The Direct Answer: How Many Amps Will Kill?
The direct answer is 0.1 Amps (100 milliamps) of alternating current (AC) at 50/60 Hz passing across the chest for just one to three seconds. This is the widely accepted physiological threshold for inducing fatal ventricular fibrillation. However, at 0.03 Amps (30 mA), current causes severe muscular paralysis and respiratory arrest, which is why modern electrical codes mandate Ground Fault Circuit Interrupters (GFCIs) to trip at just 5 mA (0.005 A) to provide a massive safety margin before you reach the 'let-go' threshold.
Substituted Values: Assuming a wet-skin body resistance (R) of 1,200 ohms, a standard 120V mains supply pushes exactly 0.1 Amps (120V / 1200Ω = 0.1A) through the body, reaching the lethal threshold.
The Physiology of Current: Neighboring Thresholds (±20%)
Human physiology does not have a single 'on/off' switch for electrocution. The damage scales non-linearly with current. Below is the physiological breakdown for a 60 Hz AC current passing hand-to-hand, covering the ±20% range around the 100 mA lethal threshold.
| Current (AC 60Hz) | Percentage of Lethal | Physiological Effect |
|---|---|---|
| 80 mA (0.08 A) | 80% | Severe shock; breathing becomes extremely difficult; strong muscular contractions. |
| 90 mA (0.09 A) | 90% | Respiratory paralysis likely; diaphragm locks up; victim cannot breathe while in contact. |
| 100 mA (0.1 A) | 100% (Baseline) | Ventricular fibrillation threshold; heart muscle quivers and stops pumping blood. |
| 110 mA (0.11 A) | 110% | Sustained ventricular fibrillation; cardiac arrest; high probability of death without immediate defibrillation. |
| 120 mA (0.12 A) | 120% | Certain ventricular fibrillation combined with severe localized tissue burns at contact points. |
How Voltage and Skin Resistance Shift the Lethal Math
The 'amps that kill' is a fixed biological limit, but the voltage determines whether that current can actually overcome your body's resistance. The assumption that fixes the 100 mA answer is a hand-to-hand or hand-to-foot current path crossing the cardiac muscle with a duration of 1 to 3 seconds at standard power frequencies.
Here is how the hazard shifts across different global power systems:
- 120V Systems (North America): To push 100 mA through the body, resistance must drop to 1,200 ohms. Dry human skin has a resistance of 100,000 ohms or more. Therefore, 120V is generally only lethal if the skin is wet, sweaty, or punctured (e.g., gripping a bare wire with a cut on your palm).
- 230V Systems (UK/EU/AU): The higher voltage pushes 100 mA through 2,300 ohms of resistance. This means 230V can be lethal even if your skin is only lightly damp or if you are wearing slightly conductive footwear, making it inherently more dangerous in typical household environments.
- 3-Phase Systems (208V/400V/480V): A line-to-line fault bypasses the neutral ground path. Contact with two phases pushes massive current through the body. At 480V, the current easily exceeds 500 mA (0.5 A). At this level, the heart clamps down completely (cardiac standstill), and severe internal thermal cooking of tissues occurs alongside fibrillation.
Decision Path: Sizing Protection for Human Survival
Because human skin resistance is highly variable, we never rely on the body to limit current. Instead, we use protective devices that detect ground faults and interrupt the circuit before the current reaches the 100 mA lethal threshold. Use this decision tree to select the correct protection:
| Environment / Application | Hazard Profile | Required Protection Threshold | Concrete Device Pick |
|---|---|---|---|
| Wet areas (Bathrooms, Kitchens, Outdoors) | Skin resistance < 1,500Ω; high risk of 100mA+ shock from 120V. | 5 mA (0.005 A) Trip | Square D HOM120GFIC (120V Class A GFCI Breaker) |
| Dry residential (Bedrooms, Living Rooms, 230V regions) | Skin resistance > 10,000Ω; risk of sustained contact and respiratory paralysis. | 30 mA (0.03 A) Trip | Siemens 5SM3316-6 (230V 30mA Type A RCCB/RCD) |
| Industrial / 3-Phase Motor Control | High voltage arc flash risk; equipment ground fault protection needed. | 300 mA to 500 mA (Equipment protection, not direct human shock) | Eaton EG3200FFG (Ground Fault Sensor with Shunt Trip) |
The Default Recommendation: If you are wiring a standard 120V residential circuit where human shock is the primary concern, always terminate the decision path with a 5 mA Class A GFCI device. It trips at roughly 5% of the lethal threshold, ensuring survival even if you are standing in a puddle of water.
Frequently Asked Questions
Is it volts or amps that kill?
Amps (current) do the biological damage, but volts (voltage) are the pressure required to push those amps through your body's resistance. Without sufficient voltage, lethal current cannot flow. For example, a car battery can supply 600 Amps, but its 12V potential cannot push even 10 mA through dry skin, making it harmless from a shock perspective.
Why do GFCIs trip at 5 mA if 100 mA is lethal?
The 5 mA (0.005 A) threshold is set just below the 'let-go' current (typically 10-15 mA). If a device trips at 100 mA, the victim's muscles would already be locked in a tetanic contraction, making it impossible to release the live conductor. The 5 mA trip ensures the circuit opens before muscular paralysis takes hold. For more on workplace safety thresholds, refer to the OSHA electrical safety guidelines.
Does wearing rubber shoes prevent lethal shock?
Rubber-soled shoes increase the resistance of the path to ground, which reduces the total current flow. However, if you touch a live 230V wire and a grounded metal pipe simultaneously with your hands, the current travels hand-to-hand, completely bypassing your shoes. Never rely on footwear as primary protection; always rely on properly installed overcurrent and ground-fault protection as outlined by NFPA 70E standards.
