Running current is arithmetic, starting current is the problem
Running current is the easy half. Watts divided by volts gives amps, so a 1,000 watt shop vacuum on a 120 volt supply draws about 8.3 amps once it is up to speed. Many tools skip the arithmetic and print the amps on the nameplate directly, which is the figure to prefer, because it comes from the manufacturer rather than from a marketing horsepower number.
What is not on most nameplates is the inrush. A motor at rest has almost no back-EMF, so at the instant power is applied it draws a great deal more than its running current and then falls back as it comes up to speed. The magnitude depends on the motor type. A universal motor of the kind in routers, circular saws and shop vacuums typically pulls a few times its running figure for a very short time. An induction motor of the kind in a table saw, a dust collector or a compressor pulls a good deal more, commonly several times running current, and the number for a specific motor comes from its nameplate or its manufacturer rather than from a rule of thumb.
Both are brief — a fraction of a second, less on a small motor with little to spin up. Protective devices are built with that in mind and tolerate a short surge rather than opening on the first peak. This is why a saw starting on a quiet supply is a non-event even though the instantaneous current looks alarming written down.
So what actually trips
Two things, and neither of them is a single motor starting.
| Cause | What it looks like | What fixes it |
|---|---|---|
| Two motors starting together | Machine and dust collector switched at the same moment | Stagger the starts, or keep them on separate supplies |
| A start on top of a heavy steady load | Heater or compressor already running, then the saw starts | Move the continuous load elsewhere |
| Steady overload | Nothing dramatic, trips after a while under load | Fewer things on the group, or a proper supply for the shop |
| A long extension cord | Motor struggles to start, gets hot, trips or stalls | Shorter or heavier cord — a voltage problem, not a current one |
The first row is the one people meet in a shop, over and over, because the dust collector and the machine it serves are meant to be on together. That is the single most useful thing this grouping does: it puts the induction motors in different groups and flags any group that ended up with two.
How the grouping works, and what it is not
The tools are sorted heaviest first and packed into the smallest number of groups that keeps each group under the running-current budget you chose. That is a straightforward bin-packing, and it is doing something modest: telling you how many separate supplies your tool list implies at the ceiling you picked, and which tools ended up sharing.
The budget is your number. This page does not suggest one, does not name a breaker size, does not tell you what conductors anything needs, and does not state any code rule, because those are determined by the code your jurisdiction has adopted and by the authority having jurisdiction, and they vary. Installing or altering circuits is licensed work in most places, and there is no wiring procedure here for the same reason. What you can do with the output is walk into a conversation with an electrician knowing what your tools draw, how they group, and where the simultaneous starts are, which is a far better conversation than describing the problem as the breaker keeps tripping.
Two related things worth checking
The first is voltage drop over cords and long runs. A motor fed at a sagging voltage draws more current to make the same power, heats up, and struggles to start, which produces symptoms that look like an undersized supply and are not. The extension cord size calculator and the voltage drop calculator handle that side of it.
The second is the steady total rather than the grouping, which is what the circuit load calculator covers if you already know what is on a given circuit and want the sum against a rating. And if the tool at the top of your list is a compressor, its duty cycle matters as much as its current, which the air compressor CFM calculator gets into.
Questions people ask
Can a table saw and a dust collector run on the same circuit?
They run together all day in plenty of shops and they trip together in plenty of others, and the difference is mostly whether they start at the same instant. Two induction motors starting simultaneously present a combined inrush several times their combined running current, which is the case protective devices are least tolerant of. If they must share, start the dust collector first, let it come up to speed, then start the machine. If you are planning rather than coping, separate supplies for the collector and the machine is the arrangement that removes the problem instead of managing it. What the actual installation should be is a question for an electrician working to your adopted code.
How many amps does a table saw draw?
Read the nameplate rather than the horsepower. A contractor or cabinet saw on 120 volts commonly draws somewhere in the low to mid teens of amps under load, and the same motor rewired for 240 volts draws roughly half that, since the same power at twice the voltage is half the current. Peak-horsepower marketing figures on the box bear little relation to continuous draw. Whatever the nameplate says is the number to plan with, and the starting surge is a multiple of it for a fraction of a second.
Why does the breaker trip when the tool starts but not while it runs?
Because the two events are completely different in magnitude. Running current is steady and modest; starting current is a brief spike of several times that as the motor accelerates from rest. Protective devices have a time-current characteristic that deliberately tolerates a short spike and responds to a sustained overload, so a start on a quiet supply passes and a sustained overload does not. When a start trips it usually means either the spike was larger than usual, because the motor was under load or the voltage was low, or something else was already drawing most of the capacity. Diagnosing which one is happening in a specific installation is work for an electrician, not for a calculator.
Does a soft start or a VFD help?
Substantially, on the inrush question specifically. Both ramp a motor up rather than connecting it across the line at rest, which flattens the current spike to something close to the running figure, and that is why the soft start option here uses a much smaller multiplier. It does not change the steady running current at all, so a soft start does not let you put more tools on a supply that is already near its limit under load. It solves start-up tripping and it does not solve overloading. Whether a given device suits a given motor is a manufacturer question.
What size circuit does a workshop need?
That is not a question this page will answer, and you should treat any tool that answers it confidently with suspicion. Circuit sizing depends on the code adopted in your jurisdiction, the load calculation performed for the installation, the conductors and the method of installation, the panel and service capacity, and how the local authority having jurisdiction interprets all of it. Adding or altering circuits is licensed work in most places. What you can bring to that conversation is exactly what this page produces: what each tool draws, what runs simultaneously, and where the starting surges land.