120/208V or 277/480V.... huh?

Caveat emptor. This whole thread has been interesting because of the many different levels of responses. Some on the money, others just plain absolutely wrong.

Bottom line, learn something about your subject if you are asking a question and be prepared to weed out poor information from good. It's kind of like the internet in general I guess.

However, I do wish the people who like to hear themselves would be more careful about their answers. Hey, if you don't know something pretty well, skip feeling the need to respond.

Robert Trask, PE
Los Angeles, CA
USA
[email protected]
 
M
Dear confused:

First of all are you talking about transformers or motors?

Transformers are typically rated to be wired by several input voltages on the primary and may provide either a fixed output or a selected output voltage (done by taps usually).

Motors will show ratings for a say 240 vac and 480 vac, because it would be possible to wire the input of the motor to accept either voltage, along with those nameplate ratings they show the respective current (FLA) But in sizing the starter, you should use the smaller voltage as that is the HP rating you are sizing for (if you sized for the higher voltage and thne had to go back to the lower voltage for some reason, you would need a new starter.)

Matt Hyatt
 
Responding to David Adams and Robert Trask replies:

I am very interested in what you identified as ... "Some on the money, others... absolutely wrong."

This is, after all, a List that members use to elicit help. Others impart knowledge. I'm sure that those with wrong answers would like to know what is right!

ps: welcome back "Johan"

Regards, Phil Corso, PE Boca Raton, FL [[email protected]] ([email protected]) {[email protected]}
 
R
Not to sound nasty or anything but... I and others posted a valid response to the original question. I do not have the time to correct every 'bad' answer I see posted to the list. Not to mention that people tend to get REAL upset
when you 'correct' them, even privately. So, no, my experience has shown me that people posting crummy answers just get real defensive, so I don't even go there anymore.

As smart engineers (or whatever you consider yourself) we have to be able to weed out good information from poor. Period.

I'm getting some good chuckles out of the responses to the 'synchronous vs asynchronous motor' thread right now. There are some real hooters in there.

Robert Trask, PE
Los Angeles, CA USA
[email protected]
 
Maybe I can help. I'm also learning about that too. I don't fully understand also. I'm still learning. I went to a trade school for electrical (it was a 20wk. deal so i had to learn as much as I could). But what 120/208v means is that the transformer from the pole outside is changing 7200v down to 120v which most of our equipment uses. The 208 comes from multiplying 120& the square root of 3 (also the same for 277/480). I think that's why we have 3 phase system. Meaning 3 lines or phases that are hot. (1st phase is black, 2nd phase is red, 3rd phase is blue and this is call low voltage. For high voltage(277/480) 1st ph. is brown, 2nd ph. is orange, 3rd ph. is yellow.

But for residential our voltage is 120/240v which is 3 wires. 2 hots (black, red, white (neutral) and of course the ground wire (green or bare). So to get 240 from 120 you'd have to get a 2 pole breaker and hook up both wires black and red to it and going to whatever equipment utilizing it such as washer/dryer, A/C, etc...
(120v+120v=240v) 277/480v is a 3phase system. 277 is usually for luminaires and the 480 can be for motors and other equipment in commercial and industrial Co.

It gets a little complicated but i hope that this will help you. Here are some more...

118/236, 120/240v, 240/480v, 120/277v, 208Y/120v
277/480v, 208/277, 208/480, 230/460v and etc...

I'm still learning as I go. I was like you wondering what the hell 120/240v, 208Y/120v, 240/480v meant. But now I know. So be careful. Remember it's not the voltage that kills you, it's the amps(I). Laters.
 
It means that your source power is 120v from a 2 wire system, or single phase 2 wire. The combination of 120v from the 2 wire system will operate at 208volts. When 120 is added to 120 it essentially equals 208, not 240.If you would like to know, why? I can give you an explanation...
 
W
I don't believe that this post is correct. 120/208 VAC is nominal 3 phase voltage (i.e. 120 VAC * SQRT 3) as is 277/480 VAC with the phase to neutral/ground voltage first.

Single phase 2-wire w/neutral like is in most people's houses is nominally 120 VAC to neutral/ground and 240 VAC phase to phase.

William (Bill) L. Mostia, Jr., PE
Sr. Consultant
SIS-Tech Solutions
 
Hello that’s not it at all. 208 is three phases defend type like motors work at 480. They are a lot of voltages out there and everyone in the world thinks all they are to life is house voltage. Many three phase has also taps in them like 120,208 277,480 always read on this this might come default lower voltage then what you are going to use. Please guys don’t answer electricity questions you have no clue about you could get someone killed. I lost a good friend over this. He went to take copper and pulled the meter. on commercial buildings they use a CT and when you pull the meter it does not kill the voltage. He got into 480 volts the service was a 2000 amp service. Yes 2000 that why commercial electricians hate residential ones. They have no clue what an electrician is.
 
Ken I hope you come back and read this.

You took the words right out of my mouth I hear people talk about electricity all the time and all they think in the whole world is 120/240 and also most of them are like 110/220. I have worked as a commercial electrician for many years I get so mad when I try to tell someone about it. I believe in doing things right. I help older people with this when someone goes to their house and hooked stuff up wrong. I tell then just because it works do not make it right. Commercial electric is no game I have seen people grab a neutral on a 277 and knock them out. Also ground and neutral is not the same. I think before someone should be able to buy with or say romex they should have to take a class. Where I work at many that thought they were electricians they were in a house but not with this. They did not know how many amps on a wire types and this all goes back to voltage. Like computer we are on if you’re at a house you can run 12/2 to a 240 and your computer will use less amps and that means less your electric bill will be. I have wired buildings that have had over 25 panels in them. Guys in this they are no romex here you make your own circuits and you have to know what you are doing. When you see something that like 120/208 or 277/480 this means something a lot of motors have taps to use these voltages. If you put 240 on the tap that’s made for 120 well say buy to it. Electricity is not something to play with. Like ken said if you don’t know what you are doing don’t tell someone else how to do it. You could kill someone or mess something up. I know I don’t spell well but you guys could learn this stuff it takes years to really understand this stuff and still some of it don’t really make any since. Like 480/277 think about it/
 
A

antonio perez

120/208V means its rated for both 120v and 208v you'd need a electrical testing meter to check the voltage on the source in other words the electrical panel. 120V is mostly for lights, plugs,and certain equipment. you have to check and see if its single phase or three phase thou. its a huge difference. same thing with 277-480V. 277 is mainly for commercial electrical lights. no plugs. 480V is for certain plugs but that are for certain mechanical, electrical machine equipment. very dangerous voltages. be very careful always use gloves and protective wear. only a licensed and experienced electrician should handle anything like this. but any type of electrical source is dangerous. depending on the situation.
 
S
> what does it mean when something is rated 120/208V or 277/480V? What do the two numbers mean?

i would like to tell you in short that the voltages that you are talking about are definitely 3 phase voltages. these voltages are derived by using the first number and multiplying it by the square root of three or 1.73. hope this helps=120x1.73+=208.....277x1.73=480 both rounded up. I'm a journeyman line tech for withlacoochee river electric. be safe, Scott L.
 
I have ordered a piece of equipment (still) from Germany that is rated at 80 amp 3 phase 240 volts. We have 208 volt 3 phase power to our building with 80 amps @ 208 volts designated for this still. The electrician will not connect this still because he thinks that the still will draw more current than he allowed for.

Heating elements. 3 - 7.5 KW heating elements (like in a water tank) and 1 - 4.5 KW heating element. The diagram shows 3 x 230/240 50/60 Hz

What will the current draw be for the heating elements if connected to 208 3 phase power? Is it true that the elements will produce about 74% of the rated KW if run on the lower voltage. ie 7.5KW + 7.5KW + 7.5KW + 4.5KW = 27KW at 240 volts

Please help. My electrician also stated that the motors will run hot at higher current if rated for 240 volts and run on 208 volts. Is he correct?

Another question.Would it be difficult to increase the voltage to 240 volts?

Please help
Tony
 
S
OK, X kw of resistance heaters rated for 240V will generate 75% X kw when operated on 208. The areal watt density will be lower, amp consumption lower, the heaters may even last longer (but if too closely sized may not create the amount of heat you need).

Motors, different story. Most 3-phase induction motors currently sold for US use will accept either 230 or 460 volts, depending on how you connect them....but 208/3/60 is VERY common in the US, probably more so than 240/3/60, so the motors are designed to be operated satisfactorily on 208 and if you look at the nameplate, they typically list the voltage as "208-230/460". Note the dash between the 208 and 230 vs. the slash between the 230 and 460. You could read the label as 208 <b>TO</b> 230 <b>OR</b> 460 (plus tolerance). Correspondingly, the FLA (Full Load Amperage) will be listed as something in the format "8.6-8.2/4.1", so the electrician is correct that the motors will draw slightly more at 208 than at 230, but only fractionally, and hopefully within the same wire/contactor/overload sizing he would have used at 230, and unless the motors are loaded at exactly 100% of their nameplate HP, may even be below the amount he calculated for consumption at 230V. PLUS...if he did the load calcs before seeing the machine, he had to get his FLA figures from somewhere, very possibly the NEC, which tend to be high vs. real world motors, so once again you may be under whatever he calculated. Besides, as far as the overall feeders to the machine, the reduced load from the resistance heaters will probably more than compensate for the slight increase from the motors.

So hopefully the Germans, knowing these things about US power distribution, have supplied motors that will run happily on 208V.

Frankly, I'm a little concerned about the competence of your electrician. This voltage is VERY common in the US and the facts I laid out above are common knowledge and should definitely be second nature to any electrician qualified to do the installation you're discussing.
 
If I have a 208V 3-phase, 4 wire incoming power source for a facility that contains motors, lighting and a few other minor controls, and am looking to install an emergency diesel generator. is the generator required to be rated at 208V or would a 600V generator be compatible with the system as well?

There is a 600V panel in the facility.
 
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