What's the theory behind this?

[EEC]

Can you show this using math? resistance of wire then resistance of lamp

 

[mivey]

Can you show this using math? resistance of wire then resistance of lamp

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.

 

[K2500]

OT_________

 

[EEC]

What's the resistance of a 25 watt lamp
What's the resistance of 25' of #12 AWG CU.?

 

[ohmhead]

Well, yes we have, courtesy of Cadpoint.

Roger

Well you must go easy on me Roger i wasnt a member then so i missed that one .

 

[mivey]

What's the resistance of a 25 watt lamp
What's the resistance of 25' of #12 AWG CU.?

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.

 

[EEC]

I want the math to show the the 120V hot conductor only (no load) to grounded neutral

 

[mivey]

current = voltage / resistance

I want the math to show the the 120V hot conductor only (no load) to grounded neutral

I already gave you the formula.

current = voltage / resistance

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)

 

[realolman]

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.

 

[Mayimbe]

PS: "Light bulb" is appropriate in most parts of the country/world.

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.

 

[mivey]

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.

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: