How Many Amps Kill? Lethal Current Thresholds and Body Resistance Math

The direct answer to how many amps kill is 0.1 Amps (100 milliamps) of alternating current (AC) passing across the heart, which is the widely accepted threshold for triggering lethal ventricular fibrillation. The formula used to determine if a specific shock scenario reaches this threshold is Ohm’s Law: I = V / R. Substituting values for a worst-case wet-skin scenario where a worker touches a 120V source with an internal body resistance of 1,000Ω: I = 120V / 1,000Ω = 0.12A (120mA). Because 120mA exceeds the 100mA fibrillation threshold, this specific fault is lethal.

Because human physiology and environmental conditions introduce massive variables, looking at the ±20% range around that 100mA threshold reveals how quickly a severe shock crosses into a fatal one.

The Assumptions That Fix the Lethal Current Answer

Stating '100mA kills' is only accurate if we lock in three critical assumptions: the current pathway, the skin's dielectric breakdown, and the frequency of the source. According to OSHA's Controlling Electrical Hazards guide, current must pass through the chest cavity to stop the heart. A 100mA shock from the left index finger to the left thumb will cause agonizing local burns, but it will not cause fibrillation because the pathway bypasses the heart.

Furthermore, the 100mA threshold applies specifically to 50Hz/60Hz AC power. Direct Current (DC) requires roughly five times the amperage (about 500mA) to induce the same fibrillation effect, which is why a 120V DC shock, while painful and dangerous, is statistically less likely to be instantly fatal than a 120V AC shock.

To calculate the actual amps flowing through a person, you must account for the stratum corneum (the outer layer of dead, dry skin), which acts as a high-value resistor. Once voltage breaks down this layer, resistance plummets, and current spikes.

How Lethality Shifts Across 120V, 230V, and 3-Phase Systems

The voltage of the source dictates how quickly the body's resistance collapses, fundamentally shifting the lethal outcome. In a standard US 120V (Line-to-Neutral) circuit, dry skin (100,000Ω) limits current to a harmless 1.2mA. However, if the worker is sweating or standing in a puddle, resistance drops to 1,000Ω, pushing the current to 120mA—crossing the lethal threshold.

When dealing with 230V (standard EU mains or US Line-to-Line 240V), the higher electrical pressure breaks down the dielectric barrier of dry skin almost instantaneously. Even with dry hands, a 230V shock will rapidly drop body resistance into the 1,000Ω to 2,000Ω range within milliseconds, driving 115mA to 230mA through the chest. This is why 230V is statistically far more lethal in residential settings than 120V.

In industrial 3-Phase systems (e.g., 480V), the conversation shifts from milliamps to whole Amps. At 480V, internal body resistance (300Ω) yields over 1.6 Amps of current. At this level, the 'let-go' threshold is bypassed instantly, muscles contract violently (often throwing the victim away from the source), and massive thermal tissue damage occurs. Furthermore, 3-phase faults frequently trigger arc flashes, where the primary cause of death shifts from electrical fibrillation to severe thermal burns and blast overpressure, as detailed in NIOSH Electrical Safety guidelines.

When Voltage-to-Current Lethality Conversions Are Meaningless

Applying Ohm's Law to determine lethality becomes entirely meaningless in three specific scenarios:

• Static Electricity and Capacitive Discharge: Dragging your feet on a carpet can generate 15,000 Volts. Using I = V/R, this looks apocalyptic. However, static shocks lack sustained charge (measured in micro-coulombs). The current exists for nanoseconds, delivering virtually zero continuous energy, making it harmless despite the massive voltage.
• High-Frequency AC (Above 10kHz): Due to the skin effect, high-frequency currents travel exclusively on the outer surface of the conductor—or in this case, the human body. Electrosurgical units operate at 500kHz+ and push several Amps through the patient to cut tissue, but the current does not penetrate deep enough to disrupt the heart's electrical nodes.
• Non-Cardiac Pathways: If a technician touches a live 480V busbar with their right index finger and the grounded cabinet with their right elbow, the current will travel down the arm. While this will cause catastrophic localized necrosis and likely require amputation, the current pathway does not cross the chest cavity, meaning the 100mA fibrillation threshold is irrelevant to the heart's survival.