3D printer power supply calculator: PSU watts, amps and wire gauge

The formula

The supply has to deliver every DC load at once. The worst moment is warm-up, when the bed and hotend heat at full power and the motors may already be homing.

bed W      = rated watts, or volts² / bed resistance
motor W    = motors x current² x phase resistance x 1.5
fan W      = fans x 3 W
load W     = bed W + hotends x hotend W + motor W + fan W + other W
load A     = load W / volts
needed W   = load W x (1 + headroom)
supply     = smallest common size at or above needed W
wall W     = load W / supply efficiency

A bed or heater cartridge is a resistor, so its power is volts squared divided by its resistance. That is why the calculator can work from a measured resistance when the rating is unknown. An AC mains bed runs from the wall through a relay and adds nothing to the DC supply.

Where the defaults come from

Input	Default	Source
Bed	150 W at 24 V	Prusa’s MK52 24 V replacement bed: 6.25 A at 24 V
Hotend	40 W	E3D standard heater cartridge
Motors	4 at 1.68 A, 1.65 Ω	LDO 42STH38-1684 NEMA 17 data
Motor allowance	x 1.5	Duet3D: add 50% for driver losses and acceleration
Fans	4 at 3 W	Duet3D: 3 W per fan is a generous allowance
Board	2 W	Duet3D’s allowance for its own board
Headroom	30%	Mean Well FAQ
Efficiency	88%	Mean Well LRS-350-24 data sheet

What size power supply does a 3D printer need?

Add up the peak DC load: the heated bed, hotend, motors, fans and electronics at full power. Then buy a supply rated about 30% above that, as Mean Well advises. A 24 V printer with a 150 W bed and a 40 W hotend peaks near 232 W. It needs 302 W with headroom, so a 350 W supply fits.

Common supply sizes

Mean Well’s LRS series of enclosed supplies covers 50 to 600 W, and its RSP and UHP series go higher. The calculator rounds up to these nominal sizes.

Model	Output at 24 V	Typical efficiency
LRS-50-24	2.2 A, 52.8 W	88%
LRS-75-24	3.2 A, 76.8 W	90%
LRS-100-24	4.5 A, 108 W	90%
LRS-150-24	6.5 A, 156 W	89%
LRS-200-24	8.8 A, 211.2 W	89.5%
LRS-350-24	14.6 A, 350.4 W	88%
LRS-450-24	18.8 A, 451.2 W	91%
LRS-600-24	25 A, 600 W	91%
UHP-750-24	31.3 A, 751.2 W	95%
RSP-1000-24	40 A, 960 W	not listed here

The LRS series also comes in 12 V versions. The LRS-350-12 gives 29 A at 85% efficiency.

What printer makers fit

Manufacturers often size closer to the load than a 30% margin:

• Prusa MK4S. Ships with a 240 W, 24 V supply made by Delta, rated 10 A. Its MK52 bed alone draws 6.25 A of that.
• Creality Ender-3 V3 SE. Rated at 350 W, with a 24 V DC mainboard.
• Voron 2.4. Specifies a Mean Well LRS-200-24 for the electronics and a mains AC bed switched by a relay. The bed never touches the DC supply, so 200 W covers the rest.

A maker can size close because it knows what its own machine draws. When you add a bigger bed, a second hotend or more fans to a printer, you lose that margin. Run the new parts through the calculator before reusing the old supply.

How do you calculate a 3D printer’s power supply wattage?

Add the bed’s rated watts, each hotend heater’s watts, the motors, and the fans and electronics, all at full power. Divide by the supply voltage to get amps. Multiply the total by 1.3 for headroom and round up to the next standard size. Count an AC mains bed as zero, because it runs from the wall.

Step by step with the defaults

Load	Working	Watts
Bed	rated	150 W
Hotend	1 x 40 W	40 W
Motors	4 x 1.68² x 1.65 Ω x 1.5	27.9 W
Fans	4 x 3 W	12 W
Board	allowance	2 W
Peak load	231.9 W / 24 V = 9.7 A	231.9 W
With 30% headroom	231.9 x 1.3	301.5 W
Supply to buy	next size up	350 W

At 88% efficiency the supply pulls about 264 W from the wall at peak, 2.2 A on a 120 V outlet. To check a whole circuit with several printers, use the circuit capacity calculator.

Motors use less than volts times amps

A stepper driver does not draw its coil current from the supply. Duet3D sizes each motor as its rated current squared times its phase resistance, which is the power it uses standing still. It then adds 50% for driver losses and acceleration. Enter the run current from your firmware if it is set below the motor’s rating, since lower current cuts the figure by the square.

Fans and electronics

Small fans draw little. Mechatronics lists its 24 V 40 x 10 mm axial fans at 1.68 to 6 W and its 50 x 15 mm blowers at 2.16 to 4.32 W, so 3 W each is a fair average. A single-board computer is bigger. Raspberry Pi specifies a 5.1 V, 3 A supply for the Pi 4 (15.3 W) and 5.1 V, 5 A for the Pi 5 (25.5 W). Add that to “Board, display and accessories” if the printer’s supply feeds it.

Why the peak counts, not the average

A printer averages far less than its peak once the heaters reach temperature. The electricity cost calculator works from that average. The supply still has to carry the peak for the minutes of warm-up. Mean Well’s LRS-350 data sheet allows 150% of rated load for up to one second. That covers a motor’s start, not a heater that runs flat out for minutes.

Is a 12V or 24V power supply better for a 3D printer?

A 24 V supply is the better choice for a new build. At the same power, a 24 V heater draws half the current of a 12 V one. Its wires, connectors and board switches then carry half the amps and waste a quarter of the heat. Every heater and fan must match the supply’s voltage.

The same printer at 12 V and 24 V

The calculator’s defaults at each voltage, with the bed and hotend swapped for parts of the same wattage built for that voltage. Wire sizes are American Wire Gauge (AWG), where a smaller number means a thicker wire.

	24 V	12 V
Peak load	231.9 W	231.9 W
Supply current	9.7 A	19.3 A
Bed current	6.25 A	12.5 A
Bed leads, thinnest gauge	22 AWG	18 AWG
Supply leads, thinnest gauge	18 AWG	14 AWG

A 12 V heater on a 24 V supply

Doubling the voltage across a heater quadruples its power. E3D lists 3.1 to 3.8 Ω for its 12 V, 40 W cartridge. On 24 V that cartridge would put out 152 to 186 W. E3D tells buyers to double-check the supply before choosing between its 12 V and 24 V versions, and to measure the cartridge before first heat. The same applies to a bed. A 12 V, 150 W bed measures about 0.96 Ω and would try to draw 600 W on 24 V. Enter a bed’s measured resistance in the calculator to see what it draws on your supply.

What wire gauge does a 3D printer power supply need?

Size each wire for the current it carries. Remington Industries’ table rates two- or three-conductor cable with 105 °C insulation at 15 A for 18 AWG and 19 A for 16 AWG. A 24 V bed drawing 6.25 A fits 22 AWG by that table. Voron’s manual still asks for 18 AWG or larger to the controller board.

Wire ratings by gauge

Copper hook-up wire at 30 °C ambient, from Remington Industries. The calculator uses the cable column.

AWG	2 to 3 conductors, 105 °C	Single wire in free air, 80 °C	Single wire in free air, 105 °C
22	7 A	8 A	11 A
20	9 A	10 A	14 A
18	15 A	15 A	20 A
16	19 A	19 A	26 A
14	29 A	27 A	39 A
12	36 A	36 A	51 A

Remington derates bundles of four to six conductors to 80% of these values. A bed cable on a moving bed also flexes with every move, so a size heavier than the table and fine-stranded wire give it more margin.

Connectors

The connector can be the weak point, not the wire. Molex rates Micro-Fit 3.0 terminals up to 8.5 A depending on the terminal. Voron’s documentation says the Micro-Fit 3.0 connectors it recommends are specified for up to 5 A. Amass rates the XT30U at 20 A and the XT60 at 35 A. Check each connector in the bed and supply path against the current the calculator shows.

The mains side

The supply’s input carries the wall current, about 2.2 A at 120 V for the defaults. Voron’s documentation asks for at least 18 AWG for mains wiring, and 16 AWG is better. Many enclosed supplies, the LRS series included, have a 115/230 V input switch. Mean Well warns that a supply set to 115 V and fed 230 V will be damaged. In the US, check that it is set to 115 V before first power-on.