Selecting Switch Gear and Components

switch gears are protective devices and i dun think it will step down !!

u can use a step down transformer , possibly an autotransformer where ur desired voltage is chosen with winding tapped from the desired value !

For 463 KVA, a 600 KVA transformer is a rational choice. You might suffice with 500 KVA, but the extra reserve is probably good. However, 600 KVA ≈ 721 A, which would suggest an 800 A service. Of course, if you are thinking of future expansion, a 1200 A service could later be expanded by increasing the transformer.

The key question here is the size of the 208-120/3 portion of this system. Most likely you would use a stand-alone 480/3 x 208-120/3 transformer. (It needs air circulation around it, which would preclude putting it in the switchboard, unless it is rather small.)

The 208/1 and 120/1 loads can be taken from the 208/3 subsystem (the 3-phase 208 V loads would use 3-pole circuit breakers; the 1-phase 208 V loads would use 2-pole CBs, and the 1-phase 120 v loads would use 1-pole CBs).

Your 480/3 CB switchboard would include one CB to feed the primary of the 480 x 208-120 transformer, which would in turn feed a separate panel with CBs for the 208 V and 120 V loads.

The 480 and 208-120 circuit breaker panels could be integrated into a single enclosure, although I don't recall seeing such a layout myself.

Bern2009,

The other answer aims you in the right direction. I'll add:

• The two voltage levels need to be in separate panelboards. None are made that combine them into a single device, and as an electrician, I wouldn't want to be responsible for wiring such a combination (because of safety).
• Many plants will place the 480/277x208/120V transformer overhead above the panel or disconnect feeding it and with the panelboard it supplies directly below. This minimizes consumption of floor space.
• I'm surprised that you don't have an electrical engineer consult on even a preliminary design and cost estimate. He/she would be able to keep you from any serious errors at this early date.

I wish you well--John M.

i would add have you checked the diversity factor before zeroing on the Transformer size you have selected and what about backup generator requirement sizing?

good luck

crm

bern2009,

I don't understand how on earth can an architect now handle the electrical things.....

My suggestion is ,GO FIND AN E.ENG Because THIS IS NOT YOUR DOMAIN !!!! tq

i would appreciate at least he is asking instead of messing up every thing on his own as it happens now a days

crm

As mentioned in earlier posts,because of the cooling it would not be posible to enclose transformer internally, your choice of 600kVA transformer is sufficient but not ample should there be a requirement to increase the loading.

Cheers

Hi Bernie, and welcome to CR4.

With a 480V service, you would need a 277/480V to 120/208 3 phase transformer for the lower voltage loads. It's possible to build that into the 480V switchgear, but it would be a custom design. If you're in the US, it would require approval of the local inspection authority (usually the electrical inspector for the city or county Construction Services Dept.).

The 480V and 208V circuits would need to be in separate cubicles meeting NEMA and NEC requirements. For more information, I would recommend contacting switchgear manufacturers. I have used Eaton, Square D, Powell and Myers Controlled Power for custom designs with great success.

Thanks everyone... I have what I need.

To Tootje: Be assured that I intend to include an Electrical Engineer. I like to know (at least in concept) what's included in the project before I start. Owners have a tendancy to purchase (or budget ) an equipment package before they know their options. In this case, the vendor has proposed an equipment package. I'm just trying to understand what's required so I can relate to the owner, and maybe simplify things before I activate the design team.

Not an EE wannabe

Bernie

Hi Bernie,

Based on your lack description, let me try to give you my opinion

1. list down 480V/3P loads
2. lists of 208V/3P load equipments
3. lists of 110V/1P loads

in case 110V/1P loads are not so big, then you may put the 208V/110V step down transformer in a cubicle. In short ,what you need are :

• 1 ea switchgear with 1 incomings 800A ACB outgoings depends on how many loads plus CB to 480V/280V transformer & spares. (1200 A is too big & you have to invest with bigger dimension of cable)
• 1 ea 600 kVA 480V/208V transformer
• 1 ea ? kVA 208V/110V transformer
• as I stated above, if your 110V/1P loads are not significant then the 208V/110V step down transformer may be put in a separate cubicle in the switchgear, otherwise you have to pull cable from the switchgear to feed this Tf and
• you gotta provide another 110V distribution Panel with 1 incoming dependant with Tf capacity and several outgoings

thank you & may this can help.

Though an auto xformer wt multi tappings is neccessary for this project,wt custom built design into a SG Panel,we must consider deeply,sizing of d xformer for d 3 phase 480v and 3 phase 208v.If d 480v gives 721.709A,would not d 208v give 1665.48A?

In a 3 phase ac netwk under load,voltage and current are in inverse proportion.Are we using seperate xformers for d 480v and 208v 3 phase or single multi tapping? We need to consider deeply if using single multi tapped xformer can handle d load,and at what kva and current rating for d 480v and 208v,3 phase.Well,for 480v,3 phase,600kva xformer at 721,7A(800A) is ok,but what about 208V,3 phase?

Lets remind us as well that 2.2kw(230v) single phase motor or pump will draw a higher current than 2.2kw(400v) 3 phase motor or pump.Its all for our discussion and consumption.

Patrick Whowha