The direct answer is 0.1 Amps (100 milliamps) of 50/60Hz AC current passing across the chest. This is the definitive physiological threshold where ventricular fibrillation (fatal cardiac arrest) begins. To calculate how easily a circuit can push this lethal current through a human body, we convert circuit parameters to biological impact using Ohm’s Law: I = V / R. Assuming a compromised, wet-skin body resistance of 1,000Ω, the substituted formula for a standard US outlet is I = 120V / 1,000Ω = 0.12A (120mA). This means a standard 120V household circuit pushes 20% more current than the minimum lethal threshold.
The Lethal Current Spectrum (±20% Variance)
Human physiology does not have a single binary "dead" switch; the damage scales with current magnitude. Below is the precise physiological conversion table for the 100mA lethal baseline, showing the ±20% neighboring values you must account for when sizing protective relays or analyzing shock hazards.
| Current (mA) | Variance from 100mA | Physiological Effect (60Hz AC) |
|---|---|---|
| 80 mA | -20% | Severe muscle tetany; "cannot let go" threshold exceeded. |
| 90 mA | -10% | Respiratory muscle paralysis begins; asphyxiation risk if prolonged. |
| 100 mA | Baseline | Ventricular fibrillation threshold (Lethal). |
| 110 mA | +10% | Sustained fibrillation; high probability of internal tissue burns. |
| 120 mA | +20% | Cardiac arrest; severe internal burns; nerve damage. |
What Assumptions Fix the Lethal Answer?
Saying "100mA is deadly" requires three strict baseline assumptions. If any of these change, the lethal threshold shifts dramatically:
- Current Path: The 100mA threshold assumes a hand-to-hand or hand-to-foot path that crosses the heart. A finger-to-finger shock on the same hand might cause severe local burns at 500mA, but will not induce fibrillation.
- AC vs. DC: The 100mA limit applies to 50/60Hz Alternating Current. Direct Current (DC) is roughly 3 to 5 times less likely to induce fibrillation; it takes about 300mA to 500mA of DC to cause the same lethal cardiac disruption.
- Skin Resistance (R): Dry, intact skin has a resistance of roughly 100,000Ω. However, sweat, moisture, or broken skin drops this to 1,000Ω. Once the skin is punctured by the electrical burn, resistance plummets to the internal body tissue baseline of just 300Ω.
How the Math Shifts: 120V vs 230V vs 3-Phase
Because current is a function of voltage and resistance, the "deadly" nature of a circuit changes based on the supply voltage. Assuming a wet-skin/contact-area resistance of 1,000Ω, here is how the lethal current calculation shifts across global and industrial voltages:
- 120V (US Single-Phase):
120V / 1,000Ω = 120mA. Exceeds the 100mA lethal baseline by 20%. Highly dangerous in wet conditions. - 230V (EU/UK Single-Phase):
230V / 1,000Ω = 230mA. More than double the lethal threshold. Causes immediate, violent muscle contractions that can throw a worker off ladders, alongside severe internal burns. - 480V 3-Phase Wye (US Industrial): The phase-to-ground voltage is 277V.
277V / 1,000Ω = 277mA. If a worker contacts two phases (480V), the current is480V / 1,000Ω = 480mA, causing massive tissue destruction and certain death without immediate CPR and defibrillation.
When the Calculation is Meaningless
Applying Ohm's law to human shock hazards becomes mathematically meaningless in three specific scenarios:
- Static Electricity: A static shock from a doorknob can be 20,000V, but the total charge is measured in micro-amps and dissipates in nanoseconds. The sustained current never approaches 1mA.
- High-Frequency AC (>10kHz): Due to the "skin effect," high-frequency currents travel over the surface of the skin rather than through internal organs. This is why electrosurgery tools and Tesla coils can push amps of current into a body without stopping the heart.
- Unknown Contact Area: If you are calculating shock risk for a specialized industrial sensor and do not know the exact surface area of human contact or the moisture level, assuming a fixed 1,000Ω resistance is a guess, not an engineering calculation. According to OSHA electrical safety guidelines, you must assume the worst-case (wet/broken skin) for any exposed conductive part.
Decision Tree: Selecting the Right Protective Device
Use this decision path to select the correct hardware to keep current below the 100mA lethal threshold.
| If your hazard is... | Then you need... | Why it works (Trip Threshold) |
|---|---|---|
| Overcurrent / Wire melting | Standard Thermal-Magnetic Breaker | Trips at 15A–20A (15,000mA). Protects property, NOT human life. |
| Ground fault / Human shock in wet or damp areas | Class A GFCI Receptacle or Breaker | Trips at 4mA to 6mA. Cuts power long before the 30mA respiratory or 100mA cardiac thresholds. |
| Parallel arc faults (fire from damaged wire insulation) | AFCI Breaker | Detects high-frequency arcing signatures. Protects against fire, not direct shock. |
| Industrial 3-Phase Arc Flash | Relay Coordination + NFPA 70E PPE | Reduces clearing time to limit incident energy (cal/cm²). See NFPA 70E standards. |
The Concrete Pick: For any circuit where human contact is possible (bathrooms, kitchens, outdoor receptacles, jobsite temporary power), the mandatory default pick is a Class A GFCI (Ground Fault Circuit Interrupter) rated to trip at 6mA. This provides a 94% safety margin below the 100mA lethal fibrillation threshold.
FAQ: Common Misconceptions About Lethal Amps
"It's the volts that kill, not the amps" — is this true?
False. Volts are simply the electrical pressure; amps (current) are what actually disrupt the heart's electrical nodes and cook tissue. However, voltage is the mechanism that forces the amps through your body's resistance. Without sufficient voltage, lethal amps cannot flow.
Can a 12V car battery kill you?
Under normal conditions, no. Using Ohm's law (12V / 1,000Ω), a 12V source can only push 12mA through wet skin—enough to feel a slight tingle, but far below the 100mA lethal threshold. The only exception is if the 12V source is applied directly to internal tissue (e.g., via an open wound or medical catheter), bypassing the skin's resistance entirely.
Why do we say 30mA is the limit for some European RCDs if 100mA is lethal?
While 100mA causes cardiac arrest, currents as low as 30mA cause respiratory paralysis and severe "cannot let go" muscle tetany. If a worker is shocked at 40mA on a ladder or near water, they may die from a secondary fall or drowning before the current ever reaches their heart. The NIOSH electrical safety guidelines emphasize that secondary injuries are a massive factor in electrical fatalities.
