208-480V Step Up XFMR for 480V motor loads

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gk351

Senior Member
Location
IL
I got a call today asking me if I would like to bid on a project. I dont know a lot right now, but I believe I have the jist of the scenario. A company is installing a new fertilizer blending system that has a total 480V motor load of about 225A. The facility currently has an 800A 208V system. The electrical project manager for this job ( I will be bidding labor only, as materials will be provided) told me a little about the scope of work over the phone. His idea is to install a dry type XFMR ( not sure what size) and step up 208V 3ph to 480V 3Ph to supply this blending system. I know for sure the heart of this system is a 60Hp 480V motor, with multiple 30hp's, 20hp's, and so on. I dont know all motor loads as of this point

I'm just curious what everyones opinion is on this. I feel they should just install new 480V 3ph service from POCO. I'm not sure what will happen on the 208V service if all these motors are running, or when they start up, etc. If its true that all 480V motor load is 225A(running), then that equates to around 500+ amps on the 208V side? Not to mention current 208V load (which I dont know yet). If they start this new blending system at any given point in time, does anybody know what the startup current will do on the 208V side? In my head, this just seems like a disaster. I may be very wrong, but it seems this might look good on paper, but cause problems in the field. Looking for any and all feedback!

GK
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Yeah, you are much better off installing a 480 volt service. Two services are allowed because your second service would be for a different voltage. (Unless they changed that exception, I haven't looked lately)
 

Electric-Light

Senior Member
Yeah, you are much better off installing a 480 volt service. Two services are allowed because your second service would be for a different voltage. (Unless they changed that exception, I haven't looked lately)

Transformer is not free upfront, and there's an efficiency penalty because it uses energizing current even when the motor is not in use and when the motor is in use, there's some conversion loss.
 

gk351

Senior Member
Location
IL
I met with the electrical project manager today, along with 2 others who are bidding. It seemed I had more questions than the others. Anyway, he spec'ed the Square D materials for the job, and they are ordering them. All other parts and labor are on us. He spec'ed a 300KVA Dry XFMR, 120/208Y-480V Delta to supply these new loads. The motors are all 480V, and as follows: 2, 3, 15, 2x20, 30, 60hp. They will start automatcially via a supplied control panel, and will start at any given time, depending on what is being called for. They have an 800A 208V service. His plan is to install a 800A CB from this panel to feed this 300KVA XFMR. His specs didnt include a secondary XFMR disco. Which will have to be changed because he specd a 400A 480V MLO Panel. The panel will be about 175ft from the XFMR.

I asked if he talked to POCO about this, since they might have a problem with the motor loads being transformed the way they are. In the end, Im not to sure about this project, but hopefully can get some clarification.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I know around here that it is much cheaper to have even a very large 208V service than a smaller 480V service. I don't now why but it is very common for plants around here to have 208 or even 240 V stepped up to 480V.
 

kwired

Electron manager
Location
NE Nebraska
If current system does not have sufficient extra capacity your choices are upgrade so there is enough capacity or install second 480 volt service.

208 volt supply to this equipment means either larger conductors, larger controllers and gear, or a transformer to change to 480 volts.

Total cost for 208 volts is probably higher than for 480, but really depends on exactly must be done.

Your current service possibly can handle the additional load, but you may want to make sure POCO transformer can handle the additional load. To have them upgrade their transformer could cost enough that other options are worth looking into.

Maybe you even consider changing incoming service to 480 volts and separately derive 208 to supply the existing loads, or even convert some existing loads to 480 volts and supply only select loads with a smaller 208 volt separately derived system.
 

gk351

Senior Member
Location
IL
If current system does not have sufficient extra capacity your choices are upgrade so there is enough capacity or install second 480 volt service.

208 volt supply to this equipment means either larger conductors, larger controllers and gear, or a transformer to change to 480 volts.

Total cost for 208 volts is probably higher than for 480, but really depends on exactly must be done.

Your current service possibly can handle the additional load, but you may want to make sure POCO transformer can handle the additional load. To have them upgrade their transformer could cost enough that other options are worth looking into.

Maybe you even consider changing incoming service to 480 volts and separately derive 208 to supply the existing loads, or even convert some existing loads to 480 volts and supply only select loads with a smaller 208 volt separately derived system.

I agree with you K. I say convert system to 480V from POCO with a large enough Service. Then supply all the existing load, which isnt 800A, probably 400A at most with a new XFMR, lets say around 150KVA, and the new blending system will be truly fed from the new 480V service. I just dont know how the 208V system will react when trying to run all these motors transformed from a dry 300KVA XFMR? Will it work fine? Inrush current?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Sounds like they have their mind made up and there may be factors beyond what is stated, but it sure looks like they are going at it backwards. From the way it sounds 75% of their load is going to be 480. I would have to give serious consideration to a 480v service and a much smaller transformer for the smaller 208/120 load.
 

gk351

Senior Member
Location
IL
Sounds like they have their mind made up and there may be factors beyond what is stated, but it sure looks like they are going at it backwards. From the way it sounds 75% of their load is going to be 480. I would have to give serious consideration to a 480v service and a much smaller transformer for the smaller 208/120 load.

Thats how I feel about it, taking 3 steps backwards to move 1 step forward. In the end, there idea may work, it just doesnt make a lot of sense. The 208V load is samll I would guess, but considering it will have around 220A 480V motor load, I dont see how someone wouldnt automatically think of new 480V service. Also, money doesnt seem to be an issue.
 

kwired

Electron manager
Location
NE Nebraska
Thats how I feel about it, taking 3 steps backwards to move 1 step forward. In the end, there idea may work, it just doesnt make a lot of sense. The 208V load is samll I would guess, but considering it will have around 220A 480V motor load, I dont see how someone wouldnt automatically think of new 480V service. Also, money doesnt seem to be an issue.

If money isn't an issue I'll take some of it too, just call it engineering fees:lol:
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
It's their design and you are only supplying labor.... where's my check :D but......
If it's like most of these situations I've encountered, although it's their design, it will be YOUR problem.
I see a multitude of possible glitches:
An 800 amp branch breaker is a 800 amp panel ???
Supplying a 300 kva transformer with that 800 amp breaker reduces the transformer potential output
Is the transformer capable of handling the motor load, especially with that input limitation (definitely be a factory or engineering question
And as you mentioned, how is is present 800 amp main going to react with this added motor load plus the existing 208 load.

I would have a firm understanding (contract) as to the scope of your work and the cost of "extras" :D
 

mike_kilroy

Senior Member
Location
United States
may be dumb, but might it be worth at least exploring extra wire cost of wiring the motors 230v instead of 480 to keep the extra 480 panels and xfmr out of the picture? KISS?

If startup current is the problem it appears it will be, did they consider sticking each motor on a VFD to eliminate it? Or at least softstart things?
 

kwired

Electron manager
Location
NE Nebraska
may be dumb, but might it be worth at least exploring extra wire cost of wiring the motors 230v instead of 480 to keep the extra 480 panels and xfmr out of the picture? KISS?

If startup current is the problem it appears it will be, did they consider sticking each motor on a VFD to eliminate it? Or at least softstart things?

Don't forget @ lower voltage you also have bigger motor controllers, bigger overcurrent devices, larger raceways, if VFD or soft starting those will be bigger also.

Advantages of going 480 volt is usually pretty obvious just looking at equipment and wire costs if you have very many motors over 10 hp.

If you have much for motors over 20-25 hp it is almost automatic that you would desire 480 volts.
 

Shoe

Senior Member
Location
USA
He spec'ed a 300KVA Dry XFMR, 120/208Y-480V Delta to supply these new loads. The motors are all 480V, and as follows: 2, 3, 15, 2x20, 30, 60hp. They will start automatcially via a supplied control panel, and will start at any given time, depending on what is being called for. They have an 800A 208V service. His plan is to install a 800A CB from this panel to feed this 300KVA XFMR. His specs didnt include a secondary XFMR disco. Which will have to be changed because he specd a 400A 480V MLO Panel. The panel will be about 175ft from the XFMR.

Yikes, the inrush on a back-fed delta-wye at 300kva is pretty significant. That 800amp breaker would make me nervous.
 

kwired

Electron manager
Location
NE Nebraska
Yikes, the inrush on a back-fed delta-wye at 300kva is pretty significant. That 800amp breaker would make me nervous.

If you use a transformer intended for step up the inrush of energizing the transformer itself is not as bad as it is when backfeeding one designed for use as a step down.
 

G._S._Ohm

Senior Member
Location
DC area
Motors want to be driven with a source that has low impedance, so check the regulation of your step-up transformer.
IIRC a 1:2 step up will give you 4x the source impedance available from the Poco lower voltage.
 

gk351

Senior Member
Location
IL
I appreciate everyone's replies. As I sit here, I wonder what happens if you put in this system, under their current design, and it doesnt work? How many fingers will be pointed, and this job is big enough, that if they blamed me, it could potentially put a small business like mine, out on the street. If there is anymore feedback, or potential problems you all see, please keep posting, as I will be calling the electrical project manager tonight. Thanks again..GK
 

kwired

Electron manager
Location
NE Nebraska
I appreciate everyone's replies. As I sit here, I wonder what happens if you put in this system, under their current design, and it doesnt work? How many fingers will be pointed, and this job is big enough, that if they blamed me, it could potentially put a small business like mine, out on the street. If there is anymore feedback, or potential problems you all see, please keep posting, as I will be calling the electrical project manager tonight. Thanks again..GK

Exactly why you need to state any concerns you have, and maybe even have signed documentation to support your views should they choose to disregard them. Maybe even consult a third party (besides this forum) that can also give documentation of the situation and present some scenarios of what could possibly happen for a particular installation.
 

Shoe

Senior Member
Location
USA
If you use a transformer intended for step up the inrush of energizing the transformer itself is not as bad as it is when backfeeding one designed for use as a step down.

I agree, but I'm skeptical they are using a transformer intended for step-up, since the high-side "secondary" is a delta. I would consider a wye secondary to be a much better choice, if you're going to order a step-up transformer specifically for this application. Makes me think they are just reusing something they had on-hand, but I could be wrong.
 
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