The direct answer to how many amps can kill a person is 100 milliamps (0.1 Amps) of 50/60Hz AC current passing across the chest for just 1 to 2 seconds, which triggers fatal ventricular fibrillation. However, under prolonged exposure, the threshold drops: as little as 30 mA (0.03 Amps) can be lethal if the shock duration exceeds a single cardiac cycle. We calculate this by treating the human body as a resistive load using Ohm's Law: I = V / Z. For a standard 120V AC touch with wet skin impedance (Z ≈ 1000Ω), the substituted formula is I = 120V / 1000Ω = 0.12A (120 mA)—firmly in the lethal zone.
The Core Formula: Converting Touch Voltage to Lethal Current
To understand electrical shock hazards, you must convert the available voltage into the actual current that will flow through the body. The formula is I = V / Z_body, where Z_body is the total impedance of the human body. According to OSHA electrical safety guidelines and the IEC 60479 standard, human impedance is not a fixed number; it is highly dependent on skin moisture, contact area, and the current path.
Dry, intact skin has an impedance of roughly 100,000Ω, but wet or broken skin drops that impedance to 1,000Ω or less. Once the skin barrier is breached, internal body impedance drops to roughly 300Ω to 500Ω. The assumption that fixes our baseline lethal calculation is a hand-to-hand current path across the chest with damp skin (1000Ω total impedance) at standard 60Hz AC frequency.
| Current (AC 60Hz) | Physiological Effect | Lethality Risk |
|---|---|---|
| 80 mA (0.08 A) | Severe shock, respiratory paralysis, breathing stops | High if prolonged |
| 100 mA (0.10 A) | Ventricular fibrillation threshold (1-2 second exposure) | Lethal |
| 120 mA (0.12 A) | Certain ventricular fibrillation, muscular lock-in | Highly Lethal |
How the Lethal Threshold Shifts: 120V vs 230V vs 3-Phase
Presenting a single current value as universally lethal ignores the voltage driving it. The available voltage dictates how much current is actually pushed through the body's impedance. Here is how the real-world danger shifts across common global power systems, assuming a damp-skin hand-to-hand impedance of 1,000Ω:
- 120V AC (US/Canada Residential): Pushes roughly 120 mA through wet skin. This crosses the 100 mA ventricular fibrillation threshold, making standard US outlets unequivocally lethal under damp conditions. With dry skin (~10,000Ω), it pushes only 12 mA—painful, but usually non-lethal.
- 230V AC (UK/EU/AU Residential): Pushes roughly 230 mA through wet skin. This is more than double the V-Fib threshold, meaning fibrillation occurs almost instantly. Even with dry skin, 230V pushes ~23 mA, which crosses the 'let-go' threshold (10-15 mA), causing muscles to contract and trapping the victim on the live conductor.
- 3-Phase 480V AC (Industrial): A line-to-line touch pushes 480 mA or more. At this level, the immediate danger shifts from just fibrillation to massive internal tissue burns, explosive muscle contractions (which can throw the victim), and severe arc flash hazards. NFPA 70E mandates strict PPE and approach boundaries for these systems because the current levels guarantee catastrophic injury without heavy protection.
Decision Path: Selecting the Right Protection
Knowing how many amps can kill a person is only useful if it drives your safety hardware choices. Use this decision tree to select the correct protective device for your environment. We terminate this path in a concrete hardware requirement for standard AC circuits.
| Condition / Environment | Calculated Risk | Mandatory Hardware Pick |
|---|---|---|
| Dry indoor, < 50V AC (e.g., 12V/24V DC or AC control circuits) | Below perception threshold (< 1 mA). Safe. | Standard fuses/breakers for wire protection only. |
| Dry indoor, 120V/230V AC (Standard branch circuits) | Let-go threshold risk (15-30 mA). V-Fib risk if skin is compromised. | Standard 15A/20A thermal-magnetic breaker + AFCI. |
| Wet/Damp locations, 120V/230V AC (Bathrooms, outdoors, kitchens) | High V-Fib risk (>100 mA) due to low skin impedance. | Class A 30mA GFCI / RCD Breaker. |
| 3-Phase Industrial (208V - 480V) | Severe burn, lock-in, and arc flash risk (>300 mA). | Ground Fault Relay (set to 30mA-300mA) + NFPA 70E PPE. |
When the Current Calculation is Meaningless
While I = V / Z is the bedrock of shock analysis, the conversion becomes meaningless—and the situation becomes universally lethal—under specific edge cases where impedance cannot be reliably estimated:
- Broken Skin or Puncture Wounds: If a live wire contacts a cut or puncture, the high-resistance outer layer of the epidermis is bypassed entirely. Internal tissue impedance is only ~300Ω. Even a 'safe' 50V source can push >160 mA directly into the bloodstream.
- Water Immersion: If a person is in a bathtub or pool, the water acts as a parallel conductive path, effectively reducing total body impedance to near zero. In these scenarios, 120V will push amps, not milliamps, causing immediate cardiac arrest.
- High-Frequency AC (>10 kHz): At high frequencies, the 'skin effect' causes current to travel along the surface of the body rather than through internal organs. This is why a person can survive touching a high-voltage Tesla coil. However, the calculation for internal V-Fib becomes meaningless here, as the primary hazard shifts to severe surface RF burns rather than cardiac arrest.
FAQ: Lethal Current Misconceptions
Is it the volts or the amps that kill you?
It is the amps (current) that disrupt the heart's electrical system and kill you. However, volts (voltage) are the pressure required to push those lethal amps through your body's natural resistance. Without sufficient voltage, lethal current cannot flow. A static shock from a doorknob involves thousands of volts but only microamps of current, making it harmless. A 120V outlet has lower voltage but can deliver hundreds of amps of available fault current, making it deadly.
Can DC current kill you, and is the threshold different?
Yes, DC current can kill you, but the human body is roughly 3 to 5 times more tolerant of DC than 60Hz AC. The ventricular fibrillation threshold for DC is generally around 300 mA to 500 mA, compared to 100 mA for AC. This is because AC's constant zero-crossing perfectly mimics and disrupts the natural pacing signals of the human heart, whereas DC tends to cause a single massive muscle contraction (often throwing the victim clear of the source) rather than inducing fibrillation.
Why do we use 30mA for GFCIs if 100mA is the lethal threshold?
GFCIs and RCDs are calibrated to 30mA (or 5mA for personal protection in some specific US applications) to provide a massive safety margin. Because shock duration, individual heart health, and exact skin impedance vary wildly, tripping at 30mA ensures the current is cut off long before it can reach the 100mA V-Fib threshold, even if the victim is wet or the contact is prolonged.
