Technical explainer

3 phase motor wiring diagram: reading the terminal box for star and delta

18 August 2026 11 minute read

A three-phase motor's six winding leads land in a small box bolted to the frame, and the choice of star or delta is made there, with removable metal links, not by anything built into the motor itself. Nothing inside the winding decides the connection. The link pattern the electrician chooses is set by comparing the two voltages printed on the nameplate against the voltage the supply actually delivers — never by habit or preference.

What the six leads are and why they are brought out

A squirrel-cage motor has three separate stator windings, one per phase, physically arranged 120 electrical degrees apart around the frame. Each winding has two ends, so six wire ends in total come out of the stator. Rather than joining them at the works, all six are routed to a terminal box on the outside of the motor, which is what lets the same physical machine be wired as star or as delta depending on where it is installed and what supply it meets.

Terminal markings: what U, V, W and the numbers mean

The letter code

The three winding phases carry the letters U, V and W — first, second and third phase, in that order. If the motor brings out a neutral point, that conductor is marked N. This is not a house convention; it is the letter-symbol rule set out in the international standard governing terminal markings for rotating machines.

The 1/2 suffix rule

Each winding also carries a pair of numbers, one per end — 1 and 2 for the first winding, and so on. The rule fixing which end gets which number is simple but easy to forget under a grimy terminal-box lid: the end nearer the supply connection always takes the lower number. That is why, when a technician forms a star point, it is always the "2" ends that get bolted together and the "1" ends that stay free for the incoming supply — the numbering was never arbitrary, it already encodes which ends belong to which connection.

Wound-rotor motors are worth a brief aside, being a different machine type rather than a stator connection choice. A squirrel-cage rotor's bars are permanently short-circuited by end rings inside the rotor. A wound-rotor motor instead carries its own polyphase secondary winding, brought out via slip rings so external resistance can be switched into the rotor circuit and cut out in steps as the machine accelerates, down to a full short at rated speed. Because that resistance can be sized generously, a wound-rotor motor starts on a comparatively low current — roughly 250–350% of full-load current — while still developing a high locked-rotor torque of roughly 200–250% of full-load torque, which is why it is chosen where a heavy starting duty meets a weak supply.

Motor connection diagram: the star and delta link patterns

For a single-voltage, six-lead motor the standard's own connection diagram sets out both patterns as a join table rather than as a drawing to be copied. Star is formed by bolting the three "common" ends of the windings together at one point inside the box — that becomes the star point — while the three remaining "line" ends go out to the supply. Delta is formed differently: each winding's end is joined to the next winding's start, so the winding forms a closed triangle, and the three junction points themselves become the line terminals. The same box, the same six leads, two different link patterns bolted on.

Wiring three-phase motors: the dual-voltage case

A motor built for two rated voltages uses the same six leads and the same terminal box to reach either one. At the lower of its two nameplate voltages, the leads are joined in three pairs to form delta. At the higher voltage, the three far ends are joined together to form the star point instead — the identical winding, run one way at the lower figure and the other way at the higher one. The reason a single winding can serve two voltages this way is the fixed √3 relationship between a star connection's line voltage and the voltage actually applied across each winding, which the worked example below turns into real numbers.

Line current and the worked example

Take a 45 kW, 400 V three-phase motor, connected in delta at that voltage, with a full-load power factor of 0.85 and an efficiency of 0.91 — ordinary figures for a mid-size industrial motor, chosen for this example rather than lifted from any source.

Full-load line current, delta connection:

• I_L = P / (√3 × V × pf × η)
• I_L = 45,000 / (1.732 × 400 × 0.85 × 0.91)
• I_L = 45,000 / 535.9
• I_L ≈ 84.0 A

Full-load line current, if the same winding were instead connected star at the same 400 V supply: in a star connection the line current and the phase current are the same value, and each winding now sees only 400/√3 ≈ 231 V rather than the full 400 V it saw in delta. Because the winding impedance has not changed, that lower voltage produces a proportionally lower phase current, and the arithmetic collapses to a single factor:

• I_L(star) = I_L(delta) / √3 = 84.0 / 1.732 ≈ 48.5 A

What the 1/√3 step means for starting

That same 1/√3 step, applied twice, is what produces the star-delta starting ratio. Take a direct-on-line starting current of six times full-load current, a representative mid-range figure — locked-rotor current runs five to eight times full-load current generally, five to seven times with a supply-voltage dip specifically for direct-on-line starting: 84.0 × 6 ≈ 504 A, drawn into a winding held at the full 400 V.

Reconnect the same winding as star for starting. Phase current drops to 1/√3 of its delta value because each winding now sees only 231 V. On top of that, star removes the √3 multiplier that delta applies between phase current and line current — in star the two are equal. Line current therefore falls by 1/√3 twice over, and 1/√3 × 1/√3 = 1/3: 504 A ÷ 3 = 168 A.

Torque reaches the same one-third figure by a different route. Locked-rotor torque varies with the square of the voltage applied to the winding, because torque tracks the square of rotor current and rotor current is proportional to applied voltage at fixed slip. Each winding in star sees 1/√3 of the delta phase voltage, so torque falls to (1/√3)² = 1/3. Current loses a factor of 1/√3 twice; torque loses one factor of (1/√3)² — the same number reached two different ways, which is why the two ratios come out identical rather than merely similar.

Starting methods and what each costs

The starter is the switching gear between the supply breaker and the six leads described above — the breaker sized ahead of the starter contactor protects the cable and the contactor, while the starter itself decides how much voltage, and therefore how much current and torque, the winding sees during the first few seconds of a start. Direct-on-line uses a single contactor and applies full voltage immediately. Star-delta uses two or three contactors and a timer to switch the same six leads from the star pattern above to the delta pattern for running. Autotransformer and stator-series-impedance starting each insert dedicated switching gear ahead of the winding to hold voltage down temporarily. Part-winding starting is a distinct method worth naming separately so it is not confused with star-delta: only half the stator winding is energised at first, with the second half brought in a moment later, each half carrying its own current and — where separate overcurrent devices are fitted — protected at roughly half the setting the full winding would need.

The methods below are drawn from a single lecture source, disclosed here rather than papered over: it is the only citable source found for these mechanisms and ratios. Where a ratio was not stated as a fixed number by that source, the table says so rather than inventing one.

Reduced-voltage starting in context: torque curves and design letters

The percentages above only mean something against a torque-speed curve, so it is worth naming three points on that curve. Locked-rotor torque is what the motor develops the instant it is energised, with the shaft still stationary. Pull-up torque is the generally lower torque produced between that instant and the point of maximum torque. Breakdown torque is the maximum torque the motor can produce at rated voltage and frequency — a load exceeding it stalls the motor rather than merely slowing it.

Motors built to the common squirrel-cage designs are grouped into design letters describing the shape of that curve. Design B, the ordinary general-purpose type, has breakdown torque above its own locked-rotor torque — roughly 150% starting torque and up to roughly 200% breakdown torque relative to full load, with slip under about 5% at rated load. Design C reverses that relationship, with starting torque higher than breakdown torque, for loads that are hardest to move at standstill. Design D puts its maximum torque at locked rotor itself, with torque falling as speed rises, and is a high-slip design suited to loads that are cyclic or shock-loaded rather than steady.

Every ratio in the starting-methods table rides on the same fact: torque tracks the square of the voltage applied to the winding, which falls out of the standard per-phase equivalent circuit, where air-gap power — and so torque — is proportional to the square of rotor current, and rotor current at fixed slip is proportional to applied voltage. It is the same square-law step the worked example used to turn a 1/√3 voltage ratio into a 1/3 torque ratio, and it is why undervoltage during a start matters: a supply running low lengthens the time a motor spends drawing high current before it reaches speed, a heating problem as much as a starting one.

Commissioning checks: rotation, phase sequence, insulation

Rotation direction and phase sequence

Direction of rotation is defined against the shaft end that carries the driven load. Wired according to the standard terminal-marking scheme, the motor turns clockwise viewed from that end whenever the supply's phase sequence matches the alphabetical order of the terminal letters. Phase sequence itself is simply the order in which the three line voltages each reach their peak positive value, and it is what actually determines which way the shaft turns — which is why it has to be checked whenever a motor is connected to a supply it has not run on before, not assumed from the wiring diagram alone.

Reversing rotation needs no change inside the motor at all: swapping any two of the three supply leads at the terminals reverses the phase sequence the winding sees, and the shaft turns the other way. A rotating-type phase-sequence indicator will spin one way for one sequence and the opposite way for the reversed one, letting a technician confirm the supply's sequence without energising the motor itself. Where no indicator is available, the standard commissioning practice is to energise the machine briefly at low or no load, watch which way the shaft turns, then power down again before making the final permanent connection — correcting the sequence by swapping two leads if the rotation is wrong.

Insulation resistance

A recognised practice for testing the insulation resistance of rotating machinery exists, referenced by its own standard designation, and is worth running before a motor already wired elsewhere is put into service on a new supply. This article does not state specific megohm thresholds for a pass or fail, because the underlying research could not independently confirm numeric figures from that standard's own text — a gap left open rather than papered over with an invented number.

Standard terminal markings and nameplate basis

The terminal-marking convention used throughout this article is not a house style; it traces to the applicable code and standard basis. The national electrical code requires a motor's nameplate to carry, among other items, rated volts and full-load amperes, rated speed, temperature rise or insulation class, rated horsepower and a locked-rotor code letter — and it is this nameplate current, not a general ampacity table, that motor-running overload protection is sized against.

For a dual-voltage motor whose locked-rotor kVA per horsepower differs between its two rated voltages, the code requires the nameplate's locked-rotor code letter to reflect whichever voltage gives the higher figure — a reminder that a dual-voltage motor's star and delta connections are not electrically interchangeable only in running current, but in starting behaviour too.

Questions buyers ask

What do the numbers on motor terminal markings mean?

Each winding is a letter — U, V or W — with two ends numbered 1 and 2. The rule fixing which end takes which number always gives the lower number to the end nearer the supply, so "1" ends are line-side and "2" ends are the ones bolted together to form a star point.

Can any three-phase motor be started star-delta?

No. The method only works on a motor built to run normally in delta at line voltage, with all six winding leads brought out so it can be temporarily reconnected to star for the start and switched back to delta once it is up to speed.

Why does a star-delta starter need six leads brought out?

Because the starter has to physically reconfigure the winding twice — into star for starting and into delta for running — and that reconfiguration happens entirely at the terminal box. A motor with only three leads brought out has no way to be switched between the two patterns.

What should the overload relay be set to on a star-delta starter?

It depends on where the relay sits. Wired into the supply line, it sees line current directly. Wired into the winding circuit instead, it sees the smaller winding (phase) current, which is derived from the line-current relationship rather than a fixed published percentage — there is no single number to quote without knowing the wiring point.

How do I check a motor is wired for the right rotation before running it up?

Confirm the supply's phase sequence with a rotating indicator, or energise the motor briefly at low or no load, watch the shaft, then power down. If the rotation is wrong, swap any two of the three supply leads at the terminals and check again before making the connection permanent.