How Many Amps Is Deadly? The Exact Current Thresholds and Shock Math

Most people asking how many amps is deadly assume there is a single, universal number. The direct answer is 100 milliamperes (0.1 Amps) of 50/60 Hz AC current passing across the chest (hand-to-hand or hand-to-foot) is the established physiological threshold for fatal ventricular fibrillation. However, because current is a dependent variable in a circuit, we must use Ohm’s Law to calculate when a specific voltage becomes lethal. The formula is I = V ÷ R. Substituting the fatal current threshold and a worst-case wet-skin body resistance of 1,000 ohms, we get: 0.1A = V ÷ 1000Ω. This means any voltage over 100V can push a fatal current through wet skin, making standard 120V and 230V mains inherently lethal under the right conditions.

The Fibrillation Threshold Window (±20% Range)

While 100mA is the textbook threshold for ventricular fibrillation, biological systems do not trip like mechanical breakers. The danger scales progressively. Below is the specific ±20% neighboring value range around the 100mA fatal threshold, illustrating how quickly the physiological outcome shifts from survivable to fatal.

For broader context, the Occupational Safety and Health Administration (OSHA) notes that the 'let-go' threshold—where muscles freeze and you cannot release the conductor—occurs much lower, between 10mA and 20mA. This is why a 'mild' shock that you cannot let go of is vastly more dangerous than a brief, higher-current static spark.

The Assumption That Fixes the Answer: Body Resistance

The conversion from a known voltage to a deadly amperage is entirely dependent on the resistance (R) in the circuit. Human internal tissue (blood, muscle, organs) is highly conductive, offering roughly 300 to 500 ohms of resistance. The variable that fixes the answer is skin resistance.

• Dry, intact skin: 10,000 to 100,000+ ohms. (High protection)
• Wet or broken skin: 1,000 ohms or less. (Minimal protection)
• Submerged in water: Skin resistance drops to near zero, leaving only the 500-ohm internal resistance.

When the Conversion is Meaningless

Calculating deadly amps using Ohm's law becomes meaningless when the power source lacks the capacity to sustain the current, or when the circuit impedance is artificially high. A static shock from a doorknob might measure 20,000V, which theoretically calculates to a massive current. However, the available charge is in the microamp range and dissipates in microseconds. Similarly, if an electrician is wearing Class 00 rubber insulating gloves rated to 500V, the circuit resistance approaches infinity. The current drops to zero regardless of the voltage, which is why NFPA 70E mandates specific PPE categories for energized work.

How the Danger Shifts: 120V vs 230V vs 3-Phase

The physiological damage scales non-linearly as voltage increases and forces more current through the body's fixed internal resistance.

• 120V (US/Canada Residential): Pushing 120V through dry skin (100kΩ) yields just 1.2mA (a harmless tingle). Pushing it through wet skin (1kΩ) yields 120mA. This crosses the 100mA threshold, causing fatal fibrillation. This is why GFCI protection is strictly required in kitchens, bathrooms, and outdoors.
• 230V (EU/UK/AU Residential): Pushes roughly 230mA through wet skin. Paradoxically, currents above 200mA often cause violent, full-body muscle contractions that can physically throw the victim clear of the source. While it causes severe internal burns, the 'throw' effect sometimes increases survival chances compared to the 100mA 'freeze and fibrillate' zone.
• 480V 3-Phase (Industrial): At this level, arc flash and explosive tissue damage become the primary threats. The current easily exceeds 1A, causing immediate cardiac clamp (the heart stops completely rather than fibrillating) and catastrophic thermal burns. Survival relies entirely on immediate medical intervention and arc-rated PPE.

Decision Path: Sizing Protection for Human Safety

Because we know the exact physiological thresholds (5mA for respiratory shock, 30mA for severe muscle freeze, 100mA for death), we can map these to specific protective devices. Use this decision tree to select the correct safety hardware for your circuit.

Frequently Asked Questions

Is DC current less deadly than AC current?

Yes, but not by a massive margin. Direct Current (DC) does not cross the zero-voltage line 120 times a second like 60Hz AC. Because of this, DC requires about 3 to 5 times more current to induce ventricular fibrillation (roughly 300mA to 500mA). However, DC is far more likely to cause a continuous muscle contraction that locks you to the circuit, and it causes severe electrolytic tissue damage (burns) at lower thresholds. According to NIOSH electrical safety guidelines, high-voltage DC (like solar arrays or EV batteries) must be treated with the same lethal respect as AC mains.

Is it true that 'it's the volts that kill, not the amps'?

No. This is a common misconception. Current (amps) is what actually disrupts the heart's electrical nodes and causes tissue burns. Voltage is simply the 'pressure' required to push that current through your body's resistance. A 20,000V static shock has high pressure but zero sustained current capacity, so it only startles you. A 12V car battery can supply 500 Amps, but it lacks the voltage pressure to push even 1mA through dry skin. The accurate phrase is: 'Volts push the amps, and amps kill.'