3 phase to single

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ElectricianJeff

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Southern Illinois
Let me start by saying that I know little if anything about 3-phase. I wired a 3-phase rotary converter about a year ago and besides that I've never had any experience with it.

I have taken a job to re-wire a former 3-bay and office auto workshop that is quite old and has been vacant for a few years. This building currently has a 100 amp. 3-phase service. I started last week by having the poco energize the building so I go tell what I had. I went by yesterday and everything in the building is working.

The 4 wire feeder to the building consists of 3 #3 copper conductors for the three legs. The neutral is tied to #4 bare copper at the weatherhead. The ground is bonded to the neutral at the meter socket which is allowed here. From there it goes to a large fused knife type disconnect. Voltage readings line to neutral/ground read 120/120/240. From the disconnect there are 2 3-phase subpanels with their loads long removed. There is also a main lug only panel servicing the 120/240 loads which consist of about a half dozen 8' flours. lights, 2 gas furnaces/ no a/c, assorted 120 volt receps. and some outside lighting. The wiring, for the most part is run in emt.

The will be no permits or inspections required where this property is but I want a compliant and safe install.

My customer has just purchased this building to store and service his business trucks and is leaving it up to me to update. My plan so for is to replace the outside service and convert to a single phase since there is no current or anticipated need for 3-phase. I haven't decided yet weather to leave it at a 100 amp or upgrade to 200 till I meet again with the owner to discuss price of each.

I plan to use a similar design with the service in to a disconnect then about 12" over to the breaker panel.

Whats bothering me is that the current wiring consists of 2 1" emt conduits headed up from the panel and into the ceiling to all the lights and receps in the building. Each pipe consists of 6-8 conductors and a #4 bare as a shared neutral.

I would like to do this without pulling new wire but I doubt this is a compliant install. Can anyone shed some light on what I need to do with these circuits? Is this or has this ever been a compliant way?

Sorry for the long post.
 
To get started.

I would never by choice replace a 3 phase service with a single phase service.

There is just no good reason to do so and so many reasons not to.
 
ElectricianJeff said:
Whats bothering me is that the current wiring consists of 2 1" emt conduits headed up from the panel and into the ceiling to all the lights and receps in the building. Each pipe consists of 6-8 conductors and a #4 bare as a shared neutral.
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You'll need to open every thing up and confirm all the wiring if your going to be responsible.
ElectricianJeff said:
I would like to do this without pulling new wire but I doubt this is a compliant install. Can anyone shed some light on what I need to do with these circuits? Is this or has this ever been a compliant way?

.
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Time for a rewire.
 
Why do you feel you need to change anything.Chances are good that his old service can handle the job.Do a new load calculation first.Changing this to single phase will devalue this building.Also its likely sharing a few neutrals (3 phase) and this would now leave you without a neutral on one of the legs.If its not broke dont fix it.
 
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John said:
Did you measure line-to-line voltages?
Are you sure that the 240-to-neutral voltage wasn't more like 208?
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I agree that something isn't right about having 120/240/120 to ground voltage measurements. Sounds like a 3 phase, 4 wire Delta system which would provide 120/208/120 to ground at the disconnect.
 
infinity said:
I agree that something isn't right about having 120/240/120 to ground voltage measurements. Sounds like a 3 phase, 4 wire Delta system which would provide 120/208/120 to ground at the disconnect.
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That's what I am thinking.
 
ElectricianJeff said:
Whats bothering me is that the current wiring consists of 2 1" emt conduits headed up from the panel and into the ceiling to all the lights and receps in the building. Each pipe consists of 6-8 conductors and a #4 bare as a shared neutral.

I would like to do this without pulling new wire but I doubt this is a compliant install.
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...a bare neutral in EMT?

That's a problem.
 
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I would not change from 3 phase to single phase.

It sounds as though the facility's use was very similar to what it's intended use will be now, so what would be the reason for any major changes? You will have to use your judgement as to what condition the wiring is in.

Look at 310.15 B 2 for derating the ampacity of the wires in the 1" conduit thing. follow the asterisks on the small wire... and I don't believe the neutral should be bare in the conduits. Is it possible there are no phase - neutral loads and the bare copper is a ground of some sort?

Seems that you need to do a little more investigating. The voltage you read does not sound correct. It should probably be closer to 200 volts to neutral on one of the phases than 240.

Working with three phase is not all that much different than single phase... Instead of having two hot phases you have three, and with the voltages you describe, one of those will be a "wild" or "high" leg, which is produced because there is more "transformer winding" between the hot and neutral on that leg. You will have to be very careful what, if anything, you hook up between that leg and the neutral. Mark the high leg with orange tape wherever it exists in the same enclosure as the neutral.

Other than that, you would go about your business in much the same way you ever would. The voltage between any two phase wires should be 240. ... the same as it is between single phase - phase wires.

When balancing loads, you have three choices as to which two phases you hook up 240v single phase loads, instead of just one. ( Now there by durn is a crystal clear statement )

You could use three phase motors for air compressors, fans... whatever and the wire sizes will be smaller .

Try to keep the load balanced as much a possible to avoid loading the neutral, just like you would anyway with single phase, watch out what you hook from the high leg to neutral, take advantage of the increased power capacity of three phase circuits, and you should be fine, and happy.

This seems like a great opportunity to learn about three phase stuff... doesn't sound like there's a lot of pressure to produce or maintain service, and leeway to explore options... Gee... sounds kinda fun.:smile:
 
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A lot of utilities charge less per KWH for single phase than three phase, which is why a lot of customers go the single phase route. Since this is probally a Delta service if your getting 240 volts phase to phase, Are there only two transformers outside? Or three with one larger than the other two? And if the 6-#8 are feeding receptacles and lights, you will probally find two of the #8 capped off in a junction box somewhere if their using the #4 for a neutral. Probally was wired originally a WYE connected service and was an "oops" . Just re-phase and reconnect these two extra wires as neutrals and use the #4 as the ground if you are going single phase.
 
A hundred amp three phase service would give almost as much power as a two hundred amp single phase service. The dif. being the load spread over three conductors instead of two. I think that my biggest concern, with the building setting empty, would be the shape that the panel and breakers are in. Unloaded bare metal corrodes at a pretty rapid pace. I would also take a good close look at the branch circuit wiring for damage caused by mice and /or other insects. Good Luck!
 
For what it is worth, I had to separate an office building's service from the warehouse it backs up to a while back. The office was fed from the warehouse and the owners wanted separate service. I was gonna switch to single phase, but the POCO made it clear they did not want to set another pole with a transformer. (tight area with tranformers on all near by poles) He offered my alternative as 120/208 single phase. Thus, I installed a new 3 phase service and panel. No big deal, but see if the POCO will work with you on the change/downgrade.

c2500
 
As a follow-up:

I met a friend of mine at the building today whom was a lineman for the utilty but is now in upper mangement. He said going back to single phase would not be a problem. There is another building being served off the same pole with single phase. He said it was being feed off a "double pot" whatever that is.

To clarify alittle, the phases go to a 3 phase disconnect. The 2 120 phases go to a panel that services all the current 120 and 240 volt loads. The three phase goes to only two abandoned subpanels, one appears it may have served a lift at the bolts are still in the floor but the lift is long gone. The other panel went to an empty corner of the building near the panel, I'm guessing for a compressor. These 2 subs are the only places the 3-phase was used.

The owner has nothing requiring 3-phase, he will be using the building for storage of materials and parking and servicing of his company pick-up trucks. He will also be moving his small 1 person office to the finished area in the front.

The electric equipment is quite dated and needs replacement, the meter socket is about ready to fall off the building and the distribution panel is about the same with 50+ years of grease covering it. My friend confirmed that the cost per killowatt hour would be higher than with single-phase.

I'm meeting with him in the morning and going to recommend he go to single phase. I don't see "devaluing" the real estate as a issue. This is a small building in a small town population 1000 +/-, he paid less than 50,000 for the property.

It looks like I will need to run new grounded conductors to the loads and get rid of the bare copper in the conduit. I need to spend some attic time to see whats going where.

I really appreciate all the input, this will be a learning opp. for me.
 
There is the other thing you may overlook one thing with most commercal building. There may a hidden rules for service size due I know most commercal customers useally ask for 200 amp or larger on both one and tri phase system.

I am well aware some POCO do charge little more on 3 ph system over single phase but in long run it may have more benfit due some of the future customers may end up want having 3 phase system in there.

Majorty of my commercal customer have wye connected system but there is Delta system around here but only used for exsting service unless you plan to upgrading they will useally ask ya to swap over to wye system.

Over my time very rare that I do install 100 amp service in commercal location at all .

Merci,Marc
 
frenchelectrican said:
Over my time very rare that I do install 100 amp service in commercal location at all .

Merci,Marc
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I don't do a lot 100's in homes except for the occasional 60 amp upgrade in the small bungalows with gas everything.

For budget reasons , I'm thinking 2" rigid with all 200 amp rated equipiment and a 100 amp main disconnect. There's actually a PVC riser on the back of the building where they were going to fly some power into another building that was not included in this deal. That building now has its own service and the #3 conductors in that riser look almost new so I will use them for this new service and save him a few hundred by not having to replace the conductors.

I have not yet done a load calc but furnaces are gas, gas HWH, no kitchen, window unit in office area etc.

If 200 amp. is ever needed we can pop in some 3/0 conductors, change the main and be good to go.

Jeff
 
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