Electric shock lethality is determined by the amount of current (amps) passing through the body, while voltage (volts) is the electromotive force required to push that current through your skin's resistance. When makers and apprentices ask what kills you amps or volts, the direct answer is that amps cause the biological damage, but volts are the necessary delivery mechanism. Think of voltage as water pressure and current as the actual flow of water; high pressure is only dangerous if it forces a lethal volume of water through the pipe, which in this case is your body. Without sufficient voltage to overcome your skin's natural resistance, lethal current cannot flow, but once that resistance is breached, it is the current that stops your heart.
The Lethal Thresholds: Current vs. Voltage
To understand what changes in a real circuit or installation regarding safety, we have to look at the exact milliamp (mA) thresholds where human biology fails. Safety devices like Ground Fault Circuit Interrupters (GFCIs) and equipment grounding schemes are engineered entirely around these current thresholds, not just the nominal system voltage.
| Current (mA) | Physiological Effect | Real-World Consequence |
|---|---|---|
| 0.5 - 1.0 mA | Threshold of perception | Slight tingling sensation; no pain or muscle reaction. |
| 1.0 - 5.0 mA | Slight shock, not painful | Startle reaction; may cause indirect injury (falling off a ladder). |
| 6.0 - 16.0 mA | Painful shock, muscle control loss | The 'let-go' threshold begins; victim cannot voluntarily release the conductor. |
| 17.0 - 99.0 mA | Extreme pain, respiratory paralysis | Severe muscle contractions; breathing stops, but heart rhythm may remain intact. |
| 100 - 200 mA | Ventricular fibrillation | Heart muscles quiver chaotically; fatal within minutes without immediate defibrillation. |
| > 2,000 mA | Cardiac arrest and severe burns | Heart stops entirely (may restart if current is removed); massive internal tissue cooking. |
According to OSHA electrical safety guidelines, the transition from a painful shock to a fatal one happens in a remarkably narrow band between 10 mA and 100 mA. This data-dense reality is why the National Electrical Code (NEC) mandates GFCI protection in wet areas; the device is designed to trip before the current ever reaches the let-go threshold.
The Worked Example: Calculating Shock Current
To see how voltage and resistance interact to deliver lethal amps, we apply Ohm's Law (I = V / R). The variable that changes most drastically in a real-world installation is R (resistance), which is highly dependent on environmental conditions.
Assume you accidentally touch a standard 120V AC live wire in a residential panel.
Scenario A: Dry, Intact Skin
Dry, calloused skin has a high resistance, typically around 100,000 ohms (100 kΩ).
- Voltage (V): 120V
- Resistance (R): 100,000 Ω
- Current (I): 120 / 100,000 = 0.0012 Amps (1.2 mA)
Result: At 1.2 mA, you are just above the threshold of perception. You feel a mild tingle, but you are not in danger of fibrillation or muscle lock.
Scenario B: Wet, Sweaty, or Broken Skin
If your hands are wet, or if the wire punctures the outer layer of skin, resistance plummets. Wet skin resistance can drop to 1,000 ohms (1 kΩ). Once the skin is breached, the internal body resistance is only about 300 to 500 ohms.
- Voltage (V): 120V
- Resistance (R): 1,000 Ω
- Current (I): 120 / 1,000 = 0.120 Amps (120 mA)
Result: At 120 mA, you have crossed directly into the ventricular fibrillation zone. The 120 volts provided exactly enough 'pressure' to push a lethal amount of current through your lowered resistance. This demonstrates why volts matter: 120V is enough to overcome wet skin resistance, whereas a 12V car battery (which can supply hundreds of amps) cannot push lethal current through your skin because the voltage is too low to overcome the resistance.
Where You Meet This in Practice
Understanding the interplay between voltage pushing and current killing dictates how we select tools, wire panels, and design safety systems on the jobsite.
GFCI and AFCI Breaker Curves
A standard Class A GFCI breaker or receptacle is engineered with a specific trip curve. It does not measure voltage; it measures the differential current between the hot and neutral conductors. The NIOSH electrical safety parameters align with the UL 943 standard, which dictates that a Class A GFCI must trip when the ground-fault current reaches
Multimeter CAT Ratings and Available Fault Current
When buying a multimeter, you will see ratings like CAT III 1000V or CAT IV 600V. Many hobbyists assume this is purely a voltage rating. In reality, it is a rating for transient overvoltage and the available short-circuit current (amps) the meter can safely withstand without exploding in your hands. A CAT IV meter is designed to survive the massive fault current (often 10,000+ amps) available at the service entrance, where the utility transformer can dump immense energy if a probe slips and causes a dead short.
Arc Flash and NFPA 70E
In industrial settings, high voltage is dangerous because it can arc across air gaps, but the resulting arc flash is driven by thousands of amps. According to NFPA 70E standards, an arc flash converts electrical energy into explosive thermal and acoustic energy. The incident energy (measured in cal/cm²) is a direct function of the available fault current (amps) and the clearing time of the upstream breaker. Here, the amps don't just stop your heart; they vaporize copper and cause third-degree burns at a distance.
Common Confusions: The High Voltage, Low Current Myth
People commonly confuse the capacity of a power source with the actual current drawn by the body. This leads to two persistent myths in electronics and safety.
The Static Electricity Shock
Walking across a carpeted room in winter can generate 20,000 volts of static electricity. When you touch a doorknob, you feel a sharp, painful snap. If amps kill, why doesn't 20,000 volts kill you? Because the total charge (Coulombs) is microscopic, and the duration is measured in nanoseconds. The voltage is incredibly high, which easily breaks down the air gap and your skin resistance, but the source simply does not have the sustained current capacity to push more than a few microamps for a fraction of a millisecond. The energy dissipates before it can disrupt cardiac rhythm.
Stun Guns and Tasers
Conducted energy weapons (CEWs) like Tasers operate on a clever manipulation of Ohm's Law. They generate roughly 50,000 volts specifically to arc through heavy clothing and cause dielectric breakdown of the skin. However, once the circuit is established, internal current-limiting circuitry restricts the actual flow to roughly 2 to 3 mA, delivered in rapid, short pulses with a very low duty cycle. The high voltage acts as the key to unlock the door (your skin), but the current-limiting circuitry ensures the flow stays well below the 100 mA ventricular fibrillation threshold, causing localized nerve and muscle disruption without stopping the heart.
Ultimately, when designing circuits, selecting PPE, or troubleshooting a live panel, remember the golden rule of electrical safety: respect the voltage for its ability to breach your defenses, but fear the amps for what they do once they are inside.
