120/208/240v questions

[ProgrammerInTraining]

I'm not a master electric or anything but I'm not completely oblivious to electrical (I think). I can calculate amps and all that good stuff but I have some questions about 3phase power, particularly when it comes to 2 phases joining to give higher voltage, i've been told two phases put together will give you 208, I've even got on the multimeter and put it hot to hot and it does give me 211v (not exactly 208 but it will have to due) I was sold on that. But now Ive been seeing some PD's that give 240v, see the picture I've added below, hmm 120+120= 240 makes sense to me. as a matter of fact I've had some fixtures that I had to use a 3 wire cam (hot, hot, ground) to 100amp 240v bates adapter, how does it get 240 and not 208? this is making me think theres some funny business going on. Also as you can see in the picture below one of the breakers is for 2 edison plugs so I made an adapter to see if it will give me 208. any clarification will be appreciated. Thank you.


http://oi41.tinypic.com/5npv9y.jpg

 

[SteveB]

It's not the magic PD, it's the transformer that is either on the street, or possibly in the building.

Typical commercial building power in the US is 120/208 volts, 3 PHASE as provided by the primary generation system and transformed down to usable voltages.

240v is known as single phase, 3 wire, with 2 hots and a neutral and is common in non-commercial applications, homes, etc... and is itself 2 taps of a transformer from a single phase of the primary 3 phase distribution. The fact that the transformer taps one of the 3 phases is why it is called single phase, even though there are 2 hot legs.

WiKi has a decent explanation:

Three-phase electric power - Wikipedia, the free encyclopedia

 

[JLNorthGA]

Do NOT try to manufacture your own three phase power. Doing so can damage any equipment which requires three phase power. Do do this properly you need a three phase converter (aka Rotary Phase Converter). Many industrial machines use three phase power (wood shop and other tools).

With three phase power each "leg" of the current is 120 degrees out of phase with the other two legs. This makes a rotating motor that is designed for three phase run smoother, with less current and prolongs the life of the motor. It also has less parts (starting capacitors).

I have an older cabinet saw with a three phase motor. Runs quite nicely - you can put a nickle on edge when starting it and it won't topple over.

 

[derekleffew]

In a logical world, one would think that two 120V hots would always add up to 240V [120+120=240], right? Wrong. If the phases are 120° apart, as they are on a three phase system, some cancellation is going to occur. The phases are 120° apart. Multiply 240 x the sine of 120° [0.866] and get 207.84; let's call it 208V. The other way to approach this is to multiply 120 by the sq.rt.(3) [1.732] to get, wait for it, 207.84. In your case, if the hot to neutral voltage is 122V rather than 120V [it varies, see here], you'll get ~211V hot to hot.
The same relationship holds for other voltages, which is why you may see equipment/connectors listed as 277/480V or 347/600V.

... Also as you can see in the picture below one of the breakers is for 2 edison plugs so I made an adapter to see if it will give me 208.

First, un-build that "adapter" immediately! It's illegal, and more importantly, unsafe. Plug it in wrong and you could have a hot male in your hands! Also, measuring across the two hots, when both males are plugged into the same phase, won't give you 240V, or even 120V, because the potential difference is zero; both are the same distance away from ground. Second, your NEMA 5-15 female Edison is a 125V (max.) connector. If you want to run 208, 220, or 240V, you need to use a NEMA 6 or L6 (250V) connector.

See also the wiki entries Three-phase, four-wire plus ground, split-phase, bi-phase, and, for extra credit, Wye connection and Delta connection.
.

 

[n1ist]

Please take that 2-male to one female edison adapter and take it apart. It's neither legal nor safe. Depending on how it is wired, you could end up with energized male pins, or even have it explode when plugging it in.
/mike

 

[JD]

Just to totally and utterly confuse you, there is also 3 phase power that is 240 volts! It is called Delta and there is 240 volts between any of the three hots. Delta power is almost useless in the lighting world as often there is no reasonable method of retrieving the 120 volts (US) that is needed to run conventionals. There is a type of Delta that has one of the three windings center-tapped to produce the 120-0-120 voltage needed to run equipment. Since it is only one winding, it effectively is a single phase power source. In this application, the third leg is known as the "wild" leg as it measures 208 volts to neutral. Enough about Delta. It is suffice to know it is out there should you run across it.

The power we use in lighting, for the most part, is known as three phase Wye. All legs are 120 volts to neutral, with 208 volts between any two hots. (see all above posts.) ok, ok, it is rarely 208 volts, or 120 volts, but somewhere in that neighborhood.

Last but not least, Take that adapter apart! You could put your eye out with that thing!

 

[SteveB]

John is correct and as I stated "Typical commercial building power in the US is 120/208 volts, 3 PHASE as provided by the primary generation system and transformed down to usable voltages."

That's "typical" as in the most common found. All kinds of other tap downs from a transformer can be found, I've seen 477v for HVAC systems, as well as some really odd stuff at the Grumman aircraft facility on Long Island, that is now a barely used movie studio.

Here's a quick Google find:

motorsanddrives.com - Electric Motors and Power Systems

 

[mstaylor]

Steve:
When you say commercial building I would take exception to 208 being the norm. It is definitely the norm in theatres and anywhere that runs commercial equipment. There are plenty of places that we put lights and sound gear that use 240v power, clubs and many outside venues.
I worked in a processing plant that had most everything. We had three phase, 208 flavor, wild leg three phase, 277v and 480v systems. It was an adventure everytime I went to work on a piece of equipment. When I was hired I was told there was no 480v in the building except for three motors. Our entire ice making operation was 480v. Found that out at 2:00 am while troubleshooting.

 

[SteveB]

Steve:
When you say commercial building I would take exception to 208 being the norm. It is definitely the norm in theatres and anywhere that runs commercial equipment..

Oh, it's not and the 2nd website I linked to indicates that. It's pretty standard for service over 200 amps (which is typically a residential supply size limit, or was) but no doubt you see all kinds of exceptions, my comment about the Grumman plant a case in point, where the voltages were matched to the tools being used.

 

[rochem]

Also as you can see in the picture below one of the breakers is for 2 edison plugs so I made an adapter to see if it will give me 208. any clarification will be appreciated. Thank you.

First, un-build that "adapter" immediately! It's illegal, and more importantly, unsafe. Plug it in wrong and you could have a hot male in your hands!

Please take that 2-male to one female edison adapter and take it apart. It's neither legal nor safe.

Last but not least, Take that adapter apart! You could put your eye out with that thing!

My turn! Around here, we call this genre of devices "suicide cables," because that's pretty much the most practical use for them. Think about what happens when you plug in the first male, then accidentally brush the prongs of the other male while plugging it in. For the same reason, you should never* make a male-to-male adapter of any kind.

*Okay, so there are extremely rare occasions when you might need to have a live exposed male connector. But this should only be done by highly qualified electricians with years of experience, and it's definitely a highly unusual exception.

 

[mstaylor]

My turn! Around here, we call this genre of devices "suicide cables," because that's pretty much the most practical use for them. Think about what happens when you plug in the first male, then accidentally brush the prongs of the other male while plugging it in. For the same reason, you should never* make a male-to-male adapter of any kind.

*Okay, so there are extremely rare occasions when you might need to have a live exposed male connector. But this should only be done by highly qualified electricians with years of experience, and it's definitely a highly unusual exception.

Never mind the fact that you can't have two legs on seperate breakers. And yes that is definitely a suicide adapter.

 

[MNicolai]

Never mind the fact that you can't have two legs on seperate breakers. And yes that is definitely a suicide adapter.

You can, it's just highly illegal and a good way to ruin some very expensive equipment (not to mention dangerous as rochem pointed out). If one leg trips one breaker, the other breaker would stay on, so your 208v ML/motor/what-have-you is now trying to run on 120v, and it'll not be very happy.

 

[mstaylor]

You can, it's just highly illegal and a good way to ruin some very expensive equipment (not to mention dangerous as rochem pointed out). If one leg trips one breaker, the other breaker would stay on, so your 208v ML/motor/what-have-you is now trying to run on 120v, and it'll not be very happy.

Sorry, My brain inserted the phrase,"Code says" you can't have two legs on seperate breakers. I did a festival earlier this year and I was tapped into by a three phase panel for show power, sound only. Turns out the venue electrician ran the drop panel to a double pole and single pole breaker. I was not happy when I found out what happened. I had forgotten that it was supposed to three phase. I turned the double pole breaker off and the sound guy said it wasn't all dead. I went to his boss and explained that the new guy was trying to kill us.

 

[ship]

Overall experience and liability. Stuff you are not sure or safe to connect to and areas that a Licenced electricitrian is only best to find the power for or specify a transformer for your loading to. Nothing as a ME unless you accept what you are not trained for and specify in job or not, what others are better qulified to be doing. You should be stepping asside and recommending a area electrictian in the area that is better qualified to deal with it.

Rather that destroyed gear on your part, or lack of ability.. at some point what you are not qualified to ME for isn't what you should be doing assuming these questions. Need local help and nothing against you in requiring your current abilities.