Easy question about voltage readings

Hi folks, In our office - we have an electrical outlet (large 3 prong electricians just installed). It is supposed to be 208 Volts...
we were having some problems with our equipment, so I ran a multimeter on it - and when I hook up the hot and neutral to the meter (analog type), I get the correct reading of 210volts +/-.
when I connect to the hot and the GROUND I get 120 volts... is that correct, or does the electrician need to come back out? I thought hot and ground should also produce the 210 volt reading...
Thanks in advance!
- Jay
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My guess is that the first measurement was between two LINES of a 208/120V three phase system rather than between line and neutral. The second measurement is between a line and neutral. The neutral of a three phase system is often connected to ground.
You need an Electrical Engineer (PE), or at least a hot shot electrician to make sure. Itg is not brain surgery but it is important.
Bill
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| In our office - we have an electrical outlet (large 3 prong | electricians just installed). It is supposed to be 208 Volts... | | we were having some problems with our equipment, so I ran a multimeter | on it - and when I hook up the hot and neutral to the meter (analog | type), I get the correct reading of 210volts +/-. | | when I connect to the hot and the GROUND I get 120 volts... is that | correct, or does the electrician need to come back out? I thought hot | and ground should also produce the 210 volt reading...
In the USA, the standard voltage for common appliances is 120 volts. That voltage is between the hot line wire and the grounded neutral wire. Most other places in the world have a higher voltage in the 220 to 240 volt range, also wired the same way.
Heavy duty appliances would draw quite a lot of current if wired to 120 volts. Even the sum of all the small usages on 120 volts would add up to a large current. The system in North America and a few other places in the world uses TWO hot line wires to achieve a higher voltage. Voltage is figured _between_ two wires. Normally in a single phase system there are two hot line wires with the AC sine wave phased exactly opposite at 180 degree phase difference. That results in the voltage BETWEEN THE TWO HOT WIRES to be 240 volts.
This method of wiring originated with Thomas Edison's DC power system. His system had a positive wire at 110 volts and a negative wire at 110 volts. Even though the light bulbs operated only at 110 volts (not 220) this scheme reduced the costs of wiring and allowed electric usage at a greater distance from the power plant. The DC system generated power at the same voltage it was used, so he got none of the advantages of high voltage distribution we have today. But his "split" system lives on in the AC power system as "split phase".
In many places, usually commercial buildings, three phase power is provided instead of single phase power. Three phase power comes with three hot line wires instead of just two. In the majority of three phase systems, all 3 of these wires are 120 volts relative to the 4th grounded neutral wire, but each with a sine wave phase shifted at 120 degree intervals. The voltage BETWEEN ANY TWO of these wires is 208 volts, not 240 volts. This is because the sine wave phase angles are somewhat closer at 120 degrees instead of 180 degrees. Labeling these wires A, B, and C for the phase hot wires, and N for the neutral, these connection pairings have these nominal voltages:
A-N, or B-N, or C-N : 120 volts A-B, or B-C, or C-A : 208 volts
If the appliance to be served only needs single phase power at 208 volts, it can be wired with only 2 of the 3 phase wires. Just omit C above and label the two hot wires with A and B.
This method of three phase power is used where most of the electric usage is single phase 120 volts, and there is a lot of usage. This allows the power loading to be balanced over all three phases.
An older three phase power system was wired somewhat differently, where the voltages would be:
A-N, or B-N : 120 volts C-N : 208 volts (referred to as "high leg" or "stinger") A-B, or B-C, or C-A : 240 volts
This older system has several disadvantages. Where there is a lot of 120 volt usage, this can cause the power loading coming in to be out of balance. It also can result in blown transformer fuses more often due to its "delta" style wiring (of the "closed delta" type).
The nominal voltage standard is 120, 208, or 240 volts, depending on what the wiring connection is. Your actual voltage could vary above and below that amount a few percent. In some countries the standard is different.
A less common industrial three phase system involves 277 volts from any hot wire and the neutral, and 480 volts between two hot wires. If 120 volts is also needed, it requires an additional transformer to get it. In Canada, these voltages may be 346 and 600 volts.
Fluorescent and high intensity discharge lighting can be run directly on voltages above 120 volts with a special ballast, and this is often done to reduce costs.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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It's also interesting to note that many of the early DC generators had a third wire connection. A set of brushes riding on the commutator midway between the normal locations provided a 'third wire return' (we'd call it a neutral now). These 'center-tapped' generators were used to provide a balanced 120 for small appliances and 240 for heavier loads. Just like in todays AC system, the center tap carried the difference in current between the positive and negative leads when an unbalanced number of 120V loads were on.
daestrom
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It Sounds like you have a 208/120 volt 3-phase wye supply with 208 volt from phase to phase and 120 volts from phase to neutral or ground. This is a common type system and does not present a real problem since most single phase equipment with resistive loads will run on either 240 volts or 208 volts. There will be some power loss when using a 240 heater, for example, at 208 volts but not enough to cause a serious problem in most cases. For three phase motors and equipment you should order for 208 volts.
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RedDwarf wrote:

1) Put the meter away.
2) Have you tried the equipment? Does it run?
3) The majority of the responses seem to agree that you have 208/120 volts. However, without knowing what type of "large three prong receptacle" you have and what its intended purpose is, we would only be guessing as to whether or not it is wired correctly. Your readings may or may not make sense.
4) Ask the electrician for more information or if you suspect any problems. You paid him for a working circuit.
5) Hire an independant consultant for more advice if you still have problems.
--
Benjamin D Miller, PE
www.bmillerengineering.com
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On Fri, 7 Mar 2008 13:19:00 -0800 (PST), RedDwarf

Why do you think there is a neutral in a "3 prong plug" that is supposed to br 208? You are looking at line to line on the hot legs and line to ground . What you see is what you should be seeing.
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| In our office - we have an electrical outlet (large 3 prong | electricians just installed). It is supposed to be 208 Volts... | | we were having some problems with our equipment, so I ran a multimeter | on it - and when I hook up the hot and neutral to the meter (analog | type), I get the correct reading of 210volts +/-. | | when I connect to the hot and the GROUND I get 120 volts... is that | correct, or does the electrician need to come back out? I thought hot | and ground should also produce the 210 volt reading...
Do you have a digital camera? If so, please photograph the outlet at a straight on angle. Save the image and upload it to one of those free image saving web sites. Post the URL for it here. We can look at it and be sure you have a proper outlet configuration. You might also do the same for the intended plug.
Or just see if you can identify it as one of these: http://phil.ipal.org/usenet/aee/2008-03-08/nema-05-06-10-14.html
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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When you say "I hook up the hot and neutral to the meter (analog > type), I get the correct reading of 210volts +/-.", I believe you going across the 2 "hot" legs. On a 3 phase delta-wye system, that would be the correct reading. When you say "I connect to the hot and the GROUND I get 120 volts", I believe you're measuring 1 of the "hot" legs to the neutral or ground. 120 volts would be the correct reading. The outlet is most likely wired correctly unless the electrician was completely brain dead.

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