23 kV Transformer Secondary Voltage: 400 V vs 416 V

Quick answer: A 400 V secondary suits a 380 V or 400 V low-voltage network. A 416 V secondary suits a network with a nominal voltage of about 400 V under load, because the extra 4% covers the voltage drop inside the transformer.

The difference looks small. But choosing the wrong value can cause overvoltage, low voltage at the load, or a failed utility inspection.

This guide explains how to choose between 400 V and 416 V on a 23 kV transformer, and how to write it correctly in your specification.

Why Two Secondary Voltages Exist

The Same Idea, Two Conventions

Both values come from the same three-phase low-voltage system. The difference is what the number on the nameplate describes.

Some buyers and utilities state the nominal system voltage. Others state the no-load secondary voltage.

Rated Voltage Is a No-Load Voltage

The rated secondary voltage on a transformer is the voltage measured with no load connected. Once current flows, the voltage falls.

A 416 V no-load rating means the utility expects about 400 V at the terminals under normal load. That is why many specifications use 416 V, 415 V or 420 V for a 400 V network.

Key Numbers: 400 V vs 416 V

Item	23 kV / 400 V	23 kV / 416 V
Voltage ratio	57.5	55.3
Phase-to-neutral voltage	About 231 V	About 240 V
Typical full-load voltage	About 388 V	About 403 V
Secondary current at 1000 kVA	About 1,443 A	About 1,388 A
Best fit	380 V or 400 V networks	400 V loaded networks, 240 V phase equipment

The full-load figures assume about 3% voltage regulation. Your actual value depends on impedance and power factor.

Understanding Voltage Regulation

What It Is

Voltage regulation is the drop in secondary voltage from no-load to full load. It comes from the resistance and leakage reactance of the windings.

Typical Values

A distribution transformer with 4% to 6% impedance usually shows 2% to 4% regulation at 0.8 power factor. At unity power factor, the drop is smaller.

A Simple Example

Take a 400 V no-load secondary with 3% regulation. At full load, the terminal voltage is about 388 V.

Now take 416 V with the same 3%. At full load, the terminal voltage is about 403 V. The cable and switchboard add more drop, so the load end sees a little less.

When 400 V Is the Better Choice

Networks Rated 380 V

Many industrial sites use equipment rated for 380 V. A 400 V no-load secondary gives about 388 V under load, which is close to 380 V and well inside tolerance.

Standard IEC Systems

IEC 60038 lists 230/400 V as a standard system voltage with a tolerance of ±10%. A 400 V transformer fits this directly, and the full-load drop stays within the allowed band.

Short Cable Runs

If the load is close to the transformer, there is little cable drop. A 400 V secondary avoids raising the voltage too high at light load.

When 416 V Is the Better Choice

Long Low-Voltage Feeders

Long cables and overhead lines add voltage drop. Starting at 416 V leaves more margin, so the far end of the line stays above the minimum limit.

Networks Using 240 V Phase Voltage

With 416 V, phase-to-neutral voltage is about 240 V. This matches single-phase equipment and lighting designed for 240 V.

Utility Specifications

Some utilities and regional standards require 415 V or 416 V no-load secondary voltage. If your tender says so, follow it exactly.

Effect on Connected Equipment

Motors Rated 380 V

A 380 V motor on a 416 V supply sees about 9.5% above its rating. This is at the edge of the usual ±10% tolerance.

The motor may draw higher no-load current, run hotter and suffer more insulation stress. At light load, the supply voltage is even higher.

Motors Rated 400 V or 415 V

These motors work well on both options. Check the nameplate before you decide.

Lighting and Electronics

Overvoltage shortens the life of lamps, drivers and power supplies. Undervoltage causes dimming, nuisance trips and slow motor starting.

Variable Frequency Drives

Most drives accept a wide input band, but always check the rated range. A high supply voltage raises DC bus voltage and increases stress on capacitors.

Voltage Limits at the Load

The ±10% Band

IEC 60038 allows ±10% around the nominal system voltage. For a 400 V system, this means about 360 V to 440 V at the point of supply.

Condition	400 V Secondary	416 V Secondary
No-load voltage	400 V	416 V
Full-load voltage (3% drop)	About 388 V	About 403 V
After 3% cable drop	About 376 V	About 391 V
Margin above 360 V limit	About 16 V	About 31 V

What the Table Shows

Both options stay in the band here. The 416 V option simply leaves twice the margin on the low side.

On the high side, a 416 V secondary at light load stays below the 440 V upper limit. Check this if your site has long periods of low demand.

Effect on Current, Cables and Breakers

Lower Current at 416 V

For the same kVA, a higher voltage means lower current. A 1000 kVA transformer delivers about 1,443 A at 400 V and 1,388 A at 416 V, which is about 4% less.

Impact on Equipment Sizing

The lower current slightly reduces cable heating and busbar loading. Breaker ratings and short-circuit levels should still be calculated for the actual voltage, impedance and system fault level.

Using Tap Changers to Fine-Tune Voltage

How Off-Circuit Taps Work

Most 23 kV distribution transformers have an off-circuit tap changer on the high-voltage side. A common range is ±2 × 2.5%.

Tap Position	HV Tap Voltage	Effect on Secondary
+5%	24,150 V	About 5% lower
+2.5%	23,575 V	About 2.5% lower
Rated	23,000 V	Rated value
-2.5%	22,425 V	About 2.5% higher
-5%	21,850 V	About 5% higher

Why It Matters Here

Taps correct for a primary network that runs above or below 23 kV. They do not replace choosing the right rated secondary voltage.

The tap is changed only when the transformer is de-energized. Set it during commissioning based on measured voltage at the site.

Worked Example: 1000 kVA Factory Transformer

The Project

A factory is fed from a 23 kV line. The main switchboard is 60 meters from the transformer, and most motors are rated 380 V.

The Analysis

The cable run is short, so the voltage drop is small. A 400 V secondary gives about 388 V at full load, which matches the 380 V motors well.

A 416 V secondary would put about 403 V on those motors. That is 6% above their rating and leaves little margin at light load.

The Result

For this site, 400 V is the better choice. If the same factory had a 400 meter overhead feeder, the answer could change to 416 V.

Regional Practice and Standards

It Depends on the Grid

Low-voltage conventions differ by country. Some grids use 380 V, some 400 V and some 415 V as the nominal level.

Always confirm the local utility rule before you order. A transformer that meets IEC but not the utility's voltage requirement may be rejected at connection.

Documents to Check

Look at the utility's technical specification, the national grid code and the project tender. If they disagree, ask the utility to confirm in writing.

How to Choose: Decision Table

Your Situation	Recommended Secondary
Equipment rated 380 V, short cable runs	400 V
Standard 230/400 V IEC network	400 V
Long feeders with high voltage drop	416 V
240 V single-phase loads	416 V
Utility tender requires 415 V or 416 V	As specified
Mixed equipment, unclear ratings	Check nameplates, then confirm with the utility

How to Write the Voltage in Your Specification

Be Exact

Do not write only "400 V" or "416 V". State what the number means.

For example: "Rated secondary voltage 416 V at no load, 23 kV ±2 × 2.5% primary, 50 Hz, vector group Dyn11."

Items to Confirm

Item	What to State
Primary voltage	23 kV and tap range
Secondary voltage	400 V or 416 V, no-load value
Frequency	50 Hz or 60 Hz
Vector group	For example Dyn11
Impedance	For example 4% to 6%
Neutral	Solidly earthed secondary neutral
Standard	IEC 60076 or local utility standard

Commissioning Checks

Measure Before You Energize

Record the primary voltage at the site. Compare it with the rated 23 kV and set the tap if the difference is more than 2%.

Measure After You Energize

Check no-load secondary voltage at the terminals on all three phases. Then check voltage at the main switchboard under normal load.

Adjust and Record

If voltage is too high or too low, de-energize and move the tap by one step. Record the final tap position on the nameplate record for future maintenance.

Common Mistakes

Confusing Nominal and No-Load Voltage

A supplier may quote 400 V while your utility means 416 V no-load. The result is a voltage about 4% lower than planned.

Ignoring Motor Nameplates

A batch of 380 V motors on a 416 V supply is a frequent cause of overheating complaints.

Using Taps to Fix the Wrong Rating

Taps give only a few percent of adjustment. They cannot turn a 400 V design into a 416 V design.

Forgetting Cable Drop

Voltage at the transformer terminals is not the voltage at the load. Always check the drop along the full circuit.

Frequently Asked Questions

Is 416 V the same as 415 V?

They are very close and often used interchangeably in practice. The exact value should follow the standard or utility specification.

Can I use a 400 V transformer on a 415 V network?

Often yes, if the load voltage stays within the tolerance of your equipment. Use the tap changer to raise the secondary voltage by a few percent if needed.

Does 416 V mean higher efficiency?

Not directly. Lower current gives slightly lower copper loss for the same kVA, but efficiency depends mainly on the core and winding design.

What is the standard IEC low-voltage level?

IEC 60038 gives 230/400 V as the common three-phase system. Local grids may use 380 V, 415 V or other values.

What should I send to the manufacturer?

Send the primary and secondary voltage, kVA rating, frequency, vector group, impedance, tap range and applicable standard. Add the site load profile if possible.

The choice between 400 V and 416 V is not about which is better. It is about matching the transformer to your network, your equipment and your voltage drop.

Use 400 V for 380 V or standard 230/400 V systems. Use 416 V for long feeders or networks that expect about 400 V under load.

Always state clearly whether the value is no-load or nominal. Need help writing the specification for your 23 kV project? Send your requirements to our team and we will recommend the right secondary voltage and tap range.