A transformer on load is a transformer whose secondary winding is connected to an electrical load, causing secondary current to flow and forcing the primary winding to draw additional current from the source to maintain the core's magnetic flux. When you transition a transformer from no-load to on-load, two immediate changes occur in your installation: the secondary terminal voltage drops (or occasionally rises) due to internal winding impedance, and the primary power factor shifts to mirror the secondary load's power factor. Beginners commonly confuse 'on load' with 'full load'—remember that 'on load' simply means any current is being drawn, whether it is 5% or 100% of the nameplate rating, whereas 'full load' specifically means the maximum rated continuous current the windings can handle without exceeding their temperature class.
The Physics of Loading: MMF Balance and Internal Impedance
When you close a switch and connect a load to the secondary terminals, secondary current (I2) begins to flow. This current creates its own magnetomotive force (MMF), defined as N2 × I2, which directly opposes the main magnetic flux in the transformer's iron core. If the primary winding did not react, the core flux would collapse, and the transformer would fail to transfer energy.
To prevent this flux collapse, the primary winding automatically draws an additional 'load component' of current (I2') from the AC source. This primary load current creates a counter-MMF (N1 × I2') that perfectly cancels out the secondary's demagnetizing effect, keeping the core flux virtually constant. The total primary current is the phasor sum of this load component and the small no-load excitation current (I0) that was already flowing to magnetize the core.
However, real transformers are not ideal. The copper windings have resistance (R), and not all magnetic flux links both windings perfectly, resulting in leakage reactance (X). When load current flows through this internal impedance, it creates a voltage drop. According to Electronics Tutorials, this internal voltage drop means the secondary terminal voltage under load is rarely exactly equal to the no-load voltage.
50 kVA Distribution Transformer: Voltage Regulation Data
The following table illustrates how a standard 50 kVA, 11kV/415V three-phase transformer behaves under varying load conditions. Notice how the power factor drastically alters the voltage regulation.
| Load Factor (%) | Secondary Power Factor | Secondary Terminal Voltage (V) | Voltage Regulation (%) | Total Copper Loss (W) |
|---|---|---|---|---|
| 25% | 0.80 Lagging | 412.4 | 0.63% | 145 |
| 50% | 0.80 Lagging | 409.8 | 1.26% | 580 |
| 100% | 0.80 Lagging | 404.5 | 2.53% | 2,320 |
| 100% | 1.00 Unity | 408.1 | 1.66% | 2,320 |
| 100% | 0.90 Leading | 419.3 | -1.04% (Rise) | 2,320 |
Data assumes standard 75°C operating temperature and balanced three-phase loading. Source: Adapted from standard IEEE C57.12.00 test parameters.
Worked Numeric Example: Calculating Secondary Voltage Drop
Let's move from theory to the workbench. Suppose you are installing a single-phase 25 kVA, 480V/120V control transformer to power a heavy contactor coil and a PLC. You need to know if the voltage will drop too low when the contactor pulls in.
Given Parameters:
- Rated Secondary Voltage (Vnl): 120V
- Equivalent Resistance referred to secondary (Req2): 0.025 Ω
- Equivalent Leakage Reactance referred to secondary (Xeq2): 0.080 Ω
- Actual Load Current (I2): 180A
- Load Power Factor: 0.85 Lagging
Step 1: Determine the trigonometric values.
Power factor (cos φ) = 0.85. Therefore, sin φ = √(1 - 0.85²) = 0.5268.
Step 2: Calculate the approximate voltage drop.
The standard formula for approximate voltage drop in a transformer is:
Vdrop ≈ I2 × (Req2 × cos φ + Xeq2 × sin φ)
Plugging in our real values:
Vdrop = 180 × (0.025 × 0.85 + 0.080 × 0.5268)
Vdrop = 180 × (0.02125 + 0.04214)
Vdrop = 180 × 0.06339 = 11.41V
Step 3: Find the final terminal voltage.
Vterminal = Vnl - Vdrop
Vterminal = 120V - 11.41V = 108.59V
At 108.6V, your PLC's power supply (which typically accepts 100-240V AC) will operate fine, but if you had a long secondary wire run adding another 3V of drop, you might start flirting with the undervoltage trip threshold of the contactor coil. This is why calculating the transformer on load behavior is critical before pulling wire.
Where You Meet Transformer on Load in Practice
You don't just encounter these concepts in textbooks; they dictate how you design and troubleshoot real electrical systems. Here is where transformer loading behavior forces you to make specific hardware choices.
1. Long Feeder Runs and Motor Starting
If you are powering a 5HP motor at the end of a 200-foot wire run from a step-down transformer, the motor's starting inrush current (often 6x full load current) will cause a massive, temporary voltage drop across both the transformer's internal impedance and the wire resistance. If the transformer is already heavily loaded by other equipment, this secondary voltage dip can cause the motor starter to chatter or drop out. The fix is either oversizing the transformer's kVA rating to lower its internal impedance percentage, or installing a soft starter to limit the inrush.
2. Capacitor Banks and the Leading Power Factor Trap
As shown in the data table above, a leading power factor (caused by over-correction with capacitor banks or long, lightly loaded underground cables) can cause the secondary terminal voltage to actually rise above the no-load voltage.
If you install a 480V power factor correction bank on a lightly loaded transformer, the leading reactive current flowing backward through the transformer's leakage reactance can push the secondary voltage up to 500V or higher. This can degrade the insulation of connected VFDs and surge protective devices (SPDs). Always size capacitor banks based on the actual measured inductive kVAR of the load, never just the transformer's total kVA capacity.
3. Non-Linear Loads and K-Factor Transformers
Modern facilities are packed with LED drivers, server power supplies, and variable frequency drives (VFDs). These non-linear loads draw current in sharp pulses rather than smooth sine waves, generating heavy harmonic currents (3rd, 5th, 7th). When a standard transformer is on load with these harmonics, the high-frequency currents cause massive eddy current losses in the core and skin-effect heating in the windings. If you are feeding a panel dominated by electronics, you must specify a K-factor rated transformer (e.g., K-4 or K-13), which features electrostatic shields, heavier gauge windings, and derated core flux densities to survive the thermal stress of harmonic loading.
Sizing, Protection, and Tap Changers
When sizing overcurrent protection for a transformer on load, the National Electrical Code (NEC) requires you to account for both the continuous load and the magnetizing inrush current. According to All About Circuits, a primary breaker sized exactly at the full-load current will often nuisance-trip the moment you energize the transformer, because the initial core magnetization can draw 10 to 15 times the normal primary current for the first few AC cycles.
To manage voltage regulation under varying loads, larger distribution transformers (typically above 112.5 kVA) are equipped with tap changers. While de-energized tap changers (NLTC) require you to shut down the system and manually move copper links on the winding to adjust the turns ratio by ±2.5% or ±5%, On-Load Tap Changers (OLTC) use complex resistor-transition switching to adjust the primary turns ratio while the transformer remains fully energized and under load. This is how your local utility keeps your house voltage locked tightly between 114V and 126V despite the neighborhood's load swinging wildly from midnight to 6:00 PM.
For further reading on how modern amorphous metal cores are reducing the baseline losses of these systems, the US Department of Energy's guide on Transformer Efficiency provides excellent baseline data on how no-load losses compare to on-load copper losses in modern grid infrastructure.
Frequently Asked Questions
Does a transformer draw current when not on load?
Yes. Even with the secondary terminals completely open, the primary winding draws a small 'no-load current' (usually 1% to 3% of the full-load rating). This current is necessary to magnetize the iron core and overcome core hysteresis and eddy current losses.
Can a transformer on load overheat if the current is below the nameplate rating?
Yes, if the load is highly non-linear (generating harmonics) or if the ambient temperature exceeds the design baseline (usually 30°C or 40°C). Harmonics cause localized heating that standard RMS clamp meters cannot detect, which is why K-rated transformers are required for heavy IT and VFD loads.
Why does the primary power factor change when I connect a secondary load?
The transformer reflects the secondary impedance back to the primary side, scaled by the square of the turns ratio. If your secondary load is a highly inductive motor (lagging PF), the primary winding will also present a lagging power factor to the utility grid, minus a tiny shift caused by the transformer's own internal magnetizing reactance.
