208 and 240

[sage ben]

what is the real difference between the two? i am installing a three phase PV system and the manufacturer asked whether i had 208 or 240. i have 208 but it made me wonder if my understanding of the two was incorrect. can anyone explain the difference to me? thank you

[ August 12, 2003, 09:43 PM: Message edited by: sage ben ]

 

[iwire]

Re: 208 and 240

I do not understand what you asking?

In general 208 equipment should not be connected to 240 volt feeds or vice versa.

Some equipment is dual rated, like electric ranges rated a certain KW at 208 and a different KW at 240.

 

[charlie b]

Re: 208 and 240

Most of the time, when I have encountered 240 volts, it was single phase. That is, a single transformer secondary coil has 240 volts from end to end. The center point of the coil is grounded, and serves as a neutral, so that voltage from either end to the center is one half the 240, or 120 volts. That is the common arrangement for household power.

On the other hand, 208 volts is most frequently the phase to phase voltage of a three phase system. You have three transformer secondary coils, and one end of each of the three is tied to a single point, with that point being grounded. Here again, voltage from the center point to any of the three other ends is 120 volts. But this time, voltage from one end to another is not 2 times that amount, but rather 1.732 times the 120 (i.e., 208 volts). Where does the 1.732 come from? It?s the square root of 3. Where does the square root of 3 come from? For that, you need to go back to high school geometry, and look at a 30-60-90 right triangle.

What?s the difference between 208 and 240? An increase of 15%, (or looking the other direction, a reduction of 13%). That is why, as iwire has said, they should not, in general, be mixed.

 

[hillbilly]

Re: 208 and 240

Charlie b, please clarify this for me. Last week I installed (1) new branch circuit in a old apartment building. This circuit was to power a new receptacle in the bathroom. Incoming power was 3 phase. Each leg was 240v read across any of the other two. The individual leg readings were 120, 120, 208v as read to the grounded conductor. The service panel was a Square D QO that would accept a standard breaker on any of the legs. Hence it was possible to power up a standard 1 pole breaker with 208v if you weren't selective about the spacing in the panel. Is this common? Still acceptable? Is this system still in use (new)?

 

[Ed MacLaren]

Re: 208 and 240

Hillbilly, the system is still in use in some areas. The "high leg" or "wild leg" as it was sometimes called resulted from connecting a grounded (neutral) conductor from the center tap of one of the secondarys, to provide for 120/240 volt single phase loads. Calculations below.

Early in my career, we were required to paint the high leg breakers (every third row in the panel) orange to warn unwary electricians about the high leg voltage.

Now (here) we are required to install a separate panel for the single phase loads, and the high leg is not brought into it.

Ed

[ August 11, 2003, 09:24 PM: Message edited by: Ed MacLaren ]

 

[tom baker]

Re: 208 and 240

120/240 Wild leg services used to be used a lot by POCO's. To me they are a little magical, as the primary is usually open delta ( 3 wires 2 hots) and only use two transformers to get 3 phase. The advantage is less cost, due to open delta primary, and the transformers provided both single phase 120/240 and three phase 120/240. The 120/240 xfmr was larger. If the load picked up the POCO would add the thrid transformer.

Disadvantages:
Voltages and currents on secondary are unbalanced
Can't use all phases for 120V
Wild leg is 208
Proper identification and delta breakers are required.

Hillbilly:Make sure your circuit breakers are straight voltage rated 240 not 120/240. A 120 V breaker can not interrupt the 208 V on a fault. A delta breaker costs 5x as much as a slash rated and are not stocked.

Charlie E: do you see many of the wild leg services, are they still being installed?

 

[ryan_618]

Re: 208 and 240

Ed: I simply CAN NOT say enough about your diagrams. Keep it up, my friend, very informitive, very helpful.

 

[frenchelectrican]

Re: 208 and 240

tom;
in this area of northeast wisconsin we still have delta service and also i will add this one for more instering twist is corner grounded delta service ( both delta service is being phased out now ) and it can compated the matter with breaker slection ( sorry for spelling ) and alot of delta have marking in center phase but not always the case and i use the voltage tester to make sure it is proper leg and one time i work on one service it say "480 volt delta" and i tested it came out wye that is diffrent matter but same idea when you look at the chart with delta and wye connetions are


merci marc

 

[charlie]

Re: 208 and 240

The open Y primary, open delta secondary is still very much alive and well. We install them every day in this area. We call the larger transformer a lighter or lighting transformer because it supplies the lighting loads as well as the 120 volt loads. The other transformer we call the kicker because it kicks up the high phase.

We can have a small three phase load develop for a service station on a corner and just add a kicker to an existing transformer to give it a three phase service. The existing single phase loads will still be on the lighter.

Ed, we have had some businesses take services as you describe, a single phase and a three phase from the same bank.

To drive home what Tom has said, be sure you use full voltage rated circuit breakers on your three phase loads. Breakers with the slash rating are fine with the single phase 240 and 120 volt loads.

If the three phase load picks up, a third transformer will have to be added to support the delta.

It is interesting to note that another electric utility in Indiana uses an open delta configuration to serve lift stations where the load is straight 240 volt delta. They corner ground the common corner and connect the service with a triplex service drop. The customer sets a straight 240 volt single phase panelboard that is rated for a corner grounded, three phase service, uses full voltage rated two pole circuit breakers, and they are in business.

Except for having two transformers, it looks like a single phase service. You have a triplex drop, only two buses for the circuit breakers, and two pole breakers. You will read 240 volts between the phases and 240 volts to the grounded conductor (not the neutral) from either phase.

I think I have rattled on too much.

 

[tom baker]

Re: 208 and 240

Our POCO will no longer install new mid point grounded delta. I have one 480V corner grounded delta. The fused disconnect has a bus bar in the b phase. Buts thats a different animal altogether.

Oh OK open Y primary, open delta secondary. Its still magic.

Just a note on a large office building locally that has midpoint grounded delta. Phase A & C run 700 amps, while B (208) runs 200 amps. You can have problems with voltage balance with a lot of single phase loads.

[ August 11, 2003, 11:50 PM: Message edited by: tom baker ]

 

[brian john]

Re: 208 and 240

Ed:

What program are you drawing in?

Do you you cut and paste tyour drawings to this forum?

Oh they are very good and a picture/drawing is worth a 1000 words, as the saying goes.

Brian John

 

[Ed MacLaren]

Re: 208 and 240

Brian, many of these drawings were produced over the years for my electrical apprentice classroom seminars.
Most of the drawings were originally made with AutoSketch, which is a kind of "lite" version of AutoCad, sold by a company called AutoDesk.
I now use Paint Shop Pro for making modifications to existing drawings, and for converting the AutoSketch files to .gif format for the internet.

You can't "cut and paste" graphics to this forum.

You will need to put the images on a "web server" where the forum will have access to them. There are some free services that will permit you to set up a web page and upload graphic files to it, or you could use one of the so-called photo "album" services.
Then you have to create a link to the image file that you wish to include in your post on the forum.
Use the .jpeg graphic format for photos, and .gif for line drawings, diagrams, etc.

For information on how to create a link to your image go to the "post reply" page on the forum, click on the link (left margin) "*UBB Code is enabled" and scroll down to the instruction "Adding images". There are instructions there explaining how to do it.

Hope this helps.

Ed

[ August 12, 2003, 06:27 PM: Message edited by: Ed MacLaren ]

 

[sage ben]

Re: 208 and 240

thank you all for the great load of info. i have not been in the trade long and have not see any of these systems. i will enjoy reading more about them. i am starting to install PV systems and i have a 3-phase system coming up and know i understand why the manufacturer of the inverters i will be using asked me about the voltages of the system.

one more question though when i posted this topic i had set it to email when someone replied. how do i unset that?

thanks again

 

[don_resqcapt19]

Re: 208 and 240

sage ben,
If you click on the "paper and pencil" above your original post, you can edit the post. You can "uncheck" the e-mail notification option.
Don

 

[lady sparks lover]

Re: 208 and 240

Question, if you do connect a 240V motor to a 208V system, would that harm or cause the motor to malfunction in anyway.

I had a debate, which makes me glad I come to ask the forum, about putting a 240V motor on a 208V system.

What do you all think??

 

[Ed MacLaren]

Re: 208 and 240

Motors that are designed to be operated on 240 volt systems normally have a nameplate rating of 230 volts.

Let's use as an example a 30 HP, 3 phase, 230 volt motor with a full-load current rating of 80 amps. Assume at some given load it draws 72 amps when connected to it's rated voltage.

If it was operated from a 208 volt supply with the same load it would now draw approximately 10% more current, or about 79.2 amps.
This does not exceed it's nameplate rating of 80 amps, so the winding temperature should not exceed the design temperature rise.

If the same motor was called upon to deliver it's full rated 30 HP, at 208 volts, it would draw approximately 88 amps, resulting in increased winding temperature and reduced winding insulation life.

Ed

 

[tom baker]

Re: 208 and 240

It may damage the motor. Motors are designed with a 10% voltage tolerance, the motor voltage is 230, not 240, 230-23 volts is 207. If the 207 is not running low you may be ok. But use of a 230 v motor on 208 is a violation of 110.3(B)
And it is a violation of 110.4 to use a 208 v motor on 230 v

 

[lady sparks lover]

Re: 208 and 240

I would think so...thanks for your help!!