Three phase power
Converts kW, kVA and amps, and shows what each winding actually carries — because in a delta that is not the line current.
The supply conductor and the winding carry different currents
Every three-phase calculator will turn kW into amps. Almost none of them tell you that the current inside the motor is not the current in the cable feeding it.
There are two ways to connect three windings, and they are mirror images of each other:
In a wye, one end of each winding ties to a common point. Each coil carries the full line current, but only sees line voltage ÷ √3 — which is why 480 V wye gives you 277 V to neutral.
In a delta, the windings form a closed triangle. Each coil sees the full line voltage, but carries only line current ÷ √3, about 58% of it.
Same power either way. Different current in the copper.
Where that becomes an expensive mistake
An overload relay protects whatever it is in series with. Fit it in the supply lines and it reads line current. Fit it in the winding legs — as a wye-delta starter does — and it reads coil current.
Size that relay off line current and it is 73% oversized. It sits there looking correct while the winding overheats, and it never trips. The motor fails and the protection was the reason.
The other number that gets swapped
Motor circuits use full-load current twice, from two different places, and they are not the same figure.
Conductors and branch-circuit protection come off the code table — a generic value for the horsepower rating. Overload protection comes off the nameplate of the specific motor being installed, which is usually lower.
Swap them and you either undersize the conductor or under-protect the motor. This tool asks for both and labels every output with which one it used.
Two protective devices, two jobs
The breaker looks absurdly large next to the conductor, and it is meant to. It is short-circuit and ground-fault protection, and it has to pass starting inrush — six to eight times running current for a few seconds — without tripping. Hence 250% for an inverse-time breaker.
Running overload is the relay's job, at 115% or 125% of nameplate. Two devices, two failure modes, deliberately not one component trying to do both.
Questions electricians actually ask
What is the difference between line current and phase current?
Line current is what flows in the supply conductor. Phase current is what flows in one winding of the motor or transformer. They are only the same in a wye.
In a delta, each coil carries the line current divided by √3 — about 58% of it. In a wye, each coil carries the full line current but only sees line voltage ÷ √3.
The two relations are mirror images, which is exactly why they get mixed up: a wye divides the voltage, a delta divides the current.
Why does that matter for sizing an overload?
Because an overload relay protects whatever it is in series with. If it sits in the supply lines it sees line current. If it sits in the winding legs — as it does inside a wye-delta starter — it sees coil current, which in a delta is 42% lower.
Size that relay on line current and it is 73% too big. It will happily let the winding cook without ever tripping, which defeats the entire point of fitting it.
This tool always shows both figures side by side so there is nothing to remember.
Why does the tool ask for full-load current twice?
Because they are genuinely different numbers used for different things, and swapping them is one of the most common motor circuit errors.
Conductors and branch-circuit protection are sized from the full-load current in the code table — a generic figure for the horsepower rating. Overload protection is sized from the amperes on the motor’s own nameplate.
The nameplate figure is usually lower. Using it for the branch circuit undersizes the conductor; using the table figure for the overload leaves the motor under-protected. So the tool takes both and labels which output came from which.
Why is the breaker so much bigger than the conductor?
Because it is not protecting against overload. Motor branch-circuit protection is short-circuit and ground-fault protection, and it has to be large enough to let starting inrush through without nuisance tripping — a motor can pull six to eight times its running current for the first few seconds.
So an inverse-time breaker is permitted up to 250% of full-load current, and a dual-element time-delay fuse up to 175%. Running overload is a separate job done by the overload relay, sized at 115% or 125% of the nameplate.
A motor circuit therefore has two protective devices doing two different jobs. That is by design, not redundancy.
Do I use 115% or 125% for the overload?
125% if the motor has a service factor of 1.15 or more, or a marked temperature rise of 40 °C or less. 115% for everything else.
Both figures are on the nameplate, so it is worth reading rather than assuming — the difference is about a 9% wider setting, which is often the difference between a motor that runs and one that trips on a warm day.
Why does efficiency change the current from horsepower?
Because horsepower is what comes out of the shaft and current is what goes into the terminals. The difference is losses.
A 10 HP motor at 90% efficiency draws about 11% more current than a perfectly efficient one would. Calculators that leave efficiency out understate the draw, which is the wrong direction for sizing anything. Power factor and efficiency are both on the nameplate.
Notes
The power relations and the delta and wye transformations are physics and are computed here. The motor full-load-current tables are copyrighted and are not reproduced — you enter the table figure from your own copy, and the nameplate figure off the motor.
The protection multipliers and the overload percentages are short statements of rules, restated in this project's own words. The list of protective devices covers the common cases rather than the full table, and motors of other types carry different figures.
A number here is arithmetic on what you typed. It is not a design, and it cannot know about duty cycle, ambient, altitude, harmonic content, or any of the other things a real motor circuit has to account for. Tell me if something disagrees with your code book.