Machine shops have some of the most demanding electrical requirements of any small commercial building. A single CNC machining center can draw 40 to 80 amps of three-phase power. A manual lathe with a 10 HP spindle motor, a milling machine with a 5 HP motor, a surface grinder, an air compressor, welders, and shop lighting add up fast. The total connected load for a modest 3-machine CNC shop easily exceeds 200 amps of three-phase power.
The first question for any new or expanding machine shop is whether three-phase power is available. If it is, the planning is straightforward: size the service, panels, and feeders for the connected load. If three-phase is not available (common in rural areas and older industrial parks), the options are a phase converter, individual VFDs on each motor, or rewinding motors for single-phase. Each option has cost, performance, and code implications. This guide covers the electrical planning process from utility service through panel layout.
Three-Phase vs Single-Phase: Why It Matters
Three-phase power delivers 73% more power than single-phase power using the same wire gauge. A 200-amp, 240V single-phase service provides about 48 kW. A 200-amp, 240V three-phase service provides about 83 kW. For motor-heavy shops, this difference is the reason three-phase is the standard.
Three-phase motors are also superior to single-phase motors in every measurable way. They produce constant torque (no pulsating like single-phase), they self-start without start capacitors or centrifugal switches, they are more efficient (1-3% better), they cost less per HP, and they are available in larger sizes. Single-phase motors top out around 15 HP for most manufacturers.
CNC machines are designed for three-phase power. The servo drives, spindle drives, and power supplies in a CNC controller expect balanced three-phase voltage with specific voltage and frequency tolerances. Running a CNC machine on converted single-phase power may work, but the voltage balance and waveform quality often do not meet the CNC manufacturer's specifications. Servo faults and drive alarms are common complaints from shops running CNC machines on rotary phase converters.
If three-phase utility power is available within a reasonable distance, the connection cost from the utility is almost always justified. Utility companies typically charge $5 to $25 per foot for overhead and $15 to $60 per foot for underground. For a shop within 500 feet of an existing three-phase line, the connection cost might be $3,000 to $10,000 one-time, which is comparable to the cost of a good rotary phase converter.
Machine Shop Power Budget Calculator
Calculate total connected load and demand for your machine shop. Enter lathes, mills, welders, grinders, plasma tables, and compressors to determine service size, panel capacity, and whether you need single-phase or three-phase power.
Phase Converter Options
When three-phase utility power is not available, phase converters create a third phase from single-phase input. There are three types: static, rotary, and electronic (VFD-based). Each has different performance characteristics, cost, and suitability for CNC applications.
Static phase converters are the cheapest option ($200 to $800) and the worst performers. They use capacitors to create a voltage on the third terminal that is approximately 90 degrees out of phase with the input. The result is unbalanced three-phase power with the generated leg 10% to 20% lower in voltage. CNC machines will not run reliably on static converters. They are acceptable only for single motors in non-critical applications.
Rotary phase converters use a spinning idler motor to produce the third phase. Quality rotary converters produce reasonably balanced three-phase power (within 3% to 5% voltage balance) and cost $1,500 to $8,000. Rotary converters work acceptably for many machine tools including some CNC machines. Size the rotary converter at 1.5 to 2 times the largest motor load.
Electronic phase converters use power electronics to synthesize a true sine wave third phase with voltage balance within 1%. They cost $3,000 to $15,000 and produce the highest quality power of any converter type. Electronic converters meet the voltage specifications of most CNC manufacturers and are the recommended option for shops running CNC equipment without utility three-phase.
Static: Cheapest. Voltage imbalance 10-20%. Not suitable for CNC.
Rotary: Mid-cost. Voltage imbalance 3-5%. Acceptable for some CNC.
Electronic: Most expensive. Voltage imbalance <1%. Recommended for CNC.
Always size rotary converters at 1.5–2× the largest motor load.
Panel Sizing for CNC Machines
CNC machines have nameplate ratings that include the spindle motor, servo drives, hydraulic pump, coolant pump, chip conveyor, and control electronics. A typical VMC (vertical machining center) with a 15 HP spindle has a total nameplate current of 40 to 60 amps at 208/230V three-phase. A horizontal machining center with a 30 HP spindle may draw 80 to 120 amps. The breaker and feeder must be sized for the nameplate full-load current, plus 125% of the largest motor per NEC 430.
The panel serving the CNC area needs bus capacity for all connected machines, but the demand factor depends on how many machines run simultaneously at peak load. In a job shop where all machines could be cutting at the same time, use 100% demand factor for the first two machines and 80% for each additional machine.
CNC machines are sensitive to power quality. They require voltage within ±10% of nominal, frequency within ±1 Hz, and voltage imbalance below 2%. Voltage sags during other large motor starts on the same panel can cause drive faults. The best practice is to serve CNC machines from a dedicated panel separate from manual machines, welders, and air compressors.
Each CNC machine should have its own disconnect switch (NEC 430.102) within sight of the machine. The disconnect must be lockable for lockout/tagout purposes. Specify NEMA 12 (dust-tight, drip-proof) enclosures for disconnects and panels in the shop to prevent coolant mist and metal chips from entering the enclosure.
VFD Installations and Grounding
Variable Frequency Drives (VFDs) convert single-phase or three-phase input to variable-frequency three-phase output. In machine shops, VFDs serve as both single-to-three-phase converters and as speed controllers for spindle motors, pumps, and fans. A VFD rated for 15 HP three-phase output from single-phase input costs $800 to $2,000 and provides excellent power quality to the motor.
VFDs create electrical noise (harmonics and common-mode voltage) that can interfere with CNC controls and measurement equipment. Mitigation measures include line reactors on the VFD input, output filters between the VFD and motor, shielded motor cables with both ends of the shield grounded, and separation of VFD power cables from signal cables by at least 12 inches.
Machine shop grounding serves three purposes: safety (fault current path), power quality (reference potential for CNC controls), and EMI control (drain path for high-frequency noise from VFDs). Every machine must have an equipment grounding conductor back to the panel per NEC Article 250. For shops with VFDs, the grounding system must handle high-frequency common-mode currents that flow through motor bearings and machine frames to ground.
Best practice for machine shop grounding: install a ground bus bar in each panel, use separate insulated EGCs for each machine circuit, bond all machine frames to the ground bus, and install a supplemental ground ring around the shop. For CNC machines with VFDs, use shielded cables and bond the shield at both ends. This approach costs $500 to $2,000 more than minimum code compliance but prevents thousands of dollars in VFD-related bearing failures and control problems.