What is the difference between 208 and 240?

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watabull

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I've been working in the field for nine years, residential new construction and have never dealt with a 208 system. I've looked through the NEC for a description/explaination and did not find one. If there is article on the matter, which one, I can take it from there. If not could someone please explain this for me, thanks. The motivation for this is for two reasons 1) Common knowledge so I will know in the future and don't have to ask silly questions. 2)Will a 208v 8kva(1 or 3 phase) griddle cook(be ruined, sorry for the pun) on a 120/240V 40 Amp circuit? This question I do not know the answer to. Thanks.
 

George Stolz

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watabull said:
1) Common knowledge so I will know in the future and don't have to ask silly questions.
This are no such thing as a silly question. However, we can provide silly answers to reasonable questions. :D

2)Will a 208v 8kva(1 or 3 phase) griddle cook(be ruined, sorry for the pun) on a 120/240V 40 Amp circuit?
An appliance must be marked according to the voltage it is rated (422.60(A)) and connected to the voltage for which it is rated (110.3(B)). To connect a 208V range to a 240V source would be a listing violation, unless the range is marked to be connected to a 240V source as well.

It may not hurt the range, but if you connect it to the wrong source you are taking responsibility for the outcome, IMO.

Edit to add: I forgot the original question!

The difference between 208V three phase, and 240V single phase, is how the voltage is derived.

240V single phase is obtained by taking a single leg of three-phase power.
208V three phase is obtained by taking two legs of three-phase power.

In a 120/240 single phase system, the midpoint of the secondary side of the tranformer is tapped and grounded to create a neutral. From the midpoint to any line reads 120V, and from line-to-line (the full voltage) reads 240V. Imagine planting a black flag in the sand, walking 120 steps in a straight line, planting a white flag, and then walking another 120 steps in the same line to plant a red flag at the end.

In a 120/208 three phase system, the neutral is at the center of three phases from the utility. In this case, using the walking in the sand analogy: A white flag is planted in the center. You walk 120 steps away from the white flag, and plant a black flag. You return to the white flag, turn 120 degrees, and walk another 120 steps, planting a red flag. You return to the white flag, turn 120 degrees, and walk 120 steps to plant a blue flag.

Once this is done, you face the red flag from the blue flag, and count your steps as you approach it. There are 208 steps between flags. :)

If that doesn't make sense, you're not alone in the universe. :D
 
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winnie

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Location
Springfield, MA, USA
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Electric motor research
As George describes above, 240V comes from a single phase source, and 208V comes from a three phase source. I like the description of walking from a flag to give a _gut_ description of how it is that 120V + 120V = 208V in these systems. For more details, search of 'vector addition'; we've gone into great detail on other discussions.

There is a bit of confusing _naming_ used, because for _residential_ use, both are called 'single phase'.

In both cases, you have two 'hot' legs that are 120V from the grounded conductor. In both cases you use a 'single phase' panel that has two 'hot' bus bars, one 'neutral' bus bar and (sometimes) one 'ground' bus bar.

In both cases, only single phase loads are served, either line to neutral loads, or line to line loads.

The differences, however are critical.

1) A shared neutral in a MWBC will _always_ carry current if either circuit is in use, even if both circuits are carrying perfectly balanced current. This means that you _always_ count the neutral as a current carrying conductor for derating.

2) The nominal line-line voltage is 208V, and this isn't a nominal utilization voltage (sometimes you will see loads designed to connect to nominal 240V supplies called 230V or 220V loads; these are utilization voltages, and essentially mean the same thing as '240V'. 208V is really a different lower voltage.)

-Jon
 

cflava

Member
Location
Greensboro, NC
Ok, looking at the ugly's book page 13 it shows just a few of the electrical distribution systems, the common ones. Does 120/208 and 277/480 come in both wye and delta configurations as far as three phase is concerned?
 

George Stolz

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cflava said:
Does 120/208 and 277/480 come in both wye and delta configurations as far as three phase is concerned?
They both represent a wye secondary.

Delta configurations create L-N voltages that are half of their L-L voltages. For example, in the picture Nova posted, the delta on the right makes a 120/240 delta configuration.
 

cflava

Member
Location
Greensboro, NC
georgestolz said:
Delta configurations create L-N voltages that are half of their L-L voltages.
So does 277/480 only come with a delta configuration or is that only on the secondary. If so does that mean that the primary was likey 208/120 wye?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
cflava said:
So does 277/480 only come with a delta configuration or is that only on the secondary. If so does that mean that the primary was likey 208/120 wye?
_Generally_ 277/480 is a _wye_ connected secondary.

Most often the primary will be delta connected at the utility distribution voltage. Note that the _source_ of the utility voltage will often be _wye_ connected.

However many other connections are possible.

For example, the secondary of the transformer could be a 480V delta, with the neutral derived using a 'zig-zag' autotransformer.

The primary could be a wye, though this is generally a bad practise.

The primary could be 208V, but this is unlikely, used perhaps in a lab or for a single machine that needs 480V where the building supply is 120/208V.

The primary of such a transformer should be a delta, but it is not uncommon to take a 480V delta to 120/208V wye transformer and use it 'in reverse' to generate the 480V.

-Jon
 

watabull

Member
Explainations and maybe alternatives

Explainations and maybe alternatives

Ok, I guess I openned a can of worms. Residential 120/240 is derived from a single leg of three phase power. 120/208 is derived from two legs of three phase power. From the pics (Nova's), it seems that the delta could essentially carry 120/208/240, with the 208 from L2 to Grn. The Wye system looks like it won't achieve anything past 208V. So in a residential setting, would the transformer be a Delta? Then if this is the case, you have L1, L3 and Grn coming into the house, where's L2?

Back on page 21 this guy asks this, "single phase to three phase converters", they talk about rotary vs. static phase converters. Are these creating three phases out of the one leg of three phase? and if they are, would it make a different again if it were a wye or a Delta Transformer?

Following the follow-up of 422.60A, I also referred to 422.61 for the listing of the elements, the unit has been modified it seems (30 Amp cord and Plug for what I'm coming up with at 38.46 Amp). I found a wiring diagram on the junction box cover plate. After clean up I can barely make out that it shows Single and Three Phase wiring. All my numbers are lost so I will refer to the manufacturer. Why I was checking is that the newer units and replacement elements come as 208 and 240. Making it work isn't the issue, now it's about phase conversion and why would one system (Wye vs. Delta, 208V vs. 240V) be better than the other and for what structure application. Is it about money and the ease of power distribution? Or is there a valid reason for the difference? Then another explaination please, some of the guys doing the multiplexes were wiring with three phase and there were others wiring with the Single phase. I didn't have the chance to install the cans so I'm wondering now what woild be the difference? If in the three phase units wouldn't be hard to get the 240V for the household appliances? Thanks, if you can.
 

watabull

Member
I just typed a book and lost it!

I just typed a book and lost it!

Ok I think I have the difference on how the voltages are derived. But in a residential application on the Delta transformer you have L1, L3 and Grn, where would L2 be? Back on page 21 "single phase to three phase converters
", this guy is creating phases. The differences between static and rotary? And does this mean that in again a residential application you could create three phases from one leg of one phase to derive at your 208V? Or would it be defeating the purpose depending on the transformer?

I guess the other question is what are the advantages to 208V vs. 240V? The ease of power distribution, cost, both? As for installing a 208V single phase appliance on a 240V single phase circuit, wouldn't the unit draw only what was needed? Thanks.
 

George Stolz

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Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Watabull, welcome to the forum. As a new member, your posts won't be immediately visible to you, so don't worry that you don't see them - they're not lost, just in need of approval behind the scenes first.
watabull said:
From the pics (Nova's), it seems that the delta could essentially carry 120/208/240, with the 208 from L2 to Grn.
That's a little deceiving. A 120/240V, three phase, midpoint-grounded transformer has what is commonly called a "high-leg" or "stinger", which does have a technical voltage of 208V from the B phase to neutral. But it can't support 208V loads, because the voltage between those two points isn't stable.

So in a residential setting, would the transformer be a Delta?
No, it is a different transformer. Take a look over at the Technical Reference Area over at ECN forum.

Single Phase Transformer has a phase and neutral on the line side:


Delta Transformer uses three phases to create three phases (and a neutral) of a different voltage:


As for the rest of what you wrote, I'm not sure what you're getting at.

What are you trying to do? Are you trying to install a commercial range in a house? :confused:
 
208 vs 240

208 vs 240

watabull said:
I've been working in the field for nine years, residential new construction and have never dealt with a 208 system. I've looked through the NEC for a description/explaination and did not find one. If there is article on the matter, which one, I can take it from there. If not could someone please explain this for me, thanks. The motivation for this is for two reasons 1) Common knowledge so I will know in the future and don't have to ask silly questions. 2)Will a 208v 8kva(1 or 3 phase) griddle cook(be ruined, sorry for the pun) on a 120/240V 40 Amp circuit? This question I do not know the answer to. Thanks.
I have the same problem, but with a commercial building, i.e. church. I need an article to present to the church so that the new construction is not changed from 208V 3P, as recommended by my PE EE, to 240V 3P as requested by the church. This assumes that my assumptions are correct. They have an existing 35,000 SF building that is 240V 3P. The new sanctuary is 22,000 SF. They want to consolidate the service and wire for 240V 3P. I do not need an opinion. I need a published article on the subject. Any help with a title and address would be appreciated.
 

George Stolz

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Location
Windsor, CO NEC: 2017
Occupation
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Reynolds White said:
I need an article to present to the church so that the new construction is not changed from 208V 3P, as recommended by my PE EE, to 240V 3P as requested by the church.
I am confused.

230.2 requires a single service unless special conditions apply. The church's request seems not only reasonable, but is arguably required by the NEC.

I do not need an opinion. I need a published article on the subject.
I am confused again. You want an article saying that 240V 3 phase power is undesirable? :-?

I deleted your post in the other thread, so that your answers would be grouped in this thread, to minimize confusion for yourself and the other forum members. In the future, you might want to start your own thread, as your question is not directly related to the original post of this thread.
 
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