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Which One is More Dangerous? 120V or 230V and Why?

Which AC Voltage Level is Safer? 230V or 120V in Domestic and Residential Applications

We all know that the domestic and residential voltage levels at the homes are 230V AC (in EU – IEC) and 120V and 240V AC (in US – NEC). The most frequently asked question is which voltage level is more dangerous (or safer) to use and what are the reasons. To clarify the confusion, let’s see the following statements and examples.

First of all, the confusion arises when we see the statement “According to Ohm’s Law, Current Increases when Voltage increases (I=V ÷ R), but Current decreases when Voltage increases according to (P = V×I) formula“.

In addition, keep in mind that Amperes are responsible for electrocution, Not the Volts. In other words, voltage is responsible to drive the current i.e. current kills, not the voltage. Keep in mind that don’t fall for the next confusion about which one is more dangerous AC or DC voltage levels as there are some differences between AC and DC but the effect of electric shock is almost the same on the human body. Now let’s move to the actual scenario.

One thing is clear that the more current is flowing in the circuit, the higher chance of worst electrocution when one of the live wires comes in contact with the human body. Now let’s see which circuit has more current flowing in it when connected to the 230V and 120V respectively.

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It really confuses the newbies when we assume a circuit of 80W bulb connected across 120V and then 230V as it clearly shows that there is more current flowing in the 120V circuit as compared to the 230V circuit. Let’s see the following example.

Suppose, there is an 80W bulb which is first connected across 120V and then 230V supply voltage. Now let’s determine the amount of flowing current in both circuits as follows.

Which AC Voltage Level is Safer - 230V or 120V

100W bulb in 120V Circuit

To calculate the current in amperes, we may use the basic power equation (P = V×I) as follow:

I = P ÷ V ⟹ I = 100W ÷ 120V ⟹ I = 833 mA

100W bulb in 230V Circuit

I = P ÷ V ⟹ I = 100W ÷ 230V ⟹ I = 435 mA

Well, we see that more current (almost double) is flowing in the 120V circuit as compared to the 230V circuit in case of 80W bulb load connected across them. Right? but this is not the actual case.

This is OK only in case when power is constant (like input and output of a transformer is the same) but the supplied power varies from the power house. In other words, the above scenario is OK if wattage is the same. If resistance is the same, the case will reverse (V=I×R) and if its current goes through the human body, the resistance is the same, so the current will double from 120V to 230V.

These different currents will flow in the bulb as they have different resistance, but when you touch with these voltages, current will depend on your body resistance and current will flow as per ohm law. The example above only happens and is suitable when the power(W) of the circuit is constant.

P = V × I … (where P = power, V = voltage, I = current)

P = V × I … if “P” in watts is constant, then P with I is inversely proportional .

As an example = (When maintaining the power of 100W as constant)

100W = 120V × 0.833A

We have to keep 100W as constant and change the value of current and voltage.

Then , 100W = 230V × 0.435A

It is similar like a transformer i.e. if we step down voltage then automatically the current is step up and vice versa).

The above statement is right when we connect the 80W light bulb in both circuits as both bulbs in both circuits will produce the same light Intensity (energy density) as the wattage rating of both bulbs are the same. But if the bulb is only rated for 120V, it will blow when connected in a 230V circuit (see the upcoming example below). Similarly, if the bulb is rated for 230V and connected to a 120V supply, it will glow dimmer as compared to the 230V supply.

Let’s go deeper. If you have a 120 Watt lamp from the US rated at 120V, it’s gonna run 1 Amp. If you take that same lamp to the UK and plug it into 230V, it’ll draw almost 2 amps, and 230 Watts (and probably blow very quickly).

Likewise a 120 Watts lamp from the UK rated for 230V will draw 0.52 Amps on 230V. Bring that to the US and it’ll draw 0.26 Amps, 60 Watts, and be very dim. The statement is OK when resistance is constant (and power is variable, same in the case of the human body). It shows 120V needs higher (which is low) current for the same wattage lamp as compared to 230V circuit (where current is almost doubled). In short, a same wattage rated light bulb drawing 1 amp on 120V will draw almost 2 amps on 230V supply voltage.

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So what is next? Keep calm and join it as it is the resistance.

Each material has its own resistance and when the manufacturer designs an appliance, they consider the available voltage level for the suitable operation as resistance comes first than the current and voltage. In this case, the resistance of the bulb’s filament will be almost half in case of 120V circuit as compared to 230V circuit. This way, lower current will flow in the 120V circuit.

If we connect a 80Ω bulb in both (120V & 230V) circuits, the bulb connected to the 230V will glow brighter than the bulb connected across 120V. This is because low current is flowing in the 120V circuit which is unable to deliver the required amount of current which is needed to glow the bulb. In other words, a bulb having the same resistance will glow dimmer in case of 120V as compared to the 230V. Let’s see the following example.

120V is Safer than 230V AC

80Ω bulb in 120V Circuit

To calculate the current in amperes, we may use the basic Ohm’s law (V = I × R) as follow:

I = V ÷ R ⟹ I = 120V ÷ 80Ω ⟹ I = 1.5 A

80Ω bulb in 230V Circuit

I = V ÷ R ⟹ I = 230V ÷ 80Ω ⟹ I = 2.875 mA

We see that more current is flowing in the bulb’s filament when connected to 230V supply voltage. That is the reason the bulb connected to the 230V circuit is brighter while the same bulb is dim when connected to the 120V circuit. In short, the more the current, the higher the power dissipation in the form of heat and light.

This shows the bulbs are rated for specific voltage levels. In short, the 230V rated bulb connected to the 120V circuit will glow dimmer and the 120V rated bulb connected to the 230V circuit will blow it at all.

Let’s move to the final stage as now we know that the things which have to be considered are voltage, current and resistance when talking about the electric shock and human body.

Good to Know:

The average resistance of a human body in dry condition is almost ≈ 100,000Ω while the resistance of a human body in wet condition is 1000Ω.

Also, the voltage above 50V (in dry condition) and 25V (in wet condition) is enough to shock a person. Also, 30 mA (RCDs are set in the UK) is enough for respiratory paralysis while 75-100 mA will cause ventricular fibrillation (rapid & ineffective heartbeat).

Anything higher than 300mA is fatal and kills in seconds. 4.5 to 10A will instantly lead to cardiac arrest, severe burns and finally death.

Overall, it is mainly the eclectic power (a mixture of current and voltage) where voltage (as a pressure) pushes electric current (as a flow of charge) is responsible for electric shock.

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Now let’s see what happens when a human comes in contact with 120V and 230V supply voltage levels. Let’s assume a person in wet condition (standing in a rain, puddle of water or sweating having a resistance of 1kΩ) makes a contact to the 230V and 120V circuit and analyze the results.

Which One is More Dangerous - 120V or 230V and Why

Human Body in Contact of 120V Supply

  • I = V ÷ R
  • I = 120V ÷ 1kΩ
  • I = 120 mA

Human Body in Contact of 230V Supply

  • I = V ÷ R
  • I = 230V ÷ 1kΩ
  • I = 230 mA

We see that more current is flowing in the human body when contacted to the 230V as compared to the 120V supply voltage. Hence, it proves that 120V supply voltage is safer than 230V supply voltage. In other words, 230V is more dangerous than the 120V voltage.

It shows the human body has a resistance, and with V = I×R, if we double the voltage, we double the current. So current kills, but you get twice the current through your body with 230V as you do with 120V.

 As shown in the first trap example, the 100W bulb has different resistances for 120V or 230V. When we touch the live conductor of 230V, the current flows 2 times higher than 120V. Remember the resistance of our body is the same in both cases as compared to the bulb or any other electrical appliance which has different values of resistances and operating voltage as well as wattage ratings. As a human body, we have only a constant value which is resistance whereas the resistance is variable (different values for specific and different voltage levels by designers and manufactures).  

To keep things simple, think of voltage as pressure, the higher the volts the more pressure that is behind it. Technically the current does the damage, yes, but you get more current from more volts. It is similar to asking which voltage level is more dangerous, 1V or 1000V? This is why you see “Warning signs” at high voltage appliances and equipment i.e. higher voltage ⟹ higher danger.

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It is the Amps (Current) which kills, not the Volts (Voltage). But keep in mind that high voltage moves more amps when the power is variable. If the resistance (like the human body) is constant (same), more current will flow in it when voltage increases. Overall, it is the energy which leads to serious damage, burn, electrical hazard and even electrocution. In short, the mixture of current and voltage (electric power) is responsible for all the bad things mentioned above (no matter 55V, 120V or 230V single phase or 415-480V thee phase).

If you come with a possible but valid question of a 12V battery where voltage is low and current is high but harmless at all. That is because the voltage level is not that high which makes it possible to flow the current into your body as the human body has very high resistance (10k Ohms in dry condition). Keep in mind 25V is enough to shock you (in wet condition) while anything higher than 50V is harmful even in dry conditions.

The amps depend on the nominal voltage and the maximum power that the thing you are touching can generate. Thus, you can safely touch a car battery that say 100A peak, because the voltage is low (12v) so the current flowing inside you is like microamps. Related to the post (high voltage but low current), a person in the inbox confirmed “taken a live contact with 7500 to 12,000 Volts a couple times! Fortunately only 30mA but it still hurts like a mofo and makes you scream like a little school girl!”.

Similar is the case in aluminum smelters (where each smelting pot is powered by very high amperage and only a few volts) or welding machines having high amperage and low voltage levels. As current find a least resistive path to flow, i.e. it flows through the base metals ⟹ the ground cable ⟹ than human body. If the human body is the only conductor (in absence of metals), you may get severe shock. In case of arc welding, there is a risk of serious electric shock when you touch two metal objects that have a voltage difference between them. In addition, never ever do welding procedures in wet, rain or snow as it will reduce the overall resistance of the human body and low voltage be able to flow current through your body.

According to (V = I ÷ R), Both currents & voltage are lethal w/o voltage no currents will flow. You cannot electrocute if the power source is not energized. Hence, the higher the voltage, the more dangerous it is.

To summarize the post, It is the energy that kills you, energy is proportional to V2, thus 230V is more dangerous, during the same duration of time.  

Keep in mind that electricity is deadly in all forms. 120V, 230V, 240V and so on makes no difference. Under the right circumstances it’s all deadly.

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