Secondary voltage is the alternating current (AC) potential difference induced across the output winding of a transformer, determined strictly by the primary input voltage and the physical turns ratio of the coils. When you wire a transformer, this output value dictates your downstream breaker sizing, wire gauge, and the exact type of loads you can connect, while simultaneously providing galvanic isolation from the primary supply. If you get this wrong, you either fry your control boards or starve your motors.
What Secondary Voltage Actually Is (And What It Isn't)
Think of a transformer's turns ratio like the gear ratio on a bicycle. The primary winding is the front chainring, and the secondary winding is the rear cog. The secondary voltage is simply the speed of the rear wheel—it is entirely dependent on how hard you pedal (primary voltage) and the physical size of the gears (turns ratio). It is not a fixed, independent power source; if your primary voltage sags by 5%, your secondary voltage sags by exactly 5%.
According to foundational electromagnetic theory outlined in resources like All About Circuits, the relationship is defined by the equation:
V_secondary = V_primary × (N_secondary / N_primary)
Where N represents the number of physical wire turns on each coil. This means secondary voltage is a passive, proportional result. You cannot "dial in" a secondary voltage on a standard fixed-ratio transformer without either changing the primary input voltage or physically altering the tap connections on the primary winding.
The Math: A Worked Numeric Example
Let's look at a real-world jobsite scenario: You are installing a 5kVA, single-phase distribution transformer to power a small 120/240V lighting and receptacle subpanel from a 480V commercial feed.
- Primary Voltage: 480V AC
- Secondary Voltage (Nameplate): 120/240V AC (Center-Tapped)
- Total Rating: 5,000 VA (5kVA)
First, we find the turns ratio. Since the full secondary winding outputs 240V:
Turns Ratio = 480V / 240V = 2:1
Next, we calculate the maximum secondary current to size our downstream Overcurrent Protective Device (OCPD) and wire gauge.
I_secondary (at 240V) = 5,000 VA / 240V = 20.83 Amps
According to NEC-style guidance for transformer secondary conductors, you size the breaker at 125% of the continuous secondary current:
20.83A × 1.25 = 26.04A
The Concrete Pick: You will use a 30A double-pole breaker and 10 AWG THHN copper conductors (rated 35A at 75°C) for the 240V secondary feeders.
Where You Meet This in Practice
You will encounter secondary voltage requirements across three main domains in electrical and electronics work:
- HVAC and Smart Home Control (24VAC): Thermostats, relays, and irrigation solenoids run on 24VAC. The transformer steps down 120VAC line voltage to a 24VAC secondary. These are typically Class 2 circuits, meaning the secondary voltage is inherently current-limited to prevent fire hazards.
- Industrial Machine Control (120VAC from 480V): Heavy machinery runs 3-phase 480V motors, but the PLCs, contactor coils, and indicator lights require 120VAC. A control transformer steps down one leg of the 480V to a 120V secondary, isolating the sensitive logic from high-voltage motor noise.
- Solar and Battery Inverters (Step-Up): Grid-tie inverters take a lower DC bus voltage (often rectified to high-voltage DC, then chopped into AC) and use a step-up transformer to achieve a 208V or 480V secondary voltage to match the commercial utility grid.
Decision Tree: Selecting the Right Transformer
Do not guess your transformer specs. Use this decision matrix to lock in the exact secondary voltage, VA rating, and part number for your application.
| If Your Load Requirement Is... | Then Select This Transformer Type | Concrete Part Number Pick |
|---|---|---|
| < 40VA, 24VAC secondary for HVAC/thermostats | Class 2 Hub-Mount Control Transformer with PTC reset | Honeywell AT874D (40VA, 120V to 24V) |
| 100VA - 500VA, 120VAC secondary for PLCs/relays on 480V machines | Industrial Control Transformer with primary/secondary fuse blocks | Hammond 527E (250VA, 480V to 120V) |
| Up to 5kVA, 120/240V secondary for residential subpanels from commercial feeds | Single-Phase Distribution Transformer, copper wound, NEMA 3R | Acme Electric T-1-50138 (5kVA, 480V to 120/240V) |
| 15kVA+, 120/208V secondary for 3-phase office/commercial loads | 3-Phase Delta-Wye Distribution Transformer (K-factor rated) | Eaton V15K0022T (15kVA, 480V Delta to 208Y/120V) |
Voltage Regulation and Tap Settings
One of the most frequent bench and jobsite complaints is measuring the secondary voltage with a multimeter and finding it "too high." You wire up a 240V to 24V HVAC transformer, measure the secondary with no load connected, and read 28V. Is the transformer defective? No. You are witnessing voltage regulation.
As detailed in Electronics Tutorials, transformers are designed with internal impedance. To ensure the secondary voltage is exactly 24V at full rated load, manufacturers intentionally wind the secondary with slightly more turns to compensate for the voltage drop that occurs when current flows. At no-load (zero current), there is no internal voltage drop, so the secondary voltage spikes above nominal.
How to fix secondary voltage sag or swell using taps:
Most transformers over 1kVA feature primary tap terminals labeled FCAN (Full Capacity Above Normal) and FCBN (Full Capacity Below Normal). These are physical connection points on the primary winding that alter the turns ratio.
- If your primary voltage is consistently high (e.g., 495V on a 480V nominal feed): Your secondary voltage will be proportionally high. Move the primary wire to the FCAN tap (e.g., 504V). This adds primary turns, lowering the turns ratio, and brings your secondary voltage back down to nameplate specs.
- If your primary voltage is consistently low (e.g., 450V on a 480V feed): Your secondary voltage will sag under load. Move the primary wire to the FCBN tap (e.g., 456V). This removes primary turns, increasing the secondary output.
Frequently Asked Questions
Can I use a step-up transformer in reverse to get a step-down secondary voltage?
Electrically, yes. A 120V to 480V step-up transformer will output 120V on the original primary winding if you feed 480V into the original secondary winding. However, you must verify that the original primary winding's wire gauge can handle the new, higher secondary current, and that the original secondary winding's insulation rating is sufficient for the new primary voltage. Furthermore, NEC and local AHJ rules often prohibit backfeeding standard distribution transformers unless they are specifically listed and marked for reverse-feed operation.
Why does my secondary voltage drop when the motor starts?
Motors draw Locked Rotor Amperage (LRA), which can be 5 to 7 times their running current, for a few hundred milliseconds during startup. This massive current spike causes a severe voltage drop across the internal impedance of the transformer windings and the supply wires. If your secondary voltage drops below 85% of nominal during startup, the motor contactor may chatter or drop out. The fix is not to change the transformer taps, but to upsize the transformer's kVA rating to lower its internal impedance percentage.
Does secondary voltage affect the grounding scheme?
Absolutely. The secondary voltage dictates your system grounding. For a standard 120/240V single-phase secondary, you must bond the center-tap (neutral) to the transformer case and the grounding electrode system. For a 480V Delta secondary, there is no neutral; you must either run it as an ungrounded system (with ground-fault monitoring) or create an artificial neutral using a corner-grounded delta or a zig-zag grounding transformer. Never assume the secondary ground reference is identical to the primary ground reference—the transformer provides galvanic isolation, meaning the secondary is a completely separate, newly derived electrical system.
