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Start here - what is a transformer?

This is Lesson 1 of XFMR Training, exactly as it appears inside Lineman's Reference - XFMR Lab. Free, no signup. Read it through, tap the purple checks to quiz yourself, and when you hit the end: the wiring happens in the app.

Part 1 - What is a transformer?

A single-bushing transformer (GRD-Y)
A single-bushing transformer (GRD-Y)

A transformer is a piece of electrical equipment that takes one specific voltage and turns it into another. In distribution linework, that almost always means stepping the high voltage on the utility line down to the lower voltage a home or business can use. Out in the field, you'll almost always hear a transformer called a 'pot.' They're the same thing, and we'll use both words from here on. By the end of this first lesson, you'll wire this transformer up yourself.

📌 Transformer — Electrical equipment that changes one voltage to another.
📌 Pot — Common slang for a Transformer.

This app is a simulation. It won't capture every wrinkle you'll hit in the field, but it's a safe environment to learn what each connection does before you do it for real.

Part 2 - The external parts

The external parts of a single-bushing transformer
The external parts of a single-bushing transformer

Let's go over what the individual parts are called, then we can get into their function.
The terminals where energized wires land, or connect, are called bushings. Bushings are typically composed of a porcelain shell that separates energized metal components from grounded ones.

  • The single bushing up top is the primary bushing, where the high voltage connects.
  • The three across the face are the secondary bushings, where the customer's service wires connect.
  • The steel body is called the tank, and the Tank Ground bonds it to the system neutral or other ground reference, preventing the tank from becoming energized.
  • The ground strap is a removable strap that is used to bond the tank to an additional ground reference.
📌 Bushing — An insulated terminal on a transformer where wired connections are made.
📌 Tank Ground — A connection bonding the steel tank to a neutral reference, preventing the metal tank from becoming energized.
? What do we call the terminals that energized wires connect to?
Answer: Bushings: one primary bushing up top, three secondary bushings on the face.

Part 3 - The primary side

The primary: H1 reads phase-to-ground
The primary: H1 reads phase-to-ground

The primary side is fed straight from the power line.
You'll also hear the line side called the "feed": "fed from," "feeding," same idea. It's field shorthand for the source: where the power comes from.
This pot has a single primary bushing, H1. Some transformers have more than one primary bushing, and they are labeled H1, H2, etc., left to right.
On a 7,200/12,470 V system, a primary meter would read ~7.2 kV from H1 to ground when energized. The number of primary bushings, and why this one is connected internally to the tank ground, will be covered soon.

📌 Primary — Used to describe items that deal with high voltage, e.g., primary coil, primary voltage, primary rubber gloves
? On a 7,200/12,470 system, what does the primary read from H1 to the tank?
Answer: 7,200 volts, phase-to-ground.

Part 4 - The secondary side

The secondary: X1, X2, X3. X2 is the neutral
The secondary: X1, X2, X3. X2 is the neutral

The secondary side feeds the customer. Typically there are three bushings: X1, X2, and X3.
X1 and X3 are both hot legs; X2 in the middle is the neutral. A voltmeter reads 120 volts from either hot to the neutral (X1-X2 or X2-X3), and 240 volts from hot to hot (X1-X3). That's the standard 120/240 service.

We'll cover the visual order and positioning of the secondary bushings, and why it's grounded at X2, later on.

📌 Secondary — Used to describe items that deal with usable, consumer voltages, e.g., secondary coil, secondary bushings, secondary voltage, secondary rubber gloves
📌 System Neutral — A conductor that carries the ground reference: the 'neutral.'
? On a 120/240 secondary, what do you read from X1 to X3?
Answer: 240 volts, hot leg to hot leg. Each hot to the neutral (X2) is 120.

Part 5 - The two coils inside

A single-bushing transformer with its coils drawn.
A single-bushing transformer with its coils drawn.

Inside a transformer are 2 tightly-wound coils, rated for different voltages:
The Primary Coil (pink) - the side using "primary" voltage from the power line.
The Secondary Coils (green) - the two coils tied to the "secondary" bushings on the face that handle usable "secondary" voltage.

📌 Coils — Wire wound into many tight, overlapping turns. Current flowing through a coil produces a magnetic field, and a changing magnetic field through a coil produces voltage. Those are the two effects a transformer is built on.
? Are the green coils that tie to the bushings on the face primary or secondary?
Answer: Secondary. That's the customer side. The pink coil is the primary, fed from the power line.

Part 6 - The turns ratio

Turns ratio = voltage ratio. This pot is 30:1
Turns ratio = voltage ratio. This pot is 30:1

So how does a transformer actually change voltage? Simply put: the energized primary coil and the power line's alternating current build a constantly-moving magnetic field in the steel core, and that moving field induces, or "pushes," voltage onto the secondary coil.

The two coils don't actually touch. The amount of voltage induced is set by the number of windings, or "turns", in each coil. The ratio of windings between the primary and secondary coils is called the transformer's Turns Ratio. To find a transformer's turns ratio, divide its primary voltage rating by its FULL secondary voltage rating: 7,200 ÷ 240 = 30 → 30:1 Turns Ratio.

IMPORTANT: Transformers step voltage in both directions! If you supply EITHER coil with voltage it WILL be stepped. If you supply this transformer's secondary coil with 240 V you WILL get 7,200 V at H1.

Turns ratio = voltage ratio and that's the basics of how transformers work.

📌 Turns Ratio — The ratio of the primary coil's windings to the secondary coil's windings.
? If a transformer's primary voltage rating is 14,400 V and the secondary voltage rating is 240/480 V, what is its turns ratio?
Answer: 30:1. Divide the primary by the FULL secondary rating: 14,400 ÷ 480 = 30.
⚠️ DANGER

A transformer works in BOTH directions. The same turns ratio that turns 7,200 V into 240 V can also turn 240 V into 7,200 V. Backfeeding the secondary on a "de-energized" pot will cause the primary to become energized at full potential.

Part 7 - The tank ground serves as your H2

On a single-bushing pot, the tank ground is the primary coil's H2
On a single-bushing pot, the tank ground is the primary coil's H2

On a single-bushing (GRD-Y) transformer there's no second primary bushing. The primary coil's far end runs to the tank ground instead, so the tank ground serves the same purpose as an H2.
That puts the tank ground right in the primary circuit. Bonded to the system neutral, it normally reads near-zero volts.

BUT IF THAT GROUND IS MISSING OR BROKEN WHILE ENERGIZED, the tank can rise to primary voltage and energize the whole steel tank. Treat it as primary-side. It's one of the most important, and most deadly, connections on the transformer.

📌 H2 — The second primary connection. On a two-bushing pot it's the H2 bushing; on a single-bushing pot the tank ground does its job.
? Why is it dangerous to leave the tank ground ungrounded?
Answer: The tank ground and tank can become energized at primary voltage.
⚠️ DANGER

No tank ground on this transformer means that the tank can become energized at primary voltage. Grounds - first on, last off.

Part 8 - The X2 ground reference

X2 is the grounded secondary bushing
X2 is the grounded secondary bushing

On most single-pot configs, you will see the X2 being used as the grounded secondary bushing. The X2 is the center point between the two secondary coils, and when grounded, a balanced 120 volts is output on both X3 and X1.
Which secondary bushing is grounded directly affects the output voltage on the other two bushings.
Always connect your ground strap to the grounded bushing. The ground strap acts as a backup ground reference in conjunction with the tank ground.

? Why is the secondary grounded at X2 and not X1 or X3?
Answer: X2 is the center point of the two coils. Grounding here gives a balanced 120 volts on each leg.

Part 9 - Polarity

Same-looking pot, two different polarities
Same-looking pot, two different polarities

Part 4 from earlier left a loose end and it's important: the left-to-right order of the secondary bushings. That order is set by the transformer's polarity. Because of how the coils are wound, the secondary lines up with the primary in one of two ways, additive or subtractive, and the nameplate tells you which.

Polarity doesn't change any voltage. It flips the bushing order and, in turn, it flips current flow.

Polarity isn't random. It's standardized in transformer manufacturing, so every pot follows the same layout: additive runs X3-X2-X1 left to right; subtractive runs X1-X2-X3.

THE POLARITY RULE: a transformer is ADDITIVE only when its primary rating is 8,660 V or less AND its size is 200 kVA or less. Exceed EITHER one and it's SUBTRACTIVE.

Our 50 kVA pot rated 7,200 V is additive, but that same pot rated 14,400 V is subtractive. This is why X3 has sat on the left in every graphic so far.

📌 Polarity — Which of two ways the secondary lines up with the primary, additive or subtractive, determined by the polarity rule. It sets the left-to-right order of the X bushings along with current flow direction.
📌 Additive / Subtractive — The two polarity types, marked on the nameplate. Additive (X3-X2-X1): primary rated 8,660 V or less AND 200 kVA or less. Exceed either and it's subtractive (X1-X2-X3).
? A replacement pot shows up with its X bushings in a different left-to-right order than the old one. How do you land the secondary wires?
Answer: By the X markings, never by position. The new pot's polarity is different, so the order is flipped.

Part 10 - Wire it

The wiring, in order: grounds ①, secondary ②, primary ③, close the cutout ④
The wiring, in order: grounds ①, secondary ②, primary ③, close the cutout ④

You've got every piece now. Time to put them together. Burn this rule in before you touch anything:

GROUND CONNECTIONS ARE FIRST ON AND LAST OFF.

1. Grounds first: wire the tank ground, X2's ground strap, and X2's system neutral jumper before anything else gets connected.
2. Secondary next: connect the hot legs to opposite secondary phases.
3. Primary: H1 gets connected once everything else is buttoned up.
4. Energize Last: close the cutout's fuse barrel (the cutout is the fused switch up on the pole). Nothing is hot until that barrel seats. Then anything connected to the bottom of the cutout becomes energized.

Wrecking a pot out runs the same process backward: open the cutout first, and the grounds come off last.

📌 First On, Last Off — The grounding rule. Ground connections are the FIRST thing you put on when building and the LAST thing you take off when wrecking out.

✅ What you should know:

▶ Now wire it.

You just did the reading. In the app, this same transformer is sitting in a lab waiting for you: ground the tank, wire up the secondary, wire the topside, close the fuse and watch the voltages come to life. Then keep going. Lesson 2 in the app picks up right where this one left off.

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XFMR Lab is $19.99, one time. All lessons, 16 labs, the Sandbox, and Death Traps included. No subscription.

Straight from the app, which runs in landscape. Rotate your phone for full-width diagrams.