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10-25-2010, 10:46 AM   #1
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my question is this, is a 3wire240volt system,really providing 208 volts,and why are heavy duty ac systems rated for either,240/208/my question is does the elimination of the neutral conductor make the rated voltage 208,and can you only have 208volts from a 3phase source,so the (b) phase is makes 208 volts with ground or one of the other (2) phases,my query do i have an option to provide 240,or 208 to the equipment,if said equipment is rated for either.if i am not connecting neutral to (B) leg then i will get 208 volts,that means i have to be aware which phase i place breaker on,no that wont matter right,i will always be on (B) phase,either b and c,or a and b,,help me understandok heres the deal,a buddy of mine is a hvac man,i was doing electrical work years earlier,we can be talking and i will say 240 needs or has a neutral,he says no 240 does not have a neutral i say thats not 240 thats 208 am i correct or are we both right

Last edited by lionking; 10-25-2010 at 11:03 AM. Reason: not finish

10-25-2010, 12:08 PM   #2
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if it is a wye system you get 120/208.if delta you get 120/240.accross phases on delta is 240,any phase to ground/neutral 120. on a wye phase to phase you get 208.any phase to ground/neutral 120.in the usa 208/240 no neutral.it is phase to phase.

 10-25-2010, 12:52 PM #3 Electrical Contractor     Join Date: Sep 2008 Location: Delmarva Posts: 3,368 Rewards Points: 2,000 There are 2 types of 3 phase services out there: Wye connected requires 3 transformers(one per leg), and provides 120 volts to ground on any phase, and 208 Volts across any 2 phases. Delta connected provides 120 Volts to ground on the "A" and "C" phase, and 208 Volts to ground on the "B" phase. Connecting across any 2 phases produces a potential difference of 240 Volts. Open delta connected uses 2 transformers - commonly found in rural areas. Here are a couple of photos of a pole with 2 transformers that are delta connected. Notice the center terminal of the 2nd transformer does not have anything attached to it. Closed delta connected uses 3 transformers. The 3rd transformer would be have its outer 2 terminals connected to the left terminal of the left transformer, and the right terminal of the larger transformer. Some utilities no longer supply delta connected 3 phase service for new installations, even if it has some such services still in use. The OP apparently has listed properties of both types (delta and wye) in this thread. You will have some, or the other, but not all on any one service. Attached Thumbnails     __________________ -KB Life is uncertain -- eat dessert first!! Last edited by kbsparky; 10-25-2010 at 12:59 PM. Reason: Added pic, further explanations

 10-25-2010, 01:08 PM #4 Member   Join Date: Aug 2010 Posts: 477 Rewards Points: 324 quite right,forgot about 208 on delta.
10-25-2010, 01:17 PM   #5
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Quote:
 Originally Posted by lionking my question is this, is a 3wire240volt system,really providing 208 volts...
No. A 3-wire 240 Volt service is a single phase service, and there should not be any 208 on such a system. BUT, if your premises is connected to a networked 3 phase wye connected service, you will have 120/208 Volts available, but not 240 Volts.

It's either one or the other, but not both on a single phase (3-wire) service

Quote:
 .... why are heavy duty ac systems rated for either,240/208/my question is does the elimination of the neutral conductor make the rated voltage 208,and can you only have 208volts from a 3phase source....
AC systems are dual rated, so they can be connected to any premises, whether it is fed with a 240 Volt service, or a 208 Volt service. The neutral connection is not used for most HVAC units rated at 240 Volts.

Quote:
 ....so the (b) phase is makes 208 volts with ground or one of the other (2) phases,my query do i have an option to provide 240,or 208 to the equipment,if said equipment is rated for either.....
If you have a delta-connected service, then the "B" phase would have 208 Volts to ground. IF you were to connect your equipment to the B phase, and the neutral, then I suppose it would be a 208 Volt circuit, but that is not recommended. You have 240 Volts available by using one or both of the other phase conductors.

Quote:
 ...if i am not connecting neutral to (B) leg then i will get 208 volts,that means i have to be aware which phase i place breaker on,no that wont matter right,i will always be on (B) phase,either b and c,or a and b,,help me understandok heres the deal,a buddy of mine is a hvac man,i was doing electrical work years earlier,we can be talking and i will say 240 needs or has a neutral,he says no 240 does not have a neutral i say thats not 240 thats 208 am i correct or are we both right
You can connect a single phase circuit to the "A" and "C" phases and still get 240 Volts on that circuit.
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10-25-2010, 01:35 PM   #6
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Quote:
 why are heavy duty ac systems rated for either,240/208
equipment is rated at any given voltage is because that is what the manufacturer designed the equipment to operate on. Nothing more, nothing less.

Quote:
 can you only have 208volts from a 3phase source
if you are talking about actual all 3 phases being used, no, you can have 240 volt 3 phase as well. The difference is how the transformer providing the power arranged.

floating delta/ 240 volts phase to phase/ no true relationship from phase to ground/ no other voltage present

delta connected corner grounded/ 240 phase to phase either single or 3 phase. voltages from phase to ground will be 240 for 2 of them and 0 for one. No other voltage present

delta connected with one winding center tapped and grounded/ 240 volts phase to phase. 120 volts to 2 of the phases to neutral/ground and 208 volts using the 3rd (Often positioned as "B" phase) to neutral/ground. The 208 should not be used as a power source.

Quote:
 so the (b) phase is makes 208 volts with ground or one of the other (2) phases,my query do i have an option to provide 240,or 208
there is no system that provides 208 volts from a phase to ground as well as 208 from phase to phase.

You can supply equipment with whatever voltage it is rated for. Equipment is often rated for dual voltages such as 208/240 so they do not have to build separate units for either of the voltages.

btw: by using transformers, you can change either type of service (wye or delta) to any other type of system. (wye/ 277/480 or 120/208; floating delta, corner grounded delta, center tapped/grounded delta). It all depends on what you need.

10-25-2010, 01:40 PM   #7
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Quote:
 =kbsparky;522744]No. A 3-wire 240 Volt service is a single phase service,
Huh? You never had a 240 volt delta 3 wire service? Yes, usually it would be 480 but I have run across where the POCO ran it down to 240 for the customer.

 10-25-2010, 02:49 PM #8 Electrical Contractor     Join Date: Sep 2008 Location: Delmarva Posts: 3,368 Rewards Points: 2,000 While I might concede that a corner-grounded delta service would be 3-wire 240 Volts, the OP in this thread indicated that there was a neutral conductor present. No such conductor would be present in the corner-grounded service. __________________ -KB Life is uncertain -- eat dessert first!!
10-25-2010, 03:10 PM   #9
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Quote:
 Originally Posted by kbsparky While I might concede that a corner-grounded delta service would be 3-wire 240 Volts, the OP in this thread indicated that there was a neutral conductor present. No such conductor would be present in the corner-grounded service.
well, there is no neutral but there is a grounded conductor and it is identified as a grounded conductor. Often times, neutral is a misnomer which we simply accept to refer to a grounded conductor. In either case, a person using incorrect nomenclature could be referring to a true neutral or simply a grounded conductor.

actually, the original is so confusing, I'm not real sure what he has or what he is really after. I was simply trying to add some information that might apply.
on top of that, since any POCO service can be changed to any type of system within the premises, what the service provided is may be irrelevant to what the OP is dealing with.

what it sounds like is he has a center tapped delta with the talk of B phase and 208 and mix that in with 240. Granted, that is a 4 wire system but due to the confusion of the entire post,...

Hell, I don't know what he's got. I'm ssooooo confused

10-25-2010, 03:23 PM   #10
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Quote:
 Originally Posted by nap .....Hell, I don't know what he's got. I'm ssooooo confused
Then the title of this thread is most appropriate.

I figure his confusion stems from dealing with different systems on different jobs, and his mind is mixing the characteristics of both at the same time...
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10-26-2010, 10:54 PM   #11
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Quote:
 Originally Posted by lionking my question is this, is a 3wire240volt system,really providing 208 volts,and why are heavy duty ac systems rated for either,240/208/my question is does the elimination of the neutral conductor make the rated voltage 208,and can you only have 208volts from a 3phase source,so the (b) phase is makes 208 volts with ground or one of the other (2) phases,my query do i have an option to provide 240,or 208 to the equipment,if said equipment is rated for either.if i am not connecting neutral to (B) leg then i will get 208 volts,that means i have to be aware which phase i place breaker on,no that wont matter right,i will always be on (B) phase,either b and c,or a and b,,help me understandok heres the deal,a buddy of mine is a hvac man,i was doing electrical work years earlier,we can be talking and i will say 240 needs or has a neutral,he says no 240 does not have a neutral i say thats not 240 thats 208 am i correct or are we both right
Your service resembles a 3 wire 120/240 volt system but because it (2 hot lines) was taken from a 120/208 volt 3 phase system you got 208 volts across the hot lines.

A lot of equipment including air conditioners is made to accept either 208 or 240 volts as the manufacturer expects that some customers will have 208 volts. But not all equipment has that flexibility.

If we have a 240 volt three phase high leg (asymmetric neutral) system and we supply one customer with just the A and C legs with neutral, then that customer ends up with a single phase 120/240 volt system. If we also give him the B leg then he will have a 3 phase system with 208 volts from B to neutral and 240 volts between any two of the three hot legs.

If a "240 volt" motor has three leads (two hots and neutral) it will probably not work on a 120/208 volt system. While the two hots represent a single phase source, the A leg to neutral represents a single phase source, and the C leg to neutral represents a single phase source, these three sources have a different phase relationship in a 120/208 volt system from a delta 3 phase line compared with a 120/240 volt system from or not from a high leg 3 phase line. Equipment using both hot wires and the neutral may or may not be sensitive to this difference. The term 208 volt may be used to refer to the phase releationships of a 120/208 volt system as well as the voltage itself. Equipment using just two leads (hots and no neutral, or one hot with neutral) is not sensitive to these differences in phase relationship. Equipment rated for both 208 and 240 volts is not sensitive to these differences in phase relationship.

If you have 120/240 volt 3 phase high leg power, you have to remember which slots in the breaker panel are for the B leg (208 volt hot to neutral). Many users of 120/240 volt high leg systems don't have any 208 volt branch circuits. Here the B slots would never have just a single breaker in them but may have double or triple breakers for 240 volt only power (no neutral) or 3 phase power respectively.

A 208 volt high leg (B leg) cannot power equipment rated for exactly 208 volts and also requiring a neutral.

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Last edited by AllanJ; 10-27-2010 at 07:49 AM.

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