What is the difference between all of these different voltages? Why are they used, what’s the purpose behind them, and where do the numbers come from?
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Low Voltage systems (50 volts and below) are typically used for communication and signaling. These are far less dangerous voltages to come into contact with which is why they’re not regulated by the National Electrical Code. These are 5, 10, 12, 16, 20, 24 volts for example. A lot of doorbell transformers, LED drivers, power supplies, and dimming controllers use these voltages in either an AC or DC pulse to control the switching on and off of other equipment.
Next is the Medium Voltages that we use as electricians, and this is anything from 100 volts to 250 volts. This is typically what you’ll see in homes and small businesses. 120 volts and 240 volts for a single-phase, and 120, 208, and 240 for three-phase. Depending on how a transformer is wound at the service, you’ll have 208/120 or 240/120, and in the case of a 3phase 4wire Delta application, you’ll have 208/240/120 all in the same service.
Last is the High Voltages that are found in a lot of larger office buildings or places with really powerful equipment like industrial factories and plants. This is the 277-600 volt range, and most commonly you’re dealing with voltages at 277/480. Just like 120/208 means that between hot and neutral you have 120 volts, and between two hots you have 240 – in a 277/480 service the voltage between hot and neutral is 277 and the voltage between two phase (hot) conductors is 480 volts. It’s just a lot stronger push.
How these numbers come to be is simply how power is generated and transformed, which could have its own video entirely, but you can see in a 120/240 environment that 120 volts are simply half of 240. This means that the incoming 240 volts are being tapped in the center so there are two 120 volt circuits now able to be used within that 240-volt system. When dealing with 3-phase systems, however, we use the square root of 3 or (1.732) in all of our calculations to account for the phase angles of all of the windings and how they’re being generated. So if you’re wondering how we get numbers like 208, 277 you use 1.732 (because they’re three-phase voltages). For example 480 / 1.732 = 277 volts, and 120 x 1.732 = 208 volts.
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