Electrical theory


Old 04-12-05, 10:52 PM
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Electrical theory

Hi there. This is my first posting so id like to say hi again.

Anyways, ive got a rather stupid question on electrical. Im an electronics hobbyist studying to one day become an electronics engineer, so you can grace me with all the complicated words and stuff and ill understand

The question that I have is this: The power that comes off of the secondary of the transformer that goes to my house (i live in US by the way) is 240vrms measured from the top to the bottom of the winding, correct? My 120v plugs would be 240v as well if they were connected from the top to the bottom of that secondary winding, but they are not. I believe that the secondary is center tapped, and that is where the neutral is. So that what I see at my 120v plug is a 60hz sinewave that goes from zero point up to +120v, back through zero point, then down to -120v. Is all of this correct so far?

If it is, my question is where is that center tap connected in my 120v plug, and where are those two 120v wires? It would seem to me that this would necessitate four holes in my plug instead of three. 1. 120v wire 2. 120v wire 3. neutral 4. ground.

Has this made sense at all? Could someone either explain this to me or point me to a good electrician tutorial, Ive searched and searched and havent been able to find one.

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Old 04-12-05, 11:05 PM
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Yes, all of that is correct.

The center-tapped neutral is the longer of the two blades on a standard 15A receptacle -- it is wired with a white wire. The shorter blade is the 120V hot (black or red), and the "D" hole is the ground (green).

You will find both 120V wires if you look at a large appliance receptacle like a cloths dryer or electric stove / oven which has 4 holes. This is 240V circuit, because there are two 120V lines in opposite phase which produce 240V phase-to-phase. A standard 120V receptacle only needs one hot and one neutral to produce 120V phase-to-neutral.

A home electrical service gets 2 120V hot wires and one neutral wire connected to the center tap of the transformer. Inside the breaker panel, the hot wires are used individually to feed a variety of 120V loads and combined to feed 240V loads.

Last edited by ibpooks; 04-12-05 at 11:10 PM. Reason: more info
Old 04-13-05, 03:46 AM
Join Date: Oct 2004
Location: USA
Posts: 719
Originally Posted by zachtheterrible
so you can grace me with all the complicated words and stuff and ill understand

what I see at my 120v plug is a 60hz sinewave that goes from zero point up to +120v, back through zero point, then down to -120v.
With an oscilloscope,
You would see a 60hz sinewave that goes from zero point up to +169.7v, back through zero point, then down to -169.7v.
Peak voltage.

RMS = .707 x peak
120v rms = .707 x 169.7v peak
240v rms = .707 x 339.5v peak
Just a note.
The insulation of wire must be rated above the peak voltage.
Old 04-14-05, 09:18 PM
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OOOOOOOOHHHHHHHHH. Now it is making sense. I hate it when it is so obvious but i wasnt realizing it.

thank you very much gwiz and ibpooks
Old 04-16-05, 07:51 AM
Join Date: Nov 2004
Location: CA
Posts: 2,041
There are 3 wires coming to your house: the 2 leads "top" and " bottom" of the secondary, which have 240RMS between them; and the center-tap lead or neutral. At the house, a connection is made to actual earth ground, and this is connected inside your main panel to the neutral.

In the panel, there are 2 "hot" bus bars. These are connected to the 2 hot leads from the pole. Each circuit breaker feeds ONE of these leads to a particular circuit in the house. Hence, each receptacle has a connection to one side of the hot, and to the neutral. 120 volts between hot and neutral at that location.

Sound like you are heavy on the electronics and a little light on basic electrical theory. Get yourself a good book on basic AC/DC. Such basic fundamentals as Ohm's law, etc. etc. etc. will serve you in good stead in any future career; and by the way will bite you in the butt if you fail to heed them!

Good luck in your career.

Old 04-16-05, 10:03 AM
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hi tough, thanx for the further explanation.

Believe it or not, I have a good understanding of basic theory, its just sometimes I get stuck on something and need some outside insight . Im not really into the digital end of things, i love analog, so i have a very very good understanding of AC/DC

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