Introduction #

This guide is for electrical engineers, facility managers, and maintenance professionals who need to understand how transformer voltage regulation works in three-phase systems. It solves the problem of knowing why secondary voltage changes from no-load to full-load, how to quantify it, and how it differs from conductor voltage drop. Use this knowledge when selecting transformers, verifying voltage at loads, interpreting nameplate impedance (Z%), or deciding when tap changers or conductor sizing is needed.

For a comprehensive overview of three-phase power systems, including how they work and power calculation methods, see our 3-Phase Power Explained.

What Is Transformer Voltage Regulation and Why It Matters #

Transformer voltage regulation is the change in secondary voltage when load goes from zero (no-load) to full load, with primary voltage held constant. It is usually expressed as a percentage of the full-load secondary voltage. Good regulation means secondary voltage stays close to nominal as load changes; poor regulation means voltage drops noticeably at full load.

Why It Matters #

  • Equipment limits: Motors, drives, and controls have voltage tolerances (often ±10% of nominal). Large regulation can push voltage outside these limits.
  • Standards: ANSI C84.1 and IEEE define acceptable service and utilization voltage ranges. Transformer regulation plus conductor drop must keep voltage within those ranges.
  • Efficiency and lifetime: Low voltage increases current for the same power, raises losses, and can shorten motor and transformer life.

In three-phase systems, regulation applies to line-to-line secondary voltage. The same transformer regulates all three phases together; unbalanced loads can cause phase-to-phase voltage differences beyond the regulation percentage.

How Transformer Voltage Regulation Works #

Regulation is caused by the transformer’s internal impedance (resistance and leakage reactance). When load current flows, this impedance creates a voltage drop inside the transformer, so secondary voltage under load is lower than at no-load.

Regulation Formula (Definition) #

Regulation % = (V_no_load - V_full_load) / V_full_load × 100%

Where:

  • V_no_load = secondary voltage with no load (primary at nominal voltage)
  • V_full_load = secondary voltage at full load (same primary voltage)

For three-phase transformers, use line-to-line voltages.

Approximate Formula (Using Impedance and Power Factor) #

A common approximation using nameplate impedance (Z%) and load power factor:

Regulation % ≈ (R_pu × cos φ + X_pu × sin φ) × 100%

Where:

  • R_pu = per-unit resistance (often ≈ 0.01–0.02 for distribution transformers)
  • X_pu = per-unit reactance; Z% ≈ √(R² + X²) from nameplate
  • φ = load power factor angle (cos φ = power factor)

For typical distribution transformers, Z% is often 2–6%. The higher Z%, the higher the regulation for a given load. Inductive (lagging) loads usually give worse regulation than resistive loads.

Nameplate Z% #

Z% (impedance voltage) is the primary voltage (as % of nominal) that, applied at rated frequency with secondary short-circuited, causes rated current to flow. It approximates the per-unit impedance. A 4% Z transformer has stronger internal impedance than a 2% Z transformer and will typically show higher regulation at full load.

Transformer Regulation vs Conductor Voltage Drop #

Transformer voltage regulation and conductor voltage drop are different:

Transformer Voltage Regulation Conductor Voltage Drop
Where Inside the transformer In cables/conductors from transformer to load
Cause Transformer impedance + load current Conductor resistance (and reactance) + load current
Depends on Transformer Z%, load, power factor Conductor size, length, material, load current

Total voltage drop at the load = effect of transformer regulation + effect of conductor voltage drop. For example, if the transformer causes 3% lower voltage at full load and the feeder adds 2% drop, the load sees about 5% below the transformer’s no-load voltage.

For formulas and NEC rules for conductor voltage drop, see Voltage Drop Calculation Guide.

Regulation Limits and Standards #

ANSI C84.1 and similar standards typically require:

  • Service voltage: within ±5% of nominal at the point of delivery.
  • Utilization voltage: within ±10% of nominal at equipment terminals.

Transformer regulation plus conductor drop (and any tap settings) must keep the load within these limits. If regulation is large, tap changers can raise no-load secondary voltage so that at full load the voltage stays within range. For how tap changers are used to achieve this, see Transformer Tap Changer: Voltage Regulation Guide.

Calculation and Example #

Example: 480 V Three-Phase Transformer, Full Load #

Given:

  • 480 V (line-to-line) secondary, no-load
  • Transformer Z% = 4%, load power factor 0.85 lagging
  • Assume R_pu ≈ 1%, X_pu ≈ 3.9% (so Z ≈ 4%)

Approximate regulation:

  • cos φ = 0.85 → sin φ ≈ 0.53
  • Regulation % ≈ (0.01 × 0.85 + 0.039 × 0.53) × 100% ≈ 2.9%

Full-load secondary voltage:

  • V_full_load ≈ 480 × (1 - 0.029) ≈ 466 V (line-to-line)

So with constant primary voltage, the 480 V secondary sags to about 466 V at full load. If downstream conductor drop is also significant, the load may see even lower voltage; tap settings or conductor sizing must be chosen to keep the load within limits.

Common Mistakes #

Mistake 1: Confusing Regulation with Conductor Voltage Drop #

Error: Using only conductor voltage drop (or only transformer regulation) when checking voltage at the load.

Reality: Load voltage is affected by both. Ignoring transformer regulation can underestimate drop; ignoring conductor drop can overestimate voltage at the load.

Correct approach: Consider both transformer regulation and conductor voltage drop when verifying voltage at equipment.

Mistake 2: Ignoring Power Factor #

Error: Assuming regulation depends only on load magnitude.

Reality: For a given kVA load, regulation is worse at lower (lagging) power factor because the reactive component of current causes more internal voltage drop. Motors at low PF can see higher regulation than resistive loads.

Correct approach: Use load power factor (or at least a realistic PF) when estimating regulation or when choosing tap or conductor size.

Mistake 3: Treating Regulation and Tap Changer Range as the Same #

Error: Equating “4% regulation” with “±4% tap range.”

Reality: Regulation is a result of load and impedance. Tap changers shift the no-load secondary voltage (e.g. ±5% or ±10%) to compensate for regulation and drop. They do not change the transformer’s inherent regulation percentage.

Correct approach: Use regulation to find the voltage change from no-load to full-load; use tap range to choose how much you can compensate.

For more on how tap changers implement voltage regulation, see Transformer Tap Changer: Voltage Regulation Guide.

Frequently Asked Questions #

Q1: What is the difference between voltage regulation and voltage drop? #

A: Transformer voltage regulation is the change in secondary voltage from no-load to full-load due to the transformer’s internal impedance. Voltage drop here usually means the drop along conductors from the transformer to the load. Both reduce voltage at the load; regulation is inside the transformer, drop is in the wiring.

Q2: How does Z% relate to voltage regulation? #

A: Z% is the transformer’s per-unit impedance. For a given load and power factor, higher Z% generally means higher regulation (more voltage drop from no-load to full-load). A 6% Z transformer typically has worse regulation than a 2% Z transformer at the same loading.

Q3: Is the regulation formula the same for single-phase and three-phase? #

A: The definition is the same: (V_no_load - V_full_load) / V_full_load × 100%. For three-phase transformers use line-to-line voltages. The physical cause (impedance and current) is analogous; only the phasor and √3 relationships differ in the detailed models.

Q4: When do I need a tap changer? #

A: When combined transformer regulation and conductor drop would push load voltage outside limits (e.g. ANSI ±5% / ±10%). Tap changers adjust no-load secondary voltage to compensate. For when to choose on-load vs off-load tap changers, see On-Load vs Off-Load Tap Changer.

Q5: Can I measure regulation without loading the transformer? #

A: Not directly. Regulation is defined as the change from no-load to full-load. You can measure no-load and full-load voltages and compute it, or estimate it from Z%, R_pu, X_pu, and load power factor using the approximate formula.

If you need to size transformers or check voltage under load, use our Transformer Size Calculator.

Conclusion #

Transformer voltage regulation is the change in secondary voltage from no-load to full-load caused by the transformer’s internal impedance. It is quantified as Regulation % = (V_no_load - V_full_load) / V_full_load × 100% and can be approximated from Z%, power factor, and load. In three-phase systems, use line-to-line voltages. Regulation is separate from conductor voltage drop; both must be considered to keep load voltage within ANSI/IEEE limits. Use tap changers when regulation and conductor drop together would exceed those limits. Understanding regulation, Z%, and the difference between regulation and conductor drop leads to better transformer and conductor selection and more reliable voltage at the load.

About the Author: James Chen, P.E. is a licensed electrical engineer with 15+ years of experience in industrial power systems design. Former Schneider Electric application engineer specializing in 3-phase motor control and power distribution. All content in this guide has been reviewed and validated by licensed engineers.