120/208 Single Phase

[t_pataz]

Can someone please explain how 120/208 single phase is derived from 208Y/120 three phase. I don't quite understand, mathematically, how it works and I have a client requesting this type of set-up; using a Meter-Pack Meter Center. Thanks for your help.

 

[charlie b]

Re: 120/208 Single Phase

I?m not sure I understand your question, since the ?slash notation? (i.e., 120/208) necessarily refers to a three-phase system. Also, I am aware of no explicit rules that require a different meaning if the low number appears first (120/208) as opposed to second (208/120).

On a three-phase panelboard, connecting a circuit to one single-pole breaker and to the grounded bus gives you 120 volts. Connecting a circuit to one double- pole breaker without a wire attached to the grounded bus gives you 208 volts. Is this what you are referring to? I can?t be sure, because it didn?t require math.

 

[t_pataz]

Re: 120/208 Single Phase

Charlie, What I am trying to ask is this, I have a three-phase service going into a terminal box and using single-phase meter stack in conjunction with the terminal box. Since I am now going from three phase to single phase, I am questioning the terminology of using 120/208v and not 120/240v on the single-phase side if the incoming voltage is three phase 120/208v? I understand how I am getting the single phase it's all about the nomenclature. I believe that once we do not use one of the legs from the terminal box and create single phase that it should be 120/240v, since there is 120v on each of the two legs and mathematically it is impossible (without some kind of load or draw) to obtain 208v on that circuit. Am I wrong?

 

[dereckbc]

Re: 120/208 Single Phase

T-pataz, the voltages on a three phase 208Y/120 system are 120 between any phase and a grounded conductor, and 208 between any two phases. There is no 240 invloved.

The mathmatical relationship is the square root of three (1.73 x 120 = 208). I will note you should be using all three legs of the service and not just two. To do so will will unbalance the system and could create a fire danger if enough load is developed.

 

[charlie]

Re: 120/208 Single Phase

What is happening is you are providing two legs of a three phase source to a meter that has a neutral potential lug (normally at the 9:00 position) so the customer (normally residential in an apartment building) will have a normal single phase service. The problem is that the two legs are separated by 120 degrees instead of 180 degrees and for water heating, electric ranges, ovens, and electric dryers, he gets 208 volts instead of 240 volts. The meter center gets all three phases and each meter gets different pairs of phases. If a meter center has 12 meters, 4 would have A and B, 4 B and C, and 4 C and A. I hope I haven't muddied the water any.

 

[t_pataz]

Re: 120/208 Single Phase

Exactly. But, according to manufacturers like Square D, I can have a multi meter set up using two of the three legs and a the neutral for each of the meters (five jaw), and then have a legitimate single phase service after that meter.

You are correct the load will not be balanced on the primary side but apparently Square D is not concerned with that, since they sell equipment that support this scenario.

So after the meter I will have 120/240v, no?

Since at that point each leg will be 120v and there will only be two plus the neutral.

I know this may all seem a bit repetitive but I have to answer to others and I need a clear understanding so I can explain what is going on to them. Or direct them in another direction.

Thanks.

 

[hbendillo]

Re: 120/208 Single Phase

You will still have a 208-volt, single-phase service for each meter despite the fact you are only using two of the three phases. No matter how you use them the voltage between any two phases is still 208.

 

[charlie b]

Re: 120/208 Single Phase

Originally posted by t_pataz:So after the meter I will have 120/240v, no?

As Dereck and ?the other Charlie? have mentioned, what you will have is 120 volts between either phase wire and the grounded wire, and you will have 208 volts between the two phase wires. What is this called? I regret to report that it is frequently (and inappropriately) called ?120/240.? We don?t have a phrase in our technical language for ?120/208 single phase.? Perhaps we should. At least the customer won?t be misled into believing that 240V is available.

 

[roger deas]

Re: 120/208 Single Phase

This may help.

This diagram is actually missing a little bit but will help show you the system.

The second green line should be between (A) and (C), and there should be a green line between (A) and (B), both showing 208v.

Now,

I know this may all seem a bit repetitive but I have to answer to others and I need a clear understanding so I can explain what is going on to them. Or direct them in another direction

This is obviously over your head and can not really be taught in a single thread.

I'm not sure what your profile means as far as "Team Leader", and this is not meant to be demeaning, but you should get a professional to assist you.

Roger

[ April 09, 2003, 01:39 PM: Message edited by: roger deas ]

 

[bennie]

Re: 120/208 Single Phase

208 sucks...Only 120 volt outlets should be operated from a 120/208 supply

 

[t_pataz]

Re: 120/208 Single Phase

No offence taken. The reason for these forums is to help each other get clarification and assistance when needed.
My understanding is quite clear now, and the root of my question has been answered.

I appreciate everyone?s help and input, thank you.

 

[roger deas]

Re: 120/208 Single Phase

Hello Bennie.

208 sucks...Only 120 volt outlets should be operated from a 120/208 supply

Haven't we been here before?

Roger

[ April 09, 2003, 01:53 PM: Message edited by: roger deas ]

 

[bennie]

Re: 120/208 Single Phase

You are correct Roger. We have visited most areas.

When 120/208 came on the scene, there was no 208 volt rated equipment. The only purpose was to change 480 to 120.

Some manufacturers began to make 208 volt space heaters.

208 volts is not a standard voltage for motors.

 

[Ed MacLaren]

Re: 120/208 Single Phase

When my students have trouble with "120/208 single phase", I get them to mentally perform the little exercise at the bottom of the sketch.

Ed

 

[frenchelectrican]

Re: 120/208 Single Phase

bennie:
you quoite " 208 volts is not standrad for motors"

yeah indeed but alot of 240 volts 3 ph motours do work on 208 and i use it alot with out side effect just check the nameplate on motour ALSO single phase too i see few single phase motour run on 208 also again just check the nameplate on one phase motour .

on single phase motour it will little more on current drawage somehow by 15 % or so but starting toqure will be little lower but if no effect with it. it will be ok but for hard starting i can do 2 way is :
one use buck and boost transfomer .
two use one size larger motour .
ether way it will work but it depend on everomet by the way if you check on office bulding most use 208 volts system also
with out side effects
my home have 208 volts here ( note i have 3 ph service tied with my shop ) (shop have 480v wye but use reguaur transfmoer to downstep to 120/208 for oulets and house power )

merci marc

 

[bennie]

Re: 120/208 Single Phase

Please refer to NEMA Standard Publications MG-1-1993 copyright 1995 by NEMA.

Motors used for intermittent duty will work for a while, depending on duration of use.

With extended use, they will smoke.

Warranties will be void.

It is a code violation to operate the motors at a reduced voltage. 110.3 (A&B)

[ April 10, 2003, 11:11 PM: Message edited by: bennie ]

 

[frenchelectrican]

Re: 120/208 Single Phase

ok bennie i did look at the nema listing about motours and yeah i forgot to metion that double check the name plate for proper operations i keep few motours catalog book and check on website to see the most recent update with motours designs
and it is my habaite to check the nameplate first to make sure it is ok to run on 200 -208 volts system and service factor listing too

merci marc

 

[bennie]

Re: 120/208 Single Phase

Marc: You have done good

 

[frenchelectrican]

Re: 120/208 Single Phase

bennie merci beaucoup (thank you very much ) i allready famimur with usa system and french system that why it have few challange along the way to compersie the idea between nec and enf ( electrcalue nationlie de france) aka french electrcal codes

if more question just drop a line i will be more than be happy to help you
ps french voltage is 220/415 both sp and 3ph

merci marc

 

[jschultz]

Re: 120/208 Single Phase

the meterbank should be specified as 3phase incoming with 1 phase metering. Not a 1phase incoming. If you do not know how 208v 1 phase is derived from a 3 phase system, you should not be designing installations that require this kind of knowledge. I do not want to offend you, but someone could get killed.