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# Why is it so? Watts/Amps/Volts

#1
10-04-17, 05:35 AM
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Why is it so? Watts/Amps/Volts

I have what is likely a very simple question about electricity. I'll preface the question by saying that the questions is not related to any project I'm undertaking. It is simply a question so that I can better understand.
Let's take this item:

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

It says it's capable of 7500 watts. It says it's 30 amps. It also says 120v/240v (either I assume).
Now, if it can handle 7500 watts of power and it can take 30 amps, I assume that's based on 240V.

Let's say I wanted to run only 6000 watts into this plug inlet, but, the power was at 120v? that's LESS watts(power?), but it would calculate to 50 amps (since it's 120v). let's also assume the power going into the plug had been jumpered so it's going through both hots on the plug inlet not just one.

Since the power inlet box is rated at 30 amps, I have to assume this is not possible. but I want to understand why. I'd be trying to input less wattage(power) over the same wires/input box.

I'm sure there is a simple explanation of why this is so, but it eludes me even after reading several sites on the relationship between amps/volts/watts.
Thanks in advance for your input!

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#2
10-04-17, 05:39 AM
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Your outlet could handle the load, Different designs are so you can only plug the rated idem in.

#3
10-04-17, 05:53 AM
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That inlet is rated for up to 30A per leg. If you had a power source of 50A at 120V to feed that inlet by the book you would need to have 30A fuses on each leg after the jumper to ensure that no more than 30A is allowed on each path. The 7500W description on the product page is really just there to indicate this is the inlet that corresponds with generators whose output is around 7500W. The actual rating is the one stamped or printed on the device, which is 30A.

In practice, inlets are not often a concern because portable generators are not capable of producing the kinds of sustained currents that would cause such damage. Generators large enough to do so are not really all that portable and typically hardwired through a large transfer switch or have some other special connection.

#4
10-04-17, 05:59 AM
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Thanks for the replies.
So, in the case of 10 gauge wire which is required for 240v 30amps. Is the reason then that 10 gauge is also required for 120v 30amp be that there is ONE hot used for 120v vs. 2 for 240v connections?

#5
10-04-17, 08:18 AM
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THe reason is that the gauge requirement of wire has nothing to do with Voltage and is simply the Amps of current needed.
Related to insulation temperature, which is a function of resistance of the wire and the temperature rating of the plastic covering.

#6
10-04-17, 08:47 AM
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For the metallic part of the wire, thicker wires can carry more amps regardless of voltage. Wires with thicker insulation coverings can carry higher voltage, however virtually all building wire is rated to at least 600V so that issue is irrelevant for residential and light commercial wiring.

A good counterexample is insulated electric fence wire -- it's about a 3/8" diameter, but almost all insulation and little metal. This wire is designed to carry low amps, high volts. Compare that to automotive battery cable. Very thick cable, almost all metal. This cable is designed to carry high amps, low volts.

#7
10-04-17, 10:06 AM
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To take a step back, remember a version of Ohm's law: watts = volts * amps

A given device (60 w light bulb for example) will pull 60w regardless of the volts. At 120v, a 60 w light bulb will pull 0.5 amps. That 0.5 amps would require only a thin wire to pass that many amps. That's one reason most light fixtures are wired with 18ga wire internally.

But plug that same 60w light bulb into a 12v car battery. 60w/12v = 5A. In a car, that same 60w (headlight) will need a heavier wire to handle that many amps.

That is why it's typically better to run at higher voltages at longer distances and higher power draw devices. You (in theory) could run an electric water heater off 120v, but it's easier to use 240v since you'll need lighter gauge wires. In industrial settings, 277, 480, and higher voltages make it so huge wiring isn't required.

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