AC voltages/phases

[NPloysa]

I need some help! I understand basic AC power but still confused on some things the electrician was teaching me yesterday.



Residential 120v



Hot to ground, 120v



Residential single phase 120v



Any leg, hot to ground, 120v



A leg to B leg, 240v



Commercial 3 phase 277



Any leg, hot to ground, 277v



A leg to B leg, 408v



Am I correct? Where does 208 come from? Where do I get 480? He told me all this yesterday, and I understood it. Now I am lost!



Nick

 

[Kimber]

I need some help! I understand basic AC power but still confused on some things the electrician was teaching me yesterday.



Residential 120v



Hot to ground, 120v



Residential single phase 120v



Any leg, hot to ground, 120v



A leg to B leg, 240v



Commercial 3 phase 277



Any leg, hot to ground, 277v



A leg to B leg, 408v



Am I correct? Where does 208 come from? Where do I get 480? He told me all this yesterday, and I understood it. Now I am lost!



Nick

3 Phase 480v

any leg to grnd..... 277v

any leg to leg... ... . 480v



208v is a 3 phase

any leg to grnd... ... 120v

any leg to leg ... ..... 208v



Depends on your xfmr if it is 240v 3ph or 208v 3ph

 

[NPloysa]

Ah ha... makes sense!



Thanks

 

[RustyJC]

If it's 3 phase, then each leg is 120* out of phase from the others. Leg-to-ground voltages will be determined by sin(120) = 0. 5806



480 x 0. 5806 = 277



208 x 0. 5806 = 120



Does this help?



Rusty

 

[NPloysa]

Yeah, the electrician said one leg to the other, is the first leg, plus about 60% of the second. Your 58% is close enough! Makes sense!



Nick

 

[cap-n-cray]

Remember the neutral, even though it goes to ground, has some load on it. I found out the hardway when I was cutting some wires. I had the hot turned off but another hot was using the same neutral (home owner wiring). Made a good notch in my side cutters and a good shock.



Cary

 

[ndennis]

NPloysa: Just to confuse you some more, here is what I have run across while repairing various types of equipment:

3 Phase in flavors of:

208... 220... 230... 240

single legs to ground in 3 phase systems include:

120/120/120 or 120/120/217(high leg)

At one house I ran across a single phase panel that had 270 volts and 135 volts to house outlets... . even used a second meter to confirm the readings. PG&E quickly changed the neighborhood transformer.

N Dennis