Need help with an electrical question (3 phase)

For any of you knowledgeable electricians out there....


I have a three phase refrigeration unit that is dismantled..  My problem is, I need 220volts single phase for some commercial food warmers in that exact spot....  I am assuming the amperage is plenty based on the gauge of wire running for the 3 phase... they look to be about an 8 gauge...

I assume that the three legs running are 120 volts each in the 3 phase wiring.  Any reason I Couldn't just use two of the legs to get 240 volts single phase, and then find a good ground for my neutral?


Thanks... [:)]

In a 3-phase 240V system the voltage from phase-to-phase is 240V, and the phase-to-neutral voltage is about 138VAC.  In a 208V three-phase system the phase to neutral voltage is 120V.  That's why 208V is still popular in commercial buildings.  You may have a 240V delta system in which there is no neutral, just three phases and a ground.  Is it a three-wire or a four-wire system?

Find the feeder for the refrigeration unit.  That should tell you what the ampacity is for the circuit (depends on the breaker or fuse).  #8 wire is protected for probably 50A max.  Use any two legs for your single-phase application, and make sure you ground the warmer properly - a ground fault to the warmer case without proper grounding will result in a potentially lethal piece of equipment.  With proper grounding it will cause the circuit breaker to trip or fuse to blow.  Also, if the warmers are much less load than the supply can provide (say 10A on a 50A circuit) provide some local overload/short circuit protection that is properly sized for the warmers.  Protect [b]both[/b] legs.

Generally speaking, no it won't work, unless there are some equipment tricks I don't know about.  You mentioned using a ground as neutral.  If you do that, then you'll only have 120V potential.  If you use one of the three phase legs as a neutral and another as the hot, you'll have 208V potential.  That might work since it sounds like you're dealing with something that is relatively voltage insensitive.  I'll reluctantly admit that I've done this before in a couple of restaurants.z

Huh?  How do you get 208V out of a 240V three-phase system?  In short, you don't.  

I'm an electrical engineer - I work with three-phase stuff all the time.  There are two types of three-phase connection, Wye and Delta.  In a Wye (4-wire) system there are three hots, a neutral and a ground.  Neutral and ground are bonded together at the distribution panel.  In a Delta (3-wire) system there are three hots and a ground - phase to ground voltage in a Delta system depends on how the ground is connected to the transformer.  (There are ungrounded Deltas but they are not legal to set up today.)  

By far in commercial systems the 4-wire Wye is most common.  Then there are three normally commercially available voltages, 208V, 240V, and 480V.  In 208V systems the hot-to-neutral is 120V, phase-to-phase is 208V, and it is used in distribution systems where some 3-phase is needed, but the majority of the loads are 120V single-phase.  In 240V systems the phase-to-neutral is about 138V (not useful), phase-to-phase is 240V.  This is used in systems where the load is primarily three-phase, or where a lot of 240V single-phase is required.  In 480V systems the phase-to-neutral voltage is 277V, phase-to-phase is 480V, and this is used where the load is primarily three-phase (though there are 277V single-phase lighting systems, usually found in industrial buildings).  

Trust me, I do this stuff for a living.  I even have a license.

Cant do that.  In 480 3-phase, phase to ground equals 277v and phase to phase is 480v.  Also, the phases are 120 degrees out of phase with one another.  

OK, assuming that realist's refrigeration unit is 480V three-phase, you would be correct (and he doesn't specifically say).  However, from the description he gives (and the fact that most commercial equipment isn't generally 480V), I had to make the assumption that he knows that his source is 240V three-phase (from the question: .  In that case, he is correct that two hots will give him 240V single-phase.  This is compatible with a 220V rated piece of equipment.  He needs to use [i]ground[/i] as opposed to [i]neutral[/i] for his ground connection - there should be a ground available in his system.  If, by chance, he has a three-phase Delta system (no neutral) he needs to determine what his phase-to-ground voltages are.  A 240V delta system is not likely in a commercial wiring system, but if that's what he has, he'd better hire an electrician.

realist
Trust kbaker he right I do electrical work myself everything he says is the truth

I am very familiar with 220 3 phase everything.

I second the above statement.

Thanks to you all for your insight...    the building once was a  rice supermarket
in Houston, and the 3 phase compressor existing was installed in 1974 and was used for the walk in meat freezer I assume..  

There are 3 legs present..  all live with my sound test..  I haven't put a meter on it
yet to see what currant they are holding..  And I don't remember there being a ground present.. so it may very well be the Delta that some of you speak of...


I may have to hire an electrician just to tell me exactly what I'm dealing with and go from there....  It's good to know our Board has so many knowledgeable people....[:)]

Realist

Where in the hell do you guys get the 138v to ground on 240v 3 phase. I have never seen it,I have seen 208v to ground on the b phase of 240v three phase. But never 138v on any phase. Maybe you guys have seen something I have not or your full of sh#t.

If you need to ask these questions perhaps you should hire an electrician to do this job for you.

Hire an electrican to do this job before you kill yourself or somebody. They need to make a living to you know.

I'm Curious as to how much is it to install a 3ph in a residential area?
NY would probably be 2x as much.
I've read about [i]converters[/i] that will mimic a 3ph so equipment needing 3ph can be run. Anyone have an opinion regarding these [i]converters[/i]?
Thanks

I don't think that you can run 3 phase in residential areas. Not because it can't be done, but in some cases is against the law.  

Its not against the law or code or whatever. I have it at my house. 3phase 480v with a stepdown trans. the 480 runs my ac. But I do have to pay the comercial rate.

I apoligize for tagging on to your question. but what advantages does 3 phase offer.I want to get a electric forklift for my buissness but most of the chargers are 3 phase.I was told it has something to do with less amp draw. Is this correct.

Hey K Baker, did you ever live in Mesa AZ?

Waterdog

You have to take some voltage measurements to know what type of 3 phase system you have.  

There a basically 2 types of 3 phase.

1 has the coils of the transformer wound in a "Y" configuration, the other a delta (visualize an equilateral triangle).

Lets look at the "Y" first.  The 3 legs, or phases are equal in length and all spaced 120 degrees apart.  The length of the legs is the phase to neutral voltage and the length from point to point is called the phase voltage. Sometimes (most) the center point is grounded.  In all "Y" systems the phase to neutral voltage is the phase voltage divided by the square root of 3.  Trust me. It's a trig. thing.  The common "Y" services provided by all utilities in this country are as follows (phase to ground voltage/phase to phase voltage),  120/208, 277/480.  As far as I know there is know such thing as a 138/240.  A lot of the more modern equipment can work on 120/208 systems.  See if there are jumper connections for operation on a 208, 3 phase for your unite.   These are common it large apartment complexes where the majority of the load is single phase but due to the total load requirements of the facility the power transformer that supplies the service voltage must be a 3 phase unit.  Most all 3 phase, secondary services fed from from underground primary faculties are "Y" type services.

In a delta system picture a triangle with 3 equal sides.  This is the delta system.  The coils are arranged in a delta configuration.  These usually come in 240 or 480 volt systems.  If you center tap one coil and ground it you can get a 120/240 single phase.  In fact you can service single phase 120/240 homes from a 3 phase bank configured this way.  Picture the triangle with one of the legs with a dot in the middle.  If this dot were grounded the voltage from the dot to the corners of the triangle would be half the length of the side it were on.  The distance from the dot to the apex opposite it would be a square root of 3 into 240, or 208.  This is commonly called the wild leg.  Common voltages are (single phase voltage/phase to phase voltage) 120/240 or 240/480.

Hope this helps more than confuses.  

that is exactly how I wire my equiptment. I forgot to mention that a neutral should already be there. How many wires toal are there? there should be at least 4.

Before you go wiring anything up it would be wise to seek the advice of a qualified electrician.  

About this ground-neutral thing,  THEY ARE NOT THE SAME!

A neutral conductor is a current carrying conductor.  The only place that this can, and must be, in connection with the ground is at the main service entrance.  

i secind this notion, electricity is a very trickey thing, just 1/10th amp, could kill you in the right situation. better to spend some extra money and be sure than to get killed trying to save that $$.

just the prespective of a survivalist.