120V × √3), not 240V.
- 2-Pole Breaker: Two parallel switch symbols tied by a dashed line (indicating common trip).
- Contactor Coil: A rectangle labeled with A1 and A2 terminals.
- Thermal Limit Switch: A circle with a diagonal slash, normally closed (NC).
- Control Transformer: Two overlapping circles (primary) and two smaller overlapping circles (secondary).
- Heating Element: A jagged resistor symbol or a straight line inside a grounded chassis box.
Node-by-Node Trace: Source to Load and Ground Path
We will trace a 6kW commercial unit heater equipped with a built-in 24V control transformer and a magnetic contactor. This is the most common configuration for hardwired commercial spaces.
The Power Path (Line-to-Line)
- Source: Power originates at the 2-pole 40A breaker in the 120/208V panelboard. L1 (Black) and L2 (Red) exit the breaker.
- Disconnect: Both conductors pass through a local fused or unfused disconnect switch (required by NEC 424.19 for servicing).
- Contactor Primary: L1 and L2 land on the line-side (L1, L2) of the heavy-duty magnetic contactor.
- Control Transformer Primary: In parallel with the contactor line-side, L1 and L2 also connect to the primary terminals (H1, H2) of the 208V-to-24V step-down transformer.
- Load Switching: When the thermostat calls for heat, the contactor coil energizes, pulling the contacts closed. Power flows through the load-side (T1, T2) of the contactor directly into the resistive heating elements.
The Control Path (24V Secondary)
The transformer secondary outputs 24V AC. The R (Red) wire goes to the mechanical wall thermostat. The thermostat acts as a simple switch. When closed, it sends 24V back via the W (White) wire to the A1 terminal of the contactor coil. The A2 terminal of the coil returns to the transformer C (Common) terminal.
The Ground Path (Explicit)
The Equipment Grounding Conductor (EGC) is a 10 AWG bare or green copper wire. It runs continuously from the panel's ground bar, through the disconnect, and lands directly on the PE (Protective Earth) or chassis ground lug of the heater. The ground path is never switched, never fused, and carries zero current during normal operation. Its sole purpose is to provide a low-impedance fault path to trip the breaker if a live wire touches the metal chassis.
Physical Terminal and Pin Mapping Table
When you open the physical heater junction box, the schematic nodes translate to specific screw terminals. Use this mapping to ensure correct termination.
| Diagram Node | Physical Terminal Label | Wire Color (NEC) | Wire Size (AWG) | Function / Polarity Note |
|---|---|---|---|---|
| Breaker Pole 1 | L1 (Line In) | Black | 8 AWG THHN | Ungrounded conductor. Polarity interchangeable on resistive load. |
| Breaker Pole 2 | L2 (Line In) | Red | 8 AWG THHN | Ungrounded conductor. 120° out of phase with L1. |
| Panel Ground Bar | PE / Ground Lug | Green / Bare | 10 AWG Copper | Equipment ground. Must be continuous and unswitched. |
| Transformer Primary | H1 / H2 | Black / Red (Taps) | 14 AWG (Internal) | 208V tap. Polarity does not matter for standard AC primary. |
| Transformer Secondary | X1 (Hot) / X2 (Com) | Red (R) / White (C) | 18 AWG | 24V AC control voltage. X1 goes to thermostat, X2 to coil. |
| Contactor Coil | A1 / A2 | White (W) / White (C) | 18 AWG | A1 receives switched 24V from thermostat. A2 returns to X2. |
Verifying Connections with a Digital Multimeter
Never assume the panel labeling is correct. Before terminating the heater, you must verify the supply voltage with a Category III or IV digital multimeter. This step prevents catastrophic failure if the circuit is actually a 240V Delta system mislabeled as 208V.
Follow this exact measurement sequence at the load side of the disconnect switch (with the heater wires disconnected and wire-nutted for safety):
- Measure L1 to L2: Set meter to AC Volts. Probe the Black and Red wires.
Expected Reading: 208V (Acceptable range: 200V - 214V). If you read 240V, you are on a Delta system. Stop. The 208V heater elements will overheat and fail on 240V. - Measure L1 to Ground: Probe Black to the bare ground wire.
Expected Reading: 120V. - Measure L2 to Ground: Probe Red to the bare ground wire.
Expected Reading: 120V.
The High-Leg Delta Trap: If you read 208V on L1-to-Ground, and 120V on L2-to-Ground, you have accidentally tapped the "high leg" (wild leg) of a 240V Delta system. A 208V single-phase heater cannot be safely wired to a high-leg delta. You must relocate the breaker to two standard phases in the panel.
Decision Tree: Sizing the Breaker and Wire for 208V Loads
Sizing for 208V requires strict adherence to NEC Article 210 (Branch Circuits) and Article 424 (Fixed Electric Space Heating Equipment). Because commercial heaters typically run for three hours or more, they are classified as continuous loads, requiring a 125% sizing multiplier.
Use this decision matrix to finalize your materials list for a 6kW (6000W) heater:
| Decision Step | Calculation / Rule | Result |
|---|---|---|
| 1. Calculate Base Amperage | I = P / V → 6000W / 208V | 28.84 Amps |
| 2. Apply Continuous Load Multiplier | NEC 424.3(B): Base Amps × 1.25 | 28.84 × 1.25 = 36.05 Amps |
| 3. Select Standard Breaker Size | NEC 240.6: Next standard size up from 36.05A | 40 Amp 2-Pole Breaker |
| 4. Size Ungrounded Conductors | NEC 310.16 (75°C column): Ampacity must exceed 36.05A | 8 AWG THHN Copper (Rated 50A) |
| 5. Size Equipment Ground | NEC 250.122: Based on 40A breaker rating | 10 AWG Copper |
The Final Pick
Do not downsize the breaker to 30A to 'save money' or 'match the wire'—a 30A breaker will nuisance-trip under continuous load, and a 35A breaker is non-standard for most residential/commercial panelboards.
Concrete Material List:
Purchase an Eaton BR240 (or Square D HOM240 for Homeline panels) 2-pole 40A breaker. Pull three strands of wire through your conduit: Black and Red 8 AWG THHN for the current-carrying conductors, and a Green 10 AWG THHN (or bare 10 AWG if local AHJ permits bare in conduit) for the equipment ground. Terminate the 8 AWG wires on the heater's L1 and L2 lugs, torque to the manufacturer's spec (typically 20-25 in-lbs for #8 wire), and land the 10 AWG on the PE chassis lug.
For further reading on commercial heating circuit requirements, refer to the NFPA National Electrical Code guidelines and Fluke's technical breakdown of three-phase and 208V power systems.
