Secondary power in welding is the electrical output that comes out of the welding machine and flows through the electrode, the arc, and the workpiece. It’s the lower-voltage, higher-amperage side of the circuit that actually creates the weld. Every welding machine takes in primary power (the electricity from your wall outlet or generator, typically 115 to 600 volts) and transforms it into secondary power at much lower voltage but much higher current, which is what melts metal together.
Primary Power vs. Secondary Power
Think of a welding machine as a translator between two electrical worlds. Primary power is the input: the standard electrical supply feeding the machine, ranging from 115 volts on a small household unit up to 600 volts on industrial equipment. The machine’s internal transformer steps that voltage down and boosts the amperage, producing secondary power.
Secondary voltage during welding typically runs between 17 and 45 volts, depending on the process and settings. Open-circuit voltage (when the machine is on but no arc is struck) can reach 80 volts or so. The amperage on the secondary side, however, is where the real energy lives. Secondary current can range from around 20 amps on a light-duty machine up to 600 amps or more on heavy industrial equipment. That combination of low voltage and high amperage is what generates the intense, focused heat needed to fuse metal.
What Makes Up the Secondary Circuit
The secondary circuit is the complete electrical loop from the welding machine’s output terminals, through the welding lead (electrode cable), through the electrode or wire, across the arc, through the workpiece, and back to the machine via the work lead (often called the ground clamp, though technically it’s the work clamp). Every component in this loop carries the secondary current.
The key physical components include the output terminals on the machine, the electrode cable, the electrode holder or welding gun, the electrode or filler wire itself, the arc, the workpiece, the work clamp, and the return cable. If any connection in this loop is loose, damaged, or poorly conducting, you get problems: voltage drops, inconsistent arcs, overheating at the connection point, or loss of welding performance.
AC and DC Secondary Output
Welding machines produce secondary power as either alternating current (AC) or direct current (DC), and some machines can switch between both. The type of secondary output you choose affects arc behavior, weld quality, and which metals you can work with.
DC secondary output produces a smoother, more stable arc with less spatter. Setting the polarity to DC negative gives faster deposition rates on thin sheet metals, while DC positive drives deeper penetration into the base metal. DC is the standard choice for most stick welding, TIG welding on steel, and work in overhead or vertical positions where arc control matters most. The tradeoff is that DC equipment costs more and can’t solve arc blow, a problem where magnetic fields in the workpiece push the arc off course.
AC secondary output reverses direction rapidly, which naturally cancels out arc blow by preventing magnetic buildup. This makes AC the go-to for welding magnetized materials and for repair work on machinery that has become magnetized through use. AC is also essential for welding aluminum with TIG, because the positive half of the AC cycle strips away the oxide layer on aluminum’s surface. Shipbuilders use AC welding for seam welds because it allows higher current settings than DC. The downsides are more spatter, a less stable arc, and a rougher weld appearance compared to DC.
How Duty Cycle Relates to Secondary Current
The amperage you set on the secondary side directly determines how long you can weld before the machine needs to cool down. This is measured as duty cycle: the percentage of a 10-minute period the machine can operate at a given amperage before reaching thermal overload.
A machine rated at 300 amps with a 60% duty cycle can weld continuously for six minutes out of every ten at that amperage, then needs four minutes to cool. Crank the amperage higher and the duty cycle drops. Run it at a lower amperage and you can weld for longer stretches. Light-duty machines typically carry a 20% duty cycle, medium-duty machines sit around 40% to 60%, and heavy-duty industrial units run at 60% to 80%. Understanding this relationship matters because exceeding the duty cycle at your set amperage can overheat and damage the machine’s transformer, which is the very component converting primary power into secondary power.
Safety on the Secondary Side
Secondary voltage is lower than primary voltage, so secondary shock is less likely to be fatal. But it’s far from harmless, especially in wet conditions or when your skin resistance is low from sweat. A secondary shock happens when your body contacts both sides of the welding circuit at the same time: touching a bare spot on the electrode cable while also touching the workpiece, for example. Your body completes the circuit and current flows through you.
OSHA regulations (standard 1910.254) set specific rules for the secondary circuit to prevent this. Welding lead terminals must be protected from accidental contact using dead-front receptacles, recessed openings with hinged covers, or heavy insulating sleeving. Cables with damaged insulation or exposed conductors must be replaced, not taped over. Any cable with a splice within 10 feet of the electrode holder can’t be used. You should never coil or loop electrode cable around your body, because if the insulation fails, the current path goes straight through you.
The regulations also restrict what can serve as part of the work-lead return path. Pipelines can be used temporarily during construction or repair, but not as a permanent part of the circuit, and never when current would pass through threaded, flanged, or caulked joints. Chains, wire ropes, cranes, hoists, and elevators are prohibited from carrying welding current entirely. Conduits containing other electrical conductors also can’t double as a work-lead path. These rules exist because improvised return paths create unpredictable resistance, localized heating, and shock hazards.
Why Secondary Power Settings Matter for Weld Quality
The voltage and amperage on the secondary side are your two primary controls over the weld itself. Amperage governs how much heat enters the joint, which controls penetration depth and deposition rate. Voltage controls arc length, which affects bead width and the overall shape of the weld pool. Setting the secondary current too high burns through thin material or creates excessive distortion. Setting it too low gives poor fusion, a weak joint, and excessive spatter as the arc struggles to stay lit.
On machines with adjustable voltage and wire feed (like MIG welders), the relationship between secondary voltage and amperage needs to stay balanced. Raising wire feed speed increases amperage because the machine works harder to melt more wire per second. If voltage doesn’t rise to match, the arc becomes erratic and buries into the workpiece. If voltage is too high relative to amperage, the arc becomes wide and flat with poor penetration. Learning to read the arc and adjust secondary power settings is one of the core skills in welding.