What is contact-tip-to-work distance (CTWD)?

CTWD is the distance from the end of the contact tip to the workpiece. It sets the electrode extension, the length of wire sticking out beyond the tip that carries current resistively before the arc. Changing CTWD changes how much that wire is preheated, which shifts current, voltage, and penetration.

How does longer stickout affect MIG welding?

On a constant-voltage machine, longer stickout means more resistive I-squared-R preheating of the wire, so the same wire-feed speed melts off with less current. The current self-regulates down, which lowers penetration and deposition. Shorter stickout does the opposite, raising current and penetration.

What causes burnback in MIG welding?

Burnback is when the wire melts back to and fuses with the contact tip. It happens when stickout is too short for the current, the wire-feed speed is too low, or the arc is struck too close to the tip. Keeping a sensible stickout and matching feed to current prevents it.

How is the resistive voltage drop calculated?

The electrode extension behaves like a resistor. Its resistance per millimetre equals the steel resistivity divided by the wire cross-sectional area, so a thinner wire has higher resistance. Multiplying that by the current and the change in CTWD gives the extra voltage dropped in the wire.

Why does penetration drop when I pull back the gun?

Pulling back lengthens the stickout, which on a constant-voltage supply lowers the arc current. Lower current means less arc force and a shallower, wider bead. Holding a consistent CTWD is one of the most important habits for repeatable MIG penetration.

CTWD / Contact-Tip-to-Work Distance Calculator

In MIG (GMAW) welding the contact-tip-to-work distance quietly controls current, penetration, and arc stability. Because the wire sticking out past the tip carries current resistively, changing that length preheats the wire differently and shifts how the constant-voltage power supply self-regulates. This tool quantifies the resistive voltage drop and the resulting current change, and warns when the settings invite burnback.

How it works

The electrode extension is a resistor whose resistance depends on wire cross-section:

R_per_mm = rho_hot / area        area = pi x (d/2)^2
V_drop   = current x R_per_mm x change_in_CTWD

A thinner wire has a smaller area, so higher resistance and a bigger voltage swing per millimetre of CTWD. On a constant-voltage machine the wire also self-regulates: more stickout means more I-squared-R preheat, so the same wire-feed speed melts with less current:

new_current ~ current x (1 - k x deltaCTWD / CTWD0)

That falling current is why pulling the gun back softens penetration even though the dial has not moved.

Tips and notes

Hold a consistent CTWD. For short-circuit transfer a stickout of roughly 6 to 13 mm is typical; spray transfer runs longer, around 19 to 25 mm.
Too short a stickout at high current invites burnback and spatter at the tip; too long makes the arc cold and wandering with poor fusion.
If you must reach into a deep joint and stickout grows, expect penetration to drop and compensate with voltage or wire-feed speed.
This is a first-order resistive model. Real behaviour also depends on shielding gas, wire chemistry, and the machine’s voltage slope.