single phase vs. three phase

[john37]

This is kind of a vague question but when would it be more beneficial to run 208/120V three phase vs. 208/120V single phase for a commercial bldg?
In reference to motors what would be the recommended point where you would want to have it at 208 3 phase? I was told anything over 3HP it would be better to have it at 3 phase vs. 1 phase because the motor would run more effecient. Is that true? Any opinions would be much appreciated. Thanks.

 

[One of a Kind]

In my experience it depends on what the building currently has in place.
If you're talking about a brand new system, I've always preferred 3-phase in commercial applications. Obviously lighting and receptacle loads will be single phase, but I think 120/208V, 3-phase is easier to accomodate mechanical loads which need mostly come in 3-phase/208V or 3-phase/480V.

 

[jtester]

If my service is over 200 amps single phase, I consider 3 phase. If the HVAC units are larger than 5 ton, I always go 3 phase. If the motors are greater than 5 hp I usually go 3 phase.

A lot depends on whether 3 phase is available from the utility.

Jim T

 

[rod1478]

Its more economical to use three phase power system smaller cable size smaller breaker ratings compare to an equivalent single phase power system .

 

[kingpb]

If 3-ph is available use it, if not, don't.

 

[LarryFine]

Its more economical to use three phase power system smaller cable size smaller breaker ratings compare to an equivalent single phase power system.

Don't forget to include the extra breaker pole and conductor count in this generalization. :wink:

 

[Johnmcca]

How does 3 phase make the cable and breaker smaller? Increasing the voltage I understand, but the increase from 1 to 3 phase?

 

[cpal]

How does 3 phase make the cable and breaker smaller? Increasing the voltage I understand, but the increase from 1 to 3 phase?

kVA = V X A X 1.73 / 1000

 

[Johnmcca]

Thanks Charlie

 

[boater bill]

If the utility will provide three-phase, make sure it isn't open-delta three phase. The owner will lose panel space since they won't be able to use all of the breaker spaces available for the 120 Volt branch circuits.

For the record-IMHO open-delta should be illegal. I HATE IT.

 

[coulter]

I'm certain my industrial experience is not a close crossover to commercial. But may provide some insight.

On that disclaimer, as kb says, if 3ph is available use it.

I'm partial to impedance grounded 480V. Hang small 208 xfmrs and panels where needed. Stay away from 277 lighting - requires grounded 480 (yuch), use 208 - gets rid of harmonic problems. Spec all motors above cord and plug as 480V, 3ph. Hang small 480, 3ph, solid grounded xfm for 277 heat trace.

Haven't ever run any economic calcs - probably should someday.

If the POCO drags their feet on supplying the service you want, offer to buy the xfmr - should be able to negotiate a better rate. It kind of fun to do that cause it really freaks out the AHJ when you tell them the substation xfmr is an SDS not a service. The service is the 138kV.

(short break for assistance by young son) thomasssss12345678901qgfhhygthjjgfghfghjhydhljugkgwqwrhyhjuyuhhjhhghupo

Consider 4160V for motors 250hp and above. I have done this calc a lot, and have had 480V be the ecomonic choice up to 600hp (once) - depends on how many, and how much distribution has to be installed.

One of our posters recently commented about there being two sides to every story. I would respectfully disagree. There are three sides - emperical data, researched facts, assertions. I really try to differentiate.

This is mostly assertions.

carl

 

[jim dungar]

If the utility will provide three-phase, make sure it isn't open-delta three phase. The owner will lose panel space since they won't be able to use all of the breaker spaces available for the 120 Volt branch circuits.

For the record-IMHO open-delta should be illegal. I HATE IT.

Just a little side track.

Boater Bill, open delta is one method of connecting transformers, it does not identify the voltage system.

What you are describing is a more accurately called a center tapped delta, or a wild-leg, or a high-leg, or even a christmas tree connection. Or the best method is to drop the slang and call it by the actual system: 240/120V 3phase 4wire.

 

[cpal]

All the 3P-4W Delta I've seen used a single phase lighting panel for the 240/120 ???


no empty poles in those!, I don't think I've ever seen the high leg in the lighting panel. But I haven't seen these systems in quite a while.

 

[john37]

In my case there is an existing 240/120V 1 phase service going to an old gas station. They are converting it to an auto repair shop and providing a new compressor, car lifts, etc. The owner is asking for a recommendation if they should change out the existing service to 208/120V 3 phase. The compressor is 5HP and two lifts at 2HP. Both are available either in 1 phase or 3 phase. The site is being fed from a utility both right in front of the site and there is 3 phase available close by (next pole over). My question is would it be beneficial for the new owner to change out the existing 240/120V 1 phase to 208/120V 3 phase and why if so? Other than being more efficient to run the motors and smaller breaker sizes/wires is there any other legitimate reasons to change out the service anyone can think of? The existing apparatus is very old. I tried to insert a picture of it but couldn't figure out how too....

 

[LarryFine]

My question is would it be beneficial for the new owner to change out the existing 240/120V 1 phase to 208/120V 3 phase and why if so?

John, in your case, I'd say the decing factor would be the existing equipment.

Will the existing/remaining line-to-line single-phase loads live well on 208 single-phase?

Would the decision also dictate whether you have to replace otherwise-usable service gear?

(short break for assistance by young son) thomasssss12345678901qgfhhygthjjgfghfghjhydhljugkgwqwrhyhjuyuhhjhhghupo

Carl, your son types like my cat! (well, except for the first part)

 

[LarryFine]

For the record-IMHO open-delta should be illegal. I HATE IT.

It has its place, as long as you understand it. Last year, I wired a restaurant after a complete demolition. The existing service was 240/120 open Delta, but the POCO wanted to deliver 208/120 if they had to change anything, and at the customer's expense ($6K+).

The only 3-phase loads were the two RTU's, so I reworked the service details so we could re-use the existing servce, including the trough and 200a and 125a fused disconnects. I ran new conductors in EMT from the discos to new panels in a new room, never even having to touch the line side of the switches.

I ran a 200a 1ph panel for everything except the RTU's, which were fed from a 125a 3-ph panel, not even needing a neutral. I simply removed the high-leg fuse from the 200a switch, and ignored its lug. A 240/120 open Delta is absolutely identical to a 240/120 1-phase system, other than the high leg.