Add motor currents and compare available VFD output ratings. Use custom allowances when your drive manufacturer or project specifies them.
Advanced options
Table of contents
How to use our VFD Size Calculator
- Enter each motor's current in its "nameplate amps (A)" field using the correct supply voltage.
- Choose "Nameplate current total" under "Current allowance method" to add those currents.
- Choose "Custom current allowances" when your project specifies per-motor current multipliers or an "Additional current allowance (%)". Use a multiplier of 1 to keep an entered current.
- In "Advanced options", enter the applicable continuous-current ratings under "Available VFD output amp ratings (A)".
- Select "Calculate" and check the candidate against the manufacturer's voltage, duty, overload and derating requirements.
For example, motors drawing 8 A and 5.5 A give a 13.5 A current total. A list containing 10 A, 15 A and 20 A selects a 15 A current-capacity candidate for further application checks.
Definitions
A VFD controls motor speed by varying output frequency and voltage.
"Current allowance method" selects a plain current total or explicit custom adjustments.
A current multiplier applies to one entered current. Use a value supplied for the application's current requirement. Motor service factor describes permitted mechanical loading under specified conditions and requires separate manufacturer interpretation.
"Additional current allowance (%)" applies to the subtotal after any per-motor multipliers.
"Planning current target" is the calculated current after the selected adjustments. The "Candidate VFD output rating" is the first entered rating at or above that target.
Common mistakes and quick fixes
Use amperes in the motor-current fields. Check the nameplate voltage associated with each current.
Use "Custom current allowances" only with a justified current multiplier or percentage. Confirm how the manufacturer handles motor service factor and temporary overload.
Replace the example "Available VFD output amp ratings (A)" list with ratings for the intended drive series and duty.
After adding or removing a motor row, recalculate and check "Motor rows included".
Limitations & Key Assumptions / Boundary Conditions
- This tool compares current-capacity candidates. Complete drive selection includes voltage, phase, motor power, duty cycle, overload duration, cooling, ambient temperature, altitude, cable and filter requirements.
- Custom allowances are user-specified planning inputs. Verify their applicability in the selected drive and motor manuals.
- Each motor on a shared drive needs individual thermal or overload protection. All motors receive the drive's common output frequency [1].
- Check input wiring, breakers, grounding and protection separately with a qualified designer.
- The default rating list is illustrative. Use the manufacturer's applicable continuous-current ratings, including any derating.
Methodology
Current total
With "Nameplate current total", add the entered motor currents.
current_target = sum(motor_current)
Custom allowances
With "Custom current allowances", multiply each entered current by its explicit current multiplier, add the results, then apply the additional percentage once.
current_target = sum(motor_current * current_multiplier) * (1 + additional_percent / 100)
For 8 A at multiplier 1.15 and 5.5 A at multiplier 1, the subtotal is 14.7 A. An explicitly entered 15% additional allowance gives 16.905 A. From 10 A, 15 A and 20 A ratings, the current-capacity candidate is 20 A.
Candidate selection
The calculator sorts the supplied positive ratings and selects the first rating at or above the target. Headroom is the difference between that rating and the target. An empty or insufficient list produces a message asking for applicable ratings.
These calculations require the manufacturer's separate application checks before equipment selection.