Three-Phase Voltage Connections and the U.S. Electrical Grid

USA is 60HZ, 120/208V or 277/488V, and the number of wires AT THESE VOLTAGES IS 4.

Hi, Thanks

But as far as I know, the number of wires at three phase is 3 or 4, not only 4.

And what's the difference betwee 120/208V and 277/488V?

B/Rgds.

Ani-Luo

There is no universal standard here unfortunately. The US electrical grid is composed of a lot of smaller grids that were connected together, but each individual power utility may or may not have the same voltage standards as their neighbor. Even within a utility's service area, there are new services and old services (called "legacy") that cannot change until the connected users are demolished. The only thing that is now relatively universal is that we are all 60Hz (with the exception of a few islands of 25Hz still in use). So here are all of the 3 phase low voltage "standards" I am aware of:

120Y208 (4 wire); New standard for commercial and light industrial

208V Delta (3 wire); rare legacy

240V Delta (3 wire); mostly legacy but still some occasional new services

120/240V Delta with a center tap between A and C phases for 120V (4 wire "stinger leg"); still provided for small commercial users with >10% 120V 1 phase loads

440V Delta (3 wire); legacy

480V Delta (3 wire); mostly legacy but some new rural services are still done this way as Open Delta or Corner Grounded Delta

277Y480V (4 wire); new standard for industrial service

575V Delta (3 wire); rare legacy, but found in the South East in textile mills and the North West in some Lumber Mills.

Hi JRaef,

Your detailed information is much appreciated!

As per your information, the voltage and number of wires for the three phase grid are very odd and variable. I thought the difference was from different states, but now it seems it is from different house, different utility service area.

We are selling wind turbines to USA and our customers are going to connect with the local three phase grid, that's reason why I ask for this information.

IS there any more informtion on this subject? is there any source can be touched?

Much appreciated!

Rgds.

Ani-Luo

The power grid in the US is pretty much uniform, and it isn't just 120/208 or 277/480, or anything else. The main distribution networks for residential areas range from over 1000 volts to over 15,000 volts. Stepdown transformers supply homes with 120/240V, single phase, center-tap-grounded, 60 Hz.

Light commercial/industrial typically are supplied with 277/480V, 3-phase or 120/208V 3-phase.

Heavy industrial usually have their own sub-stations and commonly have anywhere from 2400V to 7200V going into a building where it is then transformed to lower voltages for distribution. Large plants may have 15000V +.

Ani-Luo,

What are the specifications of your wind turbines ?

Hi,

Our wind turbines vary from 500W to 20kw, both on-grid and off-grid.

Pls. check details in our website.

www.chinahummer.cn/eng

Thank you!

B/Rgds.

Ani-Luo

All electrical work done in new structure across the USA (all states) is done under the SAME ELECTRICAL CODE, and governed as such. The NEC (National Elec Code) regulates the new wiring codex as well as any older wiring that is in homes, offices, etc. Of course, there are plenty of older homes that still have clothe covered wires and old fixtures, but as a contractor or electricians adds to the home/whatever they must use modern wiring and oftetimes will simply rewire to the most current NEC.

The UK and the USA can use the same equipment and tools as the voltage is fairly equivalent.. But the wiring is different and thus the plugs and outlets are different. Europe is the same way compared to America.

Thank you!

But what wires and plugs and voltage together should I choose in the USA for our equipments? and is it the same in different states?

Is there any relative links on it?

Thank you!

Rgds.

Ani-Luo

http://en.wikipedia.org/wiki/Electrical_plug

"We are selling wind turbines to USA and our customers are going to connect with the local three phase grid, that's reason why I ask for this information."

If this is the case, forget all the other stuff about plugs etc. 3 phase connections such as this would not be done by plugging something in, it would need to be hard wired by a licensed contractor and THEY will be responsible for the connections. You could technically supply a plug, but trust me, it would NEVER be the right type of plug for 3 phase. If you think we have a lot of different voltage variations, you will find out there are at least a half dozen different 3 phase plug configurations for eah one...

If your product is intended for residential use you have a bigger problem. 99% of residential installations in the US have no access to 3 phase power.

As to what voltage to standardize on, I would pick 208Y120 and/or 480Y277 for 3 phase and provide instructions for the installer as to how they would connect it for all the different varieties that exist out there, including listing the one's they should not connect to at all. The people with 240V 3 phase would just have to use a transformer.

Hi，

Thank you for the help from all of you!

Quite appreciated!

B/Rgds.

Ani-Luo

Ani-Luo,

Let me add a few points to consider, in addition to those from others and particularly the good ones from JRaef.

• The National Electrical Code in the USA does not dictate the voltage, phasing, or type of grounding (corner, ungrounded, center tap, etc). It gives the standards of what a completed installation must do in terms of fire safety, property safety, and personnel safety. Electricians go through years of on-the-job training to learn how to do the work and use the various materials to meet and exceed the minimum standards of the NEC.
• The power distribution wiring from the utility generating equipment up to the electric service to the customer is not governed by the NEC. There are other codes that do govern this.
• Every utility I have dealt with has a document detailing their service standards. This will specify the service sizes they will install, in terms of voltage(s), phases, amps, metering, power factor, location of service, etc.
• Depending on where you are on the distribution network of a utility, the actual service you can receive may be limited by the distribution network. This is usually more significant in older parts of urban areas and out into the rural areas.
• Utilities are generally regulated at the state level, but some may be cooperatives or city-owned. There are a few Federal rules they have to comply with, including the one that requires them to work with privately-owned generators of electricity, in a way that is reasonably fair (but not always so).
• Depending on the amount of power involved, a private generator can contract directly with a separate user, to deliver power to that user over the utility's distribution system. This is called "wheeling" and the utility will charge a rate for the wear and tear and maintenance of their lines that you are using. In practice, you are putting power into their system through a meter and the buyer is taking it out through a meter. In between it is just a portion of the total electricity carried by the distribution network and is mixed with it. If the buyer uses more electricity than you generated, the utility bills for consuming power from it. If you put in more than the buyer takes, I'm not sure what happens in terms of billing or sales.
• If your generators are putting power into the utility's system, your equipment will have to protect this system from harm. This includes matching their system's voltage and frequency and automatically disconnecting if their system "goes down" (so you are not backfeeding into their system when it has been turned off for repairs or maintenance).
• If your generators are supplying power to the customer's system, the comments immediately above still apply. However, the customer could also include a transfer switch so specified loads are exclusively powered either by your generators or the utility, without interconnection.
• Even though there are so many possible types of electrical service voltages in the USA, if you design for the four-wire 208Y120 and 480Y277 voltages at 60Hz, you will cover over 80% of the probable customers.
• There are many good sources for getting local average wind speeds. This will help you to decide where to put your advertising and your sales personnel. You could also identify the utility companies serving these areas and then obtain information from them regarding their usual service voltages and any oddball exceptions to them.
• There seems to be a lot of uncertainty regarding how well new wind generators and wind farms will be able to connect to the utilities to sell their output. It seems that many of the windier areas are remote from major transmission lines. Installing new lines can meet with local resistance if they cut across someone's view or require environmental approvals.
• There is also a lot of debate regarding the effects of the turbines on birds and bats, and the low-frequency noise they can make. How you approach these issues can have a significant effect on your ability to sell turbines.

I've run out of time to add more. This is quite a bit as-is.

--John M.

Hi, Jmueller,

Your quite detailed answer is very impressive and much appreciated by me. Thank you!

I will study it further and may new questions come, I hope you can also give me a hand.

Thank you!

B/Rgds.

Ani-Luo

Recommended system voltages for the US market can be found in ANSI standard C84.1, published by the National Electrical Manufacturers Association (NEMA). 3-wire (delta) systems should be 480V (3 phase 240V is still available, but is being phased out in most locations). 4-wire (grounded wye) systems should be either 208Y/120V or 480Y/277V. The voltage to be used should be determined by the size of the generator, or more precisely, the current at 60Hz and rated voltage. Higher current requires larger and more expensive wire and connections. You might also want to investigate using 480V out of your inverter, and provide an optional transformer at additional cost if the customer wants a different voltage.

I don't believe that any utility in the US would allow a plug connection for a generator. For on-grid use, there is no way to prevent out-of-phase connection. For off-grid, you must provide an interlock or open transition switch to prevent connecting the generator to the grid. You will need to use either a circuit breaker or a contactor, both with overcurrent protection, as a disconnecting means. For on-grid use, the disconnect must have synchronizing capability. I suggest putting compression lugs on the load side of your disconnect, and allow the installer to bring in his wire to terminate on these lugs. Note that any 3-phase installation is considered commercial by the NEC, and so all wiring must be armored or in conduit.

If you decide to go with a plug connection for a stand-alone system, it needs to be a twist-locking system, such as this Hubbell connector, to prevent inadvertent disconnection.

Everyone seems to be skipping over the really fun voltage system in the USA - the one supplied to most of rural Pa - the 120/240V Delta system with 2 normal legs and one high. Most rural farms are supplied with just a 2 leg version, both @120V, but occasionally a business gets built in a rural area and gets the Delta system 3 phase with the high leg. Lights and receptacles for small appliances in the USA are always 120V. Now the real fun starts when some electrician does not realize he is wiring to a Delta system and gets lights and receptacles on the high leg. As a young green engineer, I drew plans for the power system in a rural bank and never checked the system being supplied by the power company. Unfortunately, neither did the electrician who got the bid to wire to my well balanced three phase system for a Wye connection. Big fun the first time the lights were turned on that were wired to the high leg. I was saved by our specifications which placed the onus on the contractor to verify voltage before wiring to the drawings.

Phys,

Good anecdote and warning regarding the "high-leg" or "red leg" or "wild-leg" delta system (different areas use different terms). It was mentioned but in a single sentence in a post by JRaef. If Ani Luo is supplying equipment with a solid neutral 208Y120v and it gets connected to a high-leg delta, there will be another "poof" as equipment learns how to smoke.

--JMM

Based on what I thought I knew, there is no high leg on the three phase source voltages; the high leg is phase to neutral only on a 480-240 delta-delta system with a center tap on one secondary winding to neutral. The secondary phase to phase voltages typcially are 240 nominal, 120 phase to neutral, and 360 high leg to neutral. So where did my memory fail me?

BTW, in the NEC, other than neutral (grounded conductor) and ground (grounding conductor), the only color code for wiring is for the high leg which is to be marked orange.

Kyoto,

Not sure what you meant. I have worked on a few high-leg delta systems, but none recently. As JRaef said, they are good for a predominantly 3-phase premises load that has a limited amount of 120V 1-phase power being used also. Since the 3-phase voltage is the full 240V instead of the wye approach's 208V, motor selection and application is easier. The "B" phase (the high-leg) is 208V to neutral (I'm not sure where you got the 360V), so this type of system is in compliance with the code's requirement that a service be grounded if one can do so and keep the maximum line-ground voltage less than 300V. Depending on the utility company's metering standards, the high-leg may be in the "B" position or in the "C" position within the meter socket (the NEC has no jurisdiction on this).

If you want something a little spooky, try a legacy ungrounded 2300V delta.

--John M.

This was at the Ports Authority in SC. Imagine three 4160/240 delta-delta connected transformers, with the secondary B phase connection at the top of the triangle. Opposite across the A to C phases is a transformer with a center tap which is grounded. If the secondary voltages are 240 phase to phase, phases A and C to ground (neutral) will be 120. Phase B to ground will be 360 [240 + 120 (half the phase A to C connected transformer)].

There are many "oddball" distribution voltages. A neighboring utility had one customer with european equipment who paid an obscene amount of cash to get a 400/230Y AC service. However, for someone trying to break into the US market, the best bet is to design for the largest piece of the pie, which is 208Y/120V and 480Y/277V. Those with non-standard voltage are usually very experienced at adapting "standard" equipment to work with their service.

Hi Ani-Luo we had a look at your website which is very interesting. Just could find no prices though, to give us a guide if we should consider an installation and what size? Also, your site talks of being blocked due to spam mail... not a good idea, we were definitely not encouraged to send you an email. Sorry for sounding negative but we were wondering how many potential sales may have been lost? All the best with your endeavours and hopefully the quality of your products will render those small niggly negatives inconsequential in the long term.

Hi Guest,

Thank you for dropping by at our web. And thank you for your good advice. Pls. if you have any interest in our products, send an email to me with your specific concerns. I will reply you promptly.

Our wind turbines are very popular in the eyes of many customers from over 70 countries, with some of the customers being already our local agents. I hope we can also do something together.

My email address: hummer-607@hotmail.com

B/Rgds.

Ani-Luo

Ani-Luo,

In the USA, all 3 phase circuits that I have designed use 3 Hots and a ground. If you measure the voltage between two hot legs, you will measure ~480VAC. If you measeure the voltage between a hot leg and ground, you will measure ~277VAC. For 3 phase circuits, the US does not use have a neutral leg. Hope this helps.

Randy