The direct answer is 0.05 amps (50 milliamps) of 50/60 Hz AC current passing across the chest. This is the universally recognized threshold that can kill a human by inducing ventricular fibrillation (cardiac arrest). For DC current, the threshold is significantly higher, at approximately 0.3 amps (300 mA). However, stating a lethal amperage without defining the voltage and the body's impedance is only half the equation. To understand how a standard household outlet delivers this lethal current, we have to treat the human body as a resistor and convert touch voltage into current flow.
The 'Conversion' Formula: Touch Voltage to Lethal Current
Electrocution is not a standard power conversion like calculating kilowatts from amps; it is a physiological threshold governed by Ohm's Law. The 'conversion' from a hazardous voltage to a lethal current relies on the impedance of the human body ($Z_{body}$). According to the IEC 60479-1 standard (Effects of current on human beings and livestock), human body impedance is not a fixed value. Dry, intact skin can exhibit up to 100,000 ohms of resistance. However, in worst-case scenarios—such as wet skin, sweat, or broken skin from a puncture wound—that impedance drops drastically to 1,000 ohms or less. Once current breaches the skin, internal tissue resistance is only about 300 ohms.
To calculate the actual current flowing through a person touching a live wire, we use the formula:
$I_{body} = V_{touch} / Z_{body}$
Let's substitute real-world values for a standard US residential circuit. If a person with damp hands (1,000 Ω impedance) touches a 120V live wire while grounded:
- $I = 120V / 1000\Omega$
- $I = 0.12A$ (120 mA)
At 120 mA, the current is 2.4 times the 50 mA lethal threshold. This is why 120V is unequivocally lethal under the wrong conditions.
How Lethal Current Shifts Across 120V, 230V, and 3-Phase Systems
The assumption that fixes our baseline answer is a 1,000 Ω body impedance and a 50/60 Hz AC frequency. If we change the voltage source—such as moving from a US 120V branch circuit to a European 230V outlet or a 3-phase industrial panel—the resulting current shifts proportionally. Three-phase systems are particularly dangerous because the line-to-line voltage is significantly higher than line-to-neutral.
| System Type | Nominal Voltage | Calculated Current ($I$) | Lethality Multiplier (vs 50mA) |
|---|---|---|---|
| US Line-to-Neutral | 120V AC | 0.120 A (120 mA) | 2.4x Lethal |
| EU Line-to-Neutral / US Line-to-Line | 230V AC | 0.230 A (230 mA) | 4.6x Lethal |
| 3-Phase Line-to-Line (EU) | 400V AC | 0.400 A (400 mA) | 8.0x Lethal |
| 3-Phase Line-to-Line (US Industrial) | 480V AC | 0.480 A (480 mA) | 9.6x Lethal |
As the OSHA electrical safety guidelines note, higher voltages not only push more current through the body, but they also cause severe internal thermal burns and violent muscle contractions that can throw a worker across a room, causing secondary blunt-force trauma.
Neighboring Physiological Thresholds (±20% Range)
The 50 mA (0.05 A) threshold for ventricular fibrillation is not a cliff; it is a steep curve. Below is the physiological breakdown of the ±20% neighboring range around that lethal 50 mA mark. Understanding this narrow band explains why ground fault protection is calibrated so aggressively.
| Current (mA) | Deviation from Lethal | Physiological Effect |
|---|---|---|
| 40 mA | -20% | Severe muscle tetany; 'let-go' threshold exceeded (victim cannot release the conductor). |
| 50 mA | Baseline | Ventricular fibrillation threshold. Heart rhythm disrupted; lethal without immediate defibrillation. |
| 60 mA | +20% | Guaranteed fibrillation; onset of internal tissue heating and localized electrical burns. |
When Voltage-to-Current Conversion Becomes Meaningless
In industrial electrical engineering, we constantly use Power Factor (PF) to convert between apparent power (kVA) and real power (kW). A common mistake hobbyists make is trying to calculate 'lethal watts' by applying an assumed power factor to a human shock scenario. This conversion is physically meaningless.
Power factor represents the phase shift between voltage and current caused by inductive or capacitive loads (like motors or transformer coils). At 50/60 Hz, the human body is almost purely resistive. While the skin has a minor capacitive dielectric property, the overall phase angle is negligible, meaning the PF is effectively 1.0. Attempting to apply an industrial PF (e.g., 0.8) to a human body model yields garbage data.
Furthermore, this entire 50 mA conversion becomes meaningless if the frequency shifts. At high frequencies (above 10 kHz, such as in Tesla coils or RF heating), the 'skin effect' causes current to travel over the surface of the body rather than through the internal organs and heart. A person can survive amperages at 100 kHz that would be instantly fatal at 60 Hz. Always verify the frequency assumption before applying 50/60 Hz physiological thresholds.
Frequently Asked Questions About Lethal Current
Can 1 amp kill you if the voltage is very low?
Yes, 1 amp (1000 mA) is 20 times the lethal threshold and would be instantly fatal. However, Ohm's Law dictates that you cannot push 1 amp through a 1,000-ohm human body with a low voltage source. To force 1 amp through the body, you would need 1,000 volts ($V = I \times R$). A 12V car battery can supply hundreds of amps, but it lacks the electrical 'pressure' (voltage) to push more than a few milliamps through dry skin. The current available from a power supply is irrelevant; only the current actually pushed through the body matters.
Why do we say 'it's the amps that kill you, not the volts'?
This is a popular bench-top adage, but it is technically incomplete. Current (amps) is the physical mechanism that disrupts cellular function and stops the heart. However, voltage is the mandatory delivery mechanism. A static electricity shock from a doorknob can involve 10,000 volts, but the total charge is measured in microamps and dissipates in nanoseconds, making it harmless. Conversely, 120V is relatively low pressure, but it can sustain a continuous 120 mA flow through the chest. Voltage is the gun; amps are the bullet.
How fast does a GFCI breaker trip to prevent lethal shock?
Under NEC (NFPA 70) standards, a Class A Ground Fault Circuit Interrupter (GFCI) is designed to trip when ground-fault current reaches 4 to 6 milliamps (0.004 - 0.006 A). Crucially, it must clear this fault within 20 to 30 milliseconds. This speed is engineered specifically to interrupt the circuit before the current can induce the 'let-go' tetany or reach the 50 mA fibrillation threshold, keeping the shock painful but non-lethal.
