Floor Grinder Electrical Requirements: Phase, Generators and Cord Length
A grinder that trips the breaker, stalls on start or runs hot on a long cord costs the day. Read the rating plate, match it to what the site can supply, size the generator for starting rather than running, and keep the cord short and heavy enough.
Read the rating plate on the grinder and the extractor, and confirm before you quote that the site can supply that voltage, that phase and that current. On a generator, size for starting the motor, not for running it, and let the generator supplier do the sizing from the rating plates. Keep cords as short as the job allows and heavy enough for the current: a long, light cord loses voltage, and the motor pays for it.
Start with the rating plate
Everything on this page starts from the plate on the machine. It gives the voltage, whether the motor is single phase or three phase, and the current it draws at full load. Write those down for the grinder, the extractor and anything else that will run at the same time, because the supply has to carry all of them at once.
- Voltage. The machine is built for one supply voltage, or a short list of them. It is not interchangeable
- Phase. A three phase motor needs a three phase supply. A single phase site cannot run it without a generator or equipment an electrician specifies for that motor
- Full load current. What the circuit, the cord and the generator all have to be sized against
- Plug. If it does not fit the receptacle on site, that is the supply telling you something. Do not adapt your way round it
Ask the customer what supply the building has before you quote, not on the morning of the job. If the answer is that nobody knows, price a generator, and see how to choose a concrete floor grinder for why the supply your jobs offer should shape the machine you buy.
Single phase vs three phase
A three phase supply spreads the load across three conductors instead of two, so the same machine draws less current on each one. Less current means lighter cords will do, less voltage is lost along them and less heat is made in them.
| Supply | Amps per conductor | Where it is found |
|---|---|---|
| 120 V single phase | 83.3 A | North America |
| 240 V single phase | 41.7 A | North America |
| 208 V three phase | 27.8 A | North America |
| 480 V three phase | 12.0 A | North America |
| 230 V single phase | 43.5 A | UK, Europe, Australia |
| 400 V three phase | 14.4 A | UK, Europe, Australia |
Each row is the current in each supply conductor for a load of 10 kVA: kVA × 1,000 ÷ volts on single phase, and kVA × 1,000 ÷ (1.732 × volts) on three phase, using the nominal line to line voltage. The 10 kVA load is an illustration, not the rating of any machine. Your grinder's rating plate gives its real current.
The table shows why a machine is not simply moved between supplies: the same load needs about 6.9 times the current at 120 V single phase that it does at 480 V three phase, and every cord, breaker and plug in the chain has to be rated for it.
Generators: size for the start, not the run
An electric motor draws far more current for the moment it starts than it does once it is running. Generator manufacturers' sizing guidance puts the starting current of a motor started straight on the line at typically about six times its full load current, and higher for some high efficiency motors. A generator that runs the grinder comfortably can still sag badly or stall when the grinder starts, and the sag is felt by everything else on the generator.
- Give the supplier the rating plates. The grinder, the extractor and anything else that will share the generator, and say how each one starts
- Soft starters and drives cut the starting draw, which changes what size of generator the job needs. Tell the supplier if the grinder has one
- Match the output. The generator must supply the voltage and phase the grinder needs, on a receptacle that fits its plug
This page deliberately does not give a rule of thumb for generator size. The answer depends on the motor, how it starts and what else is on the set, and generator suppliers size it from exactly that. What the generator costs per square foot, owned or hired, is what the concrete grinding cost calculator prices, and the grinding time calculator tells you how many hours it has to run.
Voltage drop: why a long cord matters
Every foot of cord has resistance, so some of the supply voltage is lost along it before it reaches the machine. The loss grows with the current and the length, and shrinks as the cord gets heavier. A motor short of voltage has less to work with, and on a hard slab under full head pressure that is when it shows.
A worked example: 25 A through 150 ft of 10 AWG copper cord on 240 V single phase loses about 9.3 V, which is 3.9% of the supply, before the voltage already lost in the building's own wiring. A non-mandatory note in the US National Electrical Code suggests keeping a branch circuit within 3%, and 5% for the feeder and branch together. The same cord stays within 3% up to about 115 ft at that current; one size heavier, 8 AWG, reaches about 180 ft.
How far a copper cord can run before it loses 3%
| Cord size | Current drawn by the machine | |||||
|---|---|---|---|---|---|---|
| 15 A | 20 A | 25 A | 30 A | 40 A | 50 A | |
| 12 AWG | 120 ft | 90 ft | Over rating | Over rating | Over rating | Over rating |
| 10 AWG | 190 ft | 140 ft | 115 ft | Over rating | Over rating | Over rating |
| 8 AWG | 305 ft | 230 ft | 180 ft | 150 ft | Over rating | Over rating |
| 6 AWG | 485 ft | 365 ft | 290 ft | 240 ft | 180 ft | Over rating |
| 4 AWG | 775 ft | 580 ft | 465 ft | 385 ft | 290 ft | 230 ft |
Down the side: the copper cord size. Across the top: the current the machine draws, from its rating plate. In each cell: the longest one way cord length, in feet, that keeps the voltage drop within 3% of a 240 V single phase supply, rounded down to 5 ft. Computed as 2 × K × amps × length ÷ circular mils, with K = 12.9 for copper at 75°C and the circular mils from the AWG definition. “Over rating” marks a current above that cord size's ampacity in NEC Table 400.5(A)(1), column A. The chart covers the cord only, not the building wiring before it.
The chart is for 240 V single phase. The same cord carrying the same current reaches further on a higher voltage, and further on three phase:
- 240 V three phase: about 1.15× the lengths in the chart
- 480 V three phase: about 2.31× the lengths in the chart
- 120 V single phase: about 0.50× the lengths, which is why a 120 V machine on a long cord struggles first
The voltage drop chart is not a cord rating
A cord that keeps the voltage up can still be too light for the current, which is what the “Over rating” cells mark. The rating printed on the cord, and its jacket type, decide whether it can carry the load safely; the chart only says how far it can carry it. Follow the cord maker's instructions for using it on a reel.
Cords and protection on site
In the US, OSHA's construction rules require extension cords used with portable tools to be three wire and designed for hard or extra hard usage, protected from damage, and visually inspected before each day's use. 120 V, single phase, 15 and 20 A receptacles on a construction site that are not part of the building's permanent wiring need ground fault protection for the people using them. Cords are not a substitute for fixed wiring and must not be run through holes in walls, ceilings or floors.
In the UK, the Health and Safety Executive's construction guidance says to use cordless tools or tools running from a 110 V center tapped to earth (CTE) supply, so no conductor is more than 55 V from earth, and that tools, plugs and cables made for domestic use are not suitable for site conditions. How a larger machine on 230 V or 400 V is supplied is part of the site's electrical installation. In Australia and elsewhere, the construction site wiring rules and the regulator where you work decide what is required.
When to call an electrician
- The grinder trips the breaker on a circuit that should carry it
- The plug does not match any receptacle on site
- The machine is three phase and the site is single phase
- You need a temporary supply, a distribution board or a long run from the panel
- Anything on the cord, plug or machine is hot, damaged or smells of burning
This guide explains the arithmetic and the questions to ask. It does not replace the electrical code where you work, the machine's manual or a licensed electrician.
Frequently Asked Questions
Can I run a three phase grinder on single phase power?
Not directly. The motor needs a three phase supply, so on a single phase site it needs either a generator that supplies three phase or a converter or drive that an electrician has specified for that motor. Plugging an adapter in is not a fix. Check the supply before you take the job, and match the machine you buy to the sites you work on.
What size generator do I need for a floor grinder?
Size it for starting the motor, not for running it. A motor can draw several times its running current for the moment it starts, and a generator that is big enough to run the grinder can still sag or stall when it starts. Give the generator supplier the rating plate of the grinder and of everything else on the generator, and how they start, and let them size it.
How long can my extension cord be for a grinder?
It depends on the current, the voltage and the size of the cord. The chart on this page shows how far a copper cord can run on 240 V single phase before it loses 3% of the voltage, the figure a non-mandatory note in the US National Electrical Code suggests for a branch circuit. A heavier cord reaches further, and the cord must also be rated for the current.
Why does my grinder trip the breaker?
Usually because the circuit is smaller than the grinder needs, other loads share it, or the start draws more than the breaker allows. Check the rating plate against the breaker and the receptacle, run the grinder on its own circuit, and if it still trips, have an electrician look at the circuit rather than fitting a bigger breaker.