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Recently, a home owner tied a hot wire directly to a neutral and asked about it on this site. Fortunately, his unfortunate mistake did not result in anyone being injured. As an answers point out, usually a device drawing current reduces the load on the wires and prevents them from heating up.

However, this got me thinking... how to green and low power draw devices, such as a led bulb or cell phone charger draw enough current to prevent this from happening?

Are they less efficient than I imagine? Or need more current than I am thinking?

15 Amps is a lot of current, and it does not seem to me that connecting a low power-draw device would be radically different than connecting a hot and neutral wire directly.

Why did directly connecting the hot and neutral wires in the linked question draw enough current to trip the breaker?

TheCatWhisperer
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  • I'm not sure exactly what you're asking, but a device will only draw the current that it needs. The 15 amp circuit breaker is there to protect the house wiring from heating then burning, the circuit breaker does not provide overcurrent protection to a device or appliance. – Tyson Jan 13 '18 at 18:42
  • @Tyson why in the linked question, did directly connecting the live and neutral wires cause enough current draw to trip the breaker? – TheCatWhisperer Jan 13 '18 at 18:44
  • Because that's a direct short, there is nothing limiting current flow. – Tyson Jan 13 '18 at 18:45
  • @Tyson my question is, how is connecting a low power device without much resistance effectively much different than a short? – TheCatWhisperer Jan 13 '18 at 18:47
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    "without much resistance" is your error in logic. You're confusing load and resistance. – Tyson Jan 13 '18 at 18:49
  • Measuring a coil on a motor usually shows a resistance less than a few ohms , the mistake made by folks that don't understand electricity is this is a DC measurement. If DC 120v line voltage is attached to the motor coil it would trip a 120v dc breaker supply but there is reactance and this creates the equivelent of resistance so the 120v AC motor that measured 1 ohm or less dc may only draw 4.4 amps for a 1/6 HP or up to 16 amps for a 1 horse motor With very similar coil resistances because of the 120v 50/60hz reactance. – Ed Beal Jan 16 '18 at 17:39

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The last sentence in your first paragraph is wrong. A device drawing load on the wires does not reduce their load, it does not prevent them from heating up. Actually, putting a 2 amp load on a pair of wires increases its load (by 2 amps) and causes it to heat up (by W=I2R or W=2*2*R watts, with R being its resistance in ohms). So a 0.1 ohm wire is now making 0.4 watts of heat when it wasn't before.

If it helps, recognize that some loads have electrical characteristics which naturally limit how much current they flow at a design voltage. We supply them that designed constant-voltage, and they sip only the current they need. For instance a resistive "1500W" heater will limit its draw to about 12 amps.

Not all loads have this ability. Some loads cannot limit their current and will flow uncontrollable amounts of current if supplied a constant voltage. A great example is a fluorescent tube, which is why fluorescents have ballasts.

However if some outside force limits how much current they can draw, then they behave in reasonable ways: the voltage that they "drop" (consume) at that current is reasonable, and produces useful work -- like light from the fluorescent tube or LED emitter. It's possible to build a constant-current power supply which will forcibly limit current to spec. This power supply need not be constant-current; it can be varying-current, the point is, it must limit current because the device it's attached to can't.

How do you hook up multiples of these?

For constant-voltage devices of the same working voltage, for instance a bunch of iPhone chargers that want 120VAC -- you supply them all spec voltage, and you hook them up in parallel. Each device "sips" the current that it needs, and no more. The power supply must push enough current to work all the devices.

For constant-current devices of the same working current, for instance several bare LED emitters that want 350 milliamps -- you supply them all spec current, and you hook them up in series. Each device gets its design current (because everything in a series loop gets the same current), and each one "drops the voltage" it needs to function. The power supply must push out enough voltage to work all the devices.

What happens if you take a constant-voltage device like a small heater, designed to run at 117V and which will tend to draw 350ma... and you put it in series with constant-current load (like an LED emitter with rectifier) that is designed to draw 350ma and drops typically 3 volts? That will work, actually. The constant-voltage heater will act as a current-limiter for the LED, keeping the LED in spec at 350ma.

In mains power, you are dealing with devices designed to be constant-voltage, so you connect them in parallel.

Wire-nuts will flow unlimited amounts of current uncontrollably, so they simply do not work as constant-voltage devices. They are better thought of as constant-current devices with very small voltage drop - like the LED in the LED-heater combo discussed above. They must be in series with a current limiting device of some kind, like a constant-voltage appliance which is able to limit its current to within the spec of the wires.

Harper - Reinstate Monica
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  • so https://diy.stackexchange.com/questions/100511/why-dont-you-connect-unused-hot-and-neutral-wires-to-complete-the-circuit#answer-100513 is incorrect? "Normally a dishwasher is connected to both supply and drain, but it also has valves and a place to do some work" – TheCatWhisperer Jan 16 '18 at 14:20
  • @TheCatWhisperer it's a weak metaphor because it disregards what work is in that context, and I think perhaps that may also be a source of confusion for you. A dishwasher just wastes the hydraulic pressure in the water, whereas the voltage pressure is not wasted at all. Probably the purest way to study "work" in electrical terms is to look at resistive heating, because it maps very elegantly to Ohm's Law. I would explore that. – Harper - Reinstate Monica Jan 16 '18 at 20:17