What's the theory behind this?

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EEC said:
Can you show this using math? resistance of wire then resistance of lamp
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current = voltage / resistance

With a direct short, you get:

current = voltage / 0 = infinity (actually limited by the source)

The lamp has resistance so the current approaches a number less than infinity. With a given voltage, the bigger the resistance, the lower the current.
 
EEC said:
What's the resistance of a 25 watt lamp
What's the resistance of 25' of #12 AWG CU.?
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How about you give the math a try using these formulas:

watts = current^2 * R
watts = volts^2 / R

Also see Chapter 9, Tables 8 & 9 of the NEC for the resistance.
 
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mivey said:
current = voltage / resistance
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EEC said:
I want the math to show the the 120V hot conductor only (no load) to grounded neutral
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I already gave you the formula.
mivey said:
current = voltage / resistance
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What exactly are you trying to ascertain? Are you assuming a perfect source or conductor? Are you trying to calculate the exact amount of fault current that you will see (can't be done without some assumptions)?

Assuming a perfect source starting at the last grounding point:
current = voltage / resistance (or actually the impedance)

current = 120 / (impedance of: ungrounded conductor + grounded conductor + connectors/contacts + fault)
 
If you had a short circuit 50 feet from the breaker using #12 wire at about 77 degrees, you would have 100 feet of wire @ about 1.018 ohms / 1000 feet or 0.1622 ohms for the hundred feet of wire in the circuit.


voltage / resistance = current

120 / 0.1622 = 740 amps.... might trip a 20 a breaker and make a spark or two

As the wire heated up with the 740 amps, the resistance would change, but I don't care about that.

if you had a 100 watt light bulb.... er, I mean incandescent lamp:).... in series in the circuit, you'd have about 144 ohms resistance ( I'm not considering the wire's resistance or it's voltage drop because I'm too lazy) :


voltage / resistance = current

120 / 144 = 0.83 Amps ......... no trip ... no sparks... just like we like:).
 
Smart $ said:
PS: "Light bulb" is appropriate in most parts of the country/world. :D
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That world that you are refering to, speaks english.

I find lamp more adecuate. It is "appropriate" for all the world. There's no chance of getting confused. Specially if you know a few things about electricity.
 
Mayimbe said:
That world that you are refering to, speaks english.

I find lamp more adecuate. It is "appropriate" for all the world. There's no chance of getting confused. Specially if you know a few things about electricity.
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Around here, if you want a light bulb, and point to a table where upon resides one of these:
http://www.lampsusa.com/index.asp?PageAction=VIEWCATS&Category=15
and one of these:
http://www.bulbs.com/"Regular"_(A--Shape)/results.aspx

and say the following: "Please hand me that lamp", your next sentence is going to be something like: "No, the thing without the cord."

Then you will get a look as if you had just give birth to a rhinoceros. :grin:
 
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