A three-phase 480 to 120/240 transformer wiring configuration steps down 480V delta primary power to a 240V delta secondary with a center-tapped winding, providing both 240V three-phase for heavy machinery and 120V single-phase for standard receptacles, while creating a 208V 'high leg' that must be carefully managed.

In a real installation, this setup changes the panelboard layout by forcing you to physically isolate the high-leg (usually Phase B) from any single-pole 120V breakers, which eliminates the need to install and wire a separate single-phase transformer just for lighting and office loads. The most common confusion in the field is mixing this up with a 120/208V Wye (Y) system; while both provide 120V to neutral, the Wye system gives 120V on all three phases to ground, whereas the 120/240V Delta only gives 120V on two phases and a dangerous 208V to ground on the third.

The Core Architecture: Delta-Delta and the Center Tap

To understand this wiring, you have to look at the physical windings inside the transformer tank. The primary side is a standard 3-wire Delta configuration connected to the 480V utility or upstream switchgear. There is no neutral on the primary side.

On the secondary side, the three 240V windings are also wired in a Delta (triangle) configuration. However, one of those three windings has a physical wire tapped exactly at its electrical midpoint. This center tap is bonded to ground and becomes your neutral (X0). Because it splits that specific 240V winding in half, you get 120V from either end of that winding to the neutral.

Phase-to-Ground Voltage Map (Secondary Side)
  • Phase A to Neutral: 120V
  • Phase C to Neutral: 120V
  • Phase B (High Leg) to Neutral: 208V
  • Any Phase to Phase (A-B, B-C, A-C): 240V

The high leg (Phase B) is the corner of the delta triangle opposite the center-tapped winding. Using basic trigonometry, the voltage from this opposite corner to the center tap is calculated by multiplying the half-winding voltage (120V) by the square root of 3 (1.732), yielding 207.8V, which we round to 208V in the field. For a deeper dive into the vector math behind three-phase transformer winding geometries, the All About Circuits textbook on three-phase transformers provides excellent phasor diagrams.

Worked Numeric Example: Sizing and Load Balancing

Let’s size a transformer for a small CNC machine shop that requires both heavy 3-phase equipment and standard 120V shop lighting. We will calculate the required kVA and the resulting primary and secondary currents.

The Loads:

  • 3-Phase 240V Loads: Two CNC mills drawing a combined 18,000 VA (18 kVA).
  • 1-Phase 120V Loads: LED lighting and receptacles drawing a combined 6,000 VA (6 kVA).

Step 1: Calculate Total kVA
Total kVA = 18 kVA (3-phase) + 6 kVA (1-phase) = 24 kVA.
Applying a standard 125% safety margin for continuous loads and future expansion: 24 kVA × 1.25 = 30 kVA. We will select a standard 30 kVA dry-type transformer.

Step 2: Calculate Primary Current (480V Delta)
Formula: I = VA / (Voltage × √3)
I = 30,000 / (480 × 1.732) = 36.1 Amps.
Wire Sizing Note: You would typically feed this with 8 AWG THHN copper conductors (rated 50A at 75°C) protected by a 40A or 45A 3-pole breaker.

Step 3: Calculate Secondary Current (240V Delta)
I = 30,000 / (240 × 1.732) = 72.2 Amps per phase.
Wire Sizing Note: The secondary feeders to the panelboard would require 3 AWG THHN copper (rated 100A at 75°C).

Step 4: Size the Neutral Conductor
The neutral in a delta system does not carry the 3-phase motor current; it only carries the unbalanced 120V single-phase load. If the maximum unbalanced 120V load on the neutral is 4,000 VA, the neutral current is 4,000 / 120 = 33.3 Amps. A 10 AWG or 8 AWG copper neutral is sufficient, but many electricians pull a full-size 3 AWG neutral to match the phase conductors for mechanical durability and to handle future panel expansions.

Where You Meet This in Practice

You will almost exclusively encounter three phase 480 to 120/240 transformer wiring in older industrial parks, retrofitted manufacturing facilities, and specific agricultural operations. Modern commercial construction heavily favors the 480Y/277 to 120/208V Wye configuration because it eliminates the high-leg hazard and allows 277V lighting directly from the panel. However, the 120/240V Delta remains prevalent where existing 240V 3-phase motors must be preserved, but the facility still needs 120V for control circuits and standard NEMA 5-15 receptacles.

NEC Color Code and Marking Mandate
According to NEC Articles 215.8, 230.56, and 250.97, the high-leg (wild leg) conductor must be durably marked. The code explicitly requires the color Orange for the high leg. If you are wiring a new panelboard fed by this transformer, the bus stab corresponding to Phase B must be identified, and any wire landing on it must be wrapped in orange tape or use orange THHN insulation. Never use orange for a standard phase in a Wye system to avoid catastrophic cross-contamination of color codes.

When pulling wire in these facilities, you must physically arrange your single-pole 120V breakers to land only on the A and C phase bus stabs. Panelboard manufacturers often sell 'high-leg delta' specific panels where the B-phase stabs are physically blocked or spaced differently to prevent a 1-pole breaker from accidentally clipping the 208V high leg.

Common Wiring Mistakes and How to Avoid Them

1. Landing a 120V Circuit on the High Leg
This is the most frequent and destructive error. If an apprentice lands a standard 120V lighting circuit on Phase B, the breaker will not trip immediately because it is a thermal-magnetic device rated for 240V. However, the connected 120V ballasts, LED drivers, or appliances will instantly see 208V to ground. This results in immediate component failure, melted wire insulation, and potential fire. Fix: Always verify phase-to-neutral voltage with a multimeter before terminating 120V loads in an unfamiliar panel.

2. Incorrectly Bonding the Neutral
The center tap (X0) must be bonded to the transformer enclosure and the equipment grounding conductor at the transformer itself, creating the main bonding jumper for this separately derived system. If you float the neutral (fail to bond it) and a ground fault occurs on the 120V side, the fault current has no low-impedance path back to the source, meaning your 120V breakers will not trip during a short circuit. For comprehensive grounding and bonding verification steps, refer to the Fluke guide on testing high-leg delta systems.

3. Sizing the Primary Breaker Too Small
Transformers experience massive inrush currents (magnetizing current) when first energized, often 8 to 12 times the rated full-load current for a few cycles. If you size the primary 480V breaker exactly to the 36.1A full-load calculation, it will trip instantly every time you close the switch. NEC Table 450.3(B) allows primary overprotection up to 250% for transformers over 9 amps. For our 36.1A primary, a standard 80A or 90A breaker is required to handle the inrush without nuisance tripping.

Frequently Asked Questions

Can I use a 480 to 120/240 transformer for single-phase loads only?

Yes, but it is highly inefficient and unbalances the transformer. If you only need 120/240V single-phase power, you should use a single-phase 480V to 120/240V transformer (which has two primary windings and two secondary windings). Using a 3-phase delta transformer for purely single-phase loads means you are paying for the weight, copper, and core losses of a third winding that is doing no work, and you will severely unbalance the upstream 480V utility feed.

Why is the high leg 208V to ground instead of 120V?

It comes down to the physical geometry of the delta winding. The center tap divides one 240V winding into two 120V segments. The high leg is connected to the opposite corner of the triangle. The voltage from that opposite corner to the center tap forms the hypotenuse of a right triangle in a phasor diagram. Using the Pythagorean theorem and 3-phase vector angles, the math dictates that 120V multiplied by the square root of 3 (1.732) equals 207.8V. It is a mathematical inevitability of the delta configuration, not a transformer defect.

What is the NEC color code for the high leg in a 120/240V delta system?

The National Electrical Code (NEC) strictly mandates that the high-leg conductor be identified by the color Orange. This applies to the wire insulation, phase tape, and the busbar marking inside the panelboard. If the installation is in a conduit system where wire colors aren't visible, the orange identification must be applied at every termination point, splice box, and panel. This rule exists specifically to prevent electricians from accidentally using the 208V leg for 120V circuits.

How does this differ from a 480Y/277 to 120/208V transformer?

The 120/208V system uses a Wye (Y) secondary configuration. In a Wye system, all three windings share a common center point (the neutral). This means the voltage from any of the three phases to neutral is exactly 120V (208V / √3). There is no high leg, no 208V-to-ground hazard, and you can land a 120V single-pole breaker on any of the three phase bus stabs. The trade-off is that a 120/208V system only provides 208V for 3-phase motors, whereas the 120/240V Delta provides the full 240V, which is required by many older, heavy-duty industrial machines and specific HVAC compressors.