Single-Phase and Three-Phase in Practice

Why this matters

The textbook difference between single-phase and three-phase is a multiplier and a couple of connection diagrams. The field difference is something else entirely: in a single-phase circuit a weak leg announces itself on the first reading you take, and in a three-phase circuit the legs are coupled, so a fault in one of them shows up as elevated current in the other two. Your ammeter points at the healthy conductors.

That single behaviour is behind a large share of the compressors and motors that get replaced for no reason. The part comes out reading high, the new part goes in reading high, and the actual fault is a connection three feet upstream that nobody put a probe across.

The wye and delta arithmetic, the 1.732 multiplier and the common service voltages are covered by a sibling article. This one is about what changes in how you diagnose.

The call, and the reading that pointed at the wrong part

A three-phase rooftop unit on a 208 V service, tripping its overload intermittently and mostly in the afternoon. Compressor nameplate full-load amps: 14.0 A.

Before anything else on this job: it is a roof, so fall protection applies wherever the walking-working surface has an unprotected edge, at 4 feet or more above a lower level under 29 CFR 1910.28 for general industry work and at 6 feet or more under 29 CFR 1926.501 if the job falls under construction. Know which Part your work sits in before you step off the ladder. Every reading below is taken with the unit running, which is energized work: 29 CFR 1910.333(a)(1) allows it only where de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations, and diagnostic testing is the case that qualifies, with a meter and leads rated CAT III or better at or above the voltage present. Route the leads clear of the condenser fan before you close the door on them, and keep your body out of the fan plane while the unit runs. When you move from measuring to repairing, open the disconnect, lock and tag it under 29 CFR 1910.333(b)(2) for the electrical exposure and under 29 CFR 1910.147 for the unexpected-startup hazard of the fan and compressor, then prove dead live-dead-live per NFPA 70E-2021, 120.5.

The first tech clamped one conductor, read 15.9 A against a 14.0 A nameplate, called the compressor 113.6 percent of full load, and quoted a compressor.

Three numbers reframed it

Clamping all three lines takes twenty seconds longer and produces a completely different picture:

Average = (15.9 + 15.7 + 12.1) / 3 = 14.57 A, which is 104 percent of the 14.0 A nameplate. As an average load, that compressor is barely working harder than it is rated for.

Current unbalance is the largest deviation from the average, divided by the average: the deviations are +1.33, +1.13 and -2.47, so the largest is 2.47 A and the unbalance is 2.47 / 14.57 = 17.0 percent.

Then the voltages, taken at the contactor load terminals with the unit running:

Average = 606.1 / 3 = 202.03 V. Largest deviation is 208.4 - 202.03 = 6.37 V, so voltage unbalance is 6.37 / 202.03 = 3.2 percent.

NEMA MG 1 publishes a derating curve for motors on unbalanced voltage and does not recommend operation above 5 percent unbalance, so 3.2 percent is inside the curve but well into derated territory. It also gives the rule that the percentage increase in temperature rise runs about twice the square of the percentage voltage unbalance: 2 x 3.2 squared = 20.5 percent more temperature rise, for a load that the average current says is only 4 percent above nameplate. That is what has been tripping the overload on hot afternoons, when 20.5 percent extra rise stacks on top of an already high ambient.

Worth narrating honestly: the current unbalance here is 5.3 times the voltage unbalance, and the multiplier commonly quoted in motor literature is roughly six to ten times. Treat that rule as an order-of-magnitude expectation, not a calculation. What matters is the direction and the scale, both of which hold: a small voltage unbalance produced a large current unbalance.

Why the healthy legs read high

This is the part that makes three-phase diagnosis different, and it is worth understanding rather than memorising.

Add series resistance in one line - a loose lug, a tired fuse clip, a pitted contactor pole - and two things happen at once. Voltage on the windings fed through that line falls, so that line carries less current. But the motor still has the same shaft load to turn, and it pulls the missing work through the paths that are still healthy, so the other two lines carry more. The fault is on L3 and the alarming numbers are on L1 and L2.

Push that to the limit and you get single-phasing: one line opens completely, the motor keeps running on the remaining two, and their current rises steeply while the open line reads zero. A three-phase motor that loses a leg while running does not stop. It runs, hot, until something protective catches it, which is exactly why phase-loss and phase-unbalance monitor relays exist and why an overload sized for balanced running is not reliable protection against this fault on its own.

Finding the leg with the problem

Once the currents say unbalance, stop clamping and start probing across things, using the method the voltage-drop article covers in full. Take the readings under the same running load, one probe on each side of each device the line passes through.

Across the L3 line-side fuse clip: 3.9 V at the 12.1 A that line is carrying. Across the L1 clip: 0.05 V. Across the L2 clip: 0.04 V.

That is a ratio of 78 to 1 against identical hardware carrying comparable current in the same enclosure at the same instant. Converted: 3.9 / 12.1 = 0.32 ohms at that clip, dissipating 3.9 x 12.1 = 47.2 W inside a spring clip designed to dissipate none. The clip had lost tension and oxidised, and the resistance climbed with its own heat, which is why the fault got worse through the afternoon and released overnight.

The confirmation

Fuse block replaced, conductors cut back to bright copper, unit restarted, same ambient conditions, same measurement points:

Average = 14.1 A, largest deviation 0.1 A, current unbalance 0.1 / 14.1 = 0.7 percent. Voltages read 207.9, 207.4 and 207.6, averaging 207.63 with a largest deviation of 0.27 V, so voltage unbalance is 0.13 percent.

Two things in that result are worth reading carefully. First, the average current fell from 14.57 A to 14.1 A on identical mechanical work, because unbalanced operation was itself adding loss. Second, the highest single reading fell from 15.9 A to 14.2 A, which is 101 percent of nameplate: the compressor the first tech quoted was never overloaded, it was being fed badly. The whole repair was a fuse block.

The same fault on a single-phase unit

Put an equivalent 0.32-ohm connection in the one supply leg of a 240 V single-phase unit and nothing is masked. There is one current path, so the added resistance reduces the current everywhere, the load sags rather than compensating, voltage at the equipment drops in proportion to the load current, and the symptom is a motor that runs slow, starts hard, or stalls and trips on its own thermal protector. A single clamp reading and a single voltage reading at the load find it.

The trade-off is honest in both directions. Single-phase tells you about a bad connection immediately, but it stalls the equipment when it happens. Three-phase keeps running through a fault that would have stopped a single-phase machine, which is useful for the process and dangerous for the motor, and the price of that resilience is that you must read all three legs or you will read the fault backwards.

What three-phase asks of you that single-phase does not

Always take all three. Three currents and three line-to-line voltages, at the same load state, or you have not taken a measurement. One leg of a three-phase reading is not a sample, it is a coin flip on whether you are looking at a victim or a cause.

Check rotation after anything that opened line conductors. Swapping any two line conductors reverses direction. Some loads tolerate it and some are destroyed by it: a scroll compressor run backwards makes a distinctive sound, moves no refrigerant, and can be damaged within minutes, so verify rotation before you leave rather than after the callback.

Do not assume the neutral is lightly loaded on a four-wire wye system. The neutral carries the unbalance between phases, and where the loads are electronic rather than resistive the triplen harmonic currents from each phase add in the neutral instead of cancelling, so a neutral can run hotter than any single phase conductor. Clamp it like any other conductor.

Balance the loads you add. When you put single-phase loads on a three-phase panel, spread them across the phases as you go. Unbalance created at the panel produces exactly the motor-heating effect described above on every three-phase motor fed from it, and it is far easier to prevent while the panel is being filled than to correct afterwards.

References

• NEMA MG 1 for motor derating on unbalanced voltage and the temperature-rise relationship
• 29 CFR 1910.333(a)(1) for the energized-work justification and 29 CFR 1910.333(b)(2) for de-energizing electrical equipment, with 29 CFR 1910.147 for the unexpected-startup and stored-energy hazard of rotating equipment
• 29 CFR 1910.28 (general industry) and 29 CFR 1926.501 (construction) for fall protection on unprotected edges
• NFPA 70E-2021, 120.5, for the live-dead-live verification sequence
• See related: Three-Phase Power Basics Reference; How to Use Voltage Drop to Find a Bad Connection; How a Motor Draws What It Draws