480 volt single phase service

[jim dungar]

Of course, you could be looking at some version of 'single wire earth return' where you bring out two phase conductors and use the soil as the third conductor. Two wires, and with suitable transformers you could provide three phase service

I do know what I am looking at.

The only earth return systems I have had experience with are SWER - single wire earth return.

Most of the rural roads in Wisconsin are fed by (1) ungrounded and (1) grounded conductor tapped off of the 3-phase 4-wire wye distribution systems that run along the county highways.

I try very hard to always include the number of conductors when I describe a circuit, although grounding conductors are implied unless specifically mentioned.

 

[LarryFine]

Jim, this is where you threw me:

My buddy owns a machine shop 1.5 miles from the closest 3-phase, the utility says they will run the third phase if he pays for the conductors and labor.

When you said "third phase", I took it to mean there were already two. If you had said "second and third phase", I would have seen you only had a single line (and the understood grounded conductor.)

 

[jim dungar]

Jim, this is where you threw me:


When you said "third phase", I took it to mean there were already two. If you had said "second and third phase", I would have seen you only had a single line (and the understood grounded conductor.)

My apologies, it appears I did mis-speak.

 

[danickstr]

It seems to me that if you take into consideration the cost of replacing VFD's as they fail, (and could damage motors) and they are susceptible to voltage spikes, lightning, etc, not to mention circuit failure, this will cost far more than 120K over 30 years or so.

here is a guide to finding good VFD's:
http://www.usphaseconverterstandards.org/phaseconverterpurchasingchecklist.htm

 

[gar]

080406-2119 EST USA

As a wild idea one or more single phase motors driving alternators. What would that cost? The multiple system to reduce startup load on the line.

.

 

[donaldsullivan]

Update

This all started because the utility company has increased the installation cost from $55,000 to $130,000. We are trying to stay within the $55,000 budget; and if using single phase is much more than $55,000, or a manufacturer isn?t willing to ok the application then we will recommend our client to just pay the utility company the $130,000. I hope this helps you to understand why I am even looking at doing this in the first place. Some of the equipment is already on the job.

At one point I was thinking not to recommend this, but the other manufacturers were oking the application. I don?t actually feel comfortable recommending this, but I would like to get the cost options before I make that decision. The cost to go single phase may be within the $55,000 to
Justify it, but if to costly the decision will be to pay the utility cost.

We did get a budget more than a year ago, but the utility company changed the price. This is a municipal project and they take quite awhile to go from preliminary design to construction. We didn't get anything in writing, but does anyone get anthing in writing from the utility company in the state of Texas, the utility companies are deregulated in Texas. I have been in this business for 14 years and I can't get anything from them in writing.

All suppliers have given their approval for the application to use the single phase on the equipment. Currently calculating the cost.

 

[winnie]

My _hunch_ (and I'd love to see the results after you do the calculations) is that going the 'single phase + VFD' route will be considerably less expensive than the original $55K that you planned for in _up front_ costs, and certainly less expensive than the revised $130K

I am basing this _hunch_ on the following: baseline costs already include 2 VFDs for the 70HP motors, so these are being upsized, that you would be adding an additional VFD for the third motor, that you won't be paying up front for the single phase service, that you will need to increase the current rating of your transfer switch, but that your generator will remain the same.

Remember, as others have noted, a VFD that is properly sized for single phase to three phase conversion will handily run on three phase input as well, so you won't have to mod the generator for single phase operation.

I don't know enough to evaluate the issue that danickstr raised above; I presume that this is a pretty long haul overhead service, subject to random voltage disturbance, lightning strikes, etc.

You should also consider if there are different rate structures for single phase versus three phase; if these motors run a significant amount of time, then the annual operating costs could easily swamp differences in installation cost, and if there are different rate structures this might make paying the $130K cheaper over the project life.

-Jon

 

[donaldsullivan]

update

update

All suppliers approved the application. The cost looked like it was going to be just over the $55,000 budget. The utility company came back to say that they could not serve 480 volt single phase at that load level. They didn't have a transformer large enough was their excuse. They were the ones that recommended single phase in the first place. I didn't argue that they could use two 120/240 volt secondary transformers to get the 240/480 volt, but I didn't really want the project to go single phase, I hadn't designed it that way and feel that I am prone to miss something when changing my design from the cuff. I now know that I can consider it in the future, and don't see a problem with it from a design or cost standpoint, if the cost for three phase is unreasonable, but maintenance on equipment is unknown, i.e. VFD, motors, etc.

Thanks for your opinions.