The direct answer to 'how many amps kill you' is 0.1 amps (100 milliamps) passing through the chest for just one to three seconds, which triggers fatal ventricular fibrillation. However, under worst-case conditions like wet skin or prolonged contact, as little as 0.03 amps (30 mA) can be lethal. Because power sources supply voltage rather than a fixed current, we must 'convert' the available voltage into the actual current your body will draw using Ohm’s Law: I = V / R. For example, touching a 120V AC source with wet, broken skin (where resistance drops to roughly 1,200 ohms) substitutes into the formula as I = 120V / 1200Ω = 0.1A (100 mA)—instantly reaching the lethal threshold.
The Lethal Current Spectrum: Physiological Thresholds
When discussing human electrocution, 'amps' is too large a unit; electrical safety standards measure shock severity in milliamps (mA). The assumption that fixes the answer to 'how many amps kill you' is not the capacity of the breaker or the power supply, but rather the biological response of the human heart to alternating current (AC) at 50/60Hz. A 20A breaker will not 'push' 20 amps through you; it will simply supply whatever current your body's resistance demands until the breaker trips.
The table below outlines the exact physiological effects of 60Hz AC current passing hand-to-hand or hand-to-foot (crossing the heart). This data is foundational to NEC requirements for GFCI protection, which is designed to trip at 5mA—well below the 'let-go' threshold.
| Current (mA) | Current (Amps) | Physiological Effect | Safety Implication |
|---|---|---|---|
| 1 mA | 0.001 A | Threshold of perception; slight tingling. | Generally harmless; standard static shock. |
| 5 mA | 0.005 A | Slight shock felt; disturbing but not painful. | GFCI trip threshold (Class A) to prevent escalation. |
| 10 - 20 mA | 0.01 - 0.02 A | 'Let-go' threshold; severe muscle contractions. | Victim cannot release the conductor; prolonged exposure begins. |
| 50 - 100 mA | 0.05 - 0.1 A | Extreme pain; severe muscle reactions; respiratory arrest. | High risk of secondary falls; approaching cardiac disruption. |
| 100 - 200 mA | 0.1 - 0.2 A | Ventricular fibrillation; heart muscles quiver uselessly. | LETHAL. Death occurs within minutes without defibrillation. |
| > 2,000 mA | > 2.0 A | Cardiac standstill; severe internal organ burns. | Heart may actually restart if shock is brief, but severe tissue necrosis occurs. |
Note: Direct Current (DC) requires roughly 3 to 5 times more amperage (300-500mA) to induce fibrillation, though DC shocks cause violent, single-direction muscle contractions that can throw a worker across a room, leading to secondary blunt-force trauma.
Converting Voltage to Lethal Current: The Resistance Variable
To understand how a standard wall outlet becomes lethal, we look at body resistance (R). Dry, intact skin has a high resistance—typically 10,000Ω to 100,000Ω. However, moisture, sweat, broken skin, or standing in water can drop that resistance to 1,200Ω or even 500Ω. The internal resistance of the human body (excluding skin) is only about 300Ω to 500Ω. Once the skin's dielectric barrier is broken down by voltage or moisture, current flows almost unimpeded.
Below is a conversion table showing how slight variations in skin condition (a ±20% shift in resistance) alter the current drawn from a standard 120V AC source, shifting the outcome from a severe shock to instant lethality.
| Skin Condition | Body Resistance (Ω) | Calculated Current (I = 120V / R) | Outcome |
|---|---|---|---|
| Damp / Sweaty Skin (+20% R) | 1,440 Ω | 83 mA (0.083 A) | Respiratory arrest; extreme pain; high risk if prolonged. |
| Wet / Broken Skin (Baseline) | 1,200 Ω | 100 mA (0.1 A) | LETHAL. Ventricular fibrillation threshold reached. |
| Soaked / Submerged (-20% R) | 960 Ω | 125 mA (0.125 A) | LETHAL. Instant cardiac disruption; severe internal burns. |
This math is exactly why the Occupational Safety and Health Administration (OSHA) mandates strict lockout/tagout procedures and ground-fault protection in damp environments. A 120V circuit is not inherently 'safe' just because it is low voltage; its lethality is entirely dictated by the resistance of the path it takes.
When the Math Fails: Pathway, Duration, and Voltage Shifts
The simple I = V/R conversion becomes meaningless or highly variable in three specific real-world scenarios:
- The Current Pathway: If 150mA enters your right index finger and exits your right thumb, it will cause severe localized tissue burns and necrosis, but it will not trigger ventricular fibrillation because the current does not cross the heart. Lethality requires a transthoracic pathway (hand-to-hand, or hand-to-opposite-foot).
- High-Voltage Arc Flashes: In industrial 480V or 4kV systems, the air itself can ionize before you even touch the busbar. The current flows through the plasma arc, not through your body's internal resistance. The hazard shifts from electrocution (internal current) to catastrophic thermal burns and blast pressure, governed by NFPA 70E incident energy calculations rather than simple Ohm's Law.
- Duration of Shock: The 100mA threshold assumes a shock duration of 1 to 3 seconds. If a shock lasts less than 0.1 seconds (such as a static discharge or a very fast-acting electronic breaker), the heart can tolerate much higher peak currents without fibrillating. Conversely, a 30mA shock held for several minutes can cause fatal respiratory paralysis.
How the Answer Shifts: 120V vs 230V vs 3-Phase
Voltage acts as the 'pressure' forcing current through your resistance. At 120V (North American residential), dry skin (100,000Ω) limits current to a harmless 1.2mA. But at 230V (European/Asian residential), that same dry skin allows 2.3mA—a noticeable, startling shock. If skin is wet (1,200Ω), 120V pushes 100mA (lethal), while 230V pushes 191mA, causing massive internal burns and instant cardiac arrest.
In 3-phase systems, the danger shifts dramatically. Touching a single phase to ground yields the standard phase-to-neutral voltage (e.g., 120V or 230V). However, touching two phases simultaneously exposes you to line-to-line voltage (e.g., 208V, 400V, or 480V). This higher voltage easily breaks down the skin's dielectric barrier in milliseconds, dropping your resistance to the internal 300Ω baseline and pushing well over 1 Amp through your torso—guaranteeing deep tissue destruction and immediate cardiac standstill.
