30 kva transformer?

[mav01]

Hello everyone!! I just join this forum and it seem a very good one!!

Any way I have a dilema...I am designing a power distribution panel wich will feed a few 3p 480v motors and a few single phase 120v motors.

I need to a transformer for the 120v motors my FLA is 133.82 amps my calculation for the 120v transformer is

kva=(V*Am*1.732)/1000

(120*133.82*1.732)/1000=27.8kva, so I went for the 30kva dry-type transformer general purpose.

can I get a transformer 240/120 volts so i can divide the load into two legs A=120v and B=120v so i can have tree phase 480v to 240v transformer.

and can I use 50 amp primary fuse and
two single phase 50 amp on the secondary?

can anyone help me?

thanks for all your help!!!

mav

 

[infinity]

Since your loads are only 120 volts why not use a 480-208Y/120 volt transformer? This would allow you to closely balance your 120 volt loads across all three phases.

 

[mav01]

thank you for your advice but if i go with 208y/120v then i have to use a bigger secondary circuit breaker...125amp. I guess what I am trying to to do is use a small circuit breaker tree legs one to neutral and two 120v...

any advice would be appreciated!!


the transformer is big enough for the fla?

 

[charlie b]

kva=(V*Am*1.732)/1000

(120*133.82*1.732)/1000=27.8kva, so I went for the 30kva dry-type transformer general purpose.

You have the formula wrong. The 120 volt motors are all single phase. So you get the KVA for each by multiplying by 120, and dividing by 1000. The factor of the square root of three (1.732) does not enter the picture at that point.

Your transformer load is, (120*133.82)/1000=16.1 KVA

Now if you want to find out how much current is on each leg of a well-balanced secondary, then you are talking about three phases, and the 1.732 does come into play.

Presuming you do choose a 120/208V secondary, then,
Current = 16.1 KVA / (208 volts times 1.732) = 44.6 amps.

You can protect the secondary with a breaker rated 125% of this value, and even take it up to the next higher standard size. 125% of 44.6 is 55.7, so I would go with a 60 amp breaker.

 

[mav01]

Hello charlie b, thank you for your reply...now I am confused.

if i use a 3p transformer for the primary line, on the secondary will be single phase?

see the incoming is 3p 480v line incoming to 120v out...all the single phase motors and plc that i am powering are 120v so, I heard that i was able to use a 3p transformer to 240/120v is this true? have tree wires on the secondary one neutral and balance two legs at 120v 104amps.
is this right?

thanks for all your help!!

 

[charlie b]

A 240/120 is confusing to me as well. I don't think I can solve that for you.

But for a 120/208 system, each 120 volt load on the secondary side gets added as a single phase load. Use 120 times the current, and don't use the 1.732. When you add them all up, then you use the total KVA to figure out the current on each leg.

Someone else is going to have to finish the story for you.

 

[Smart $]

if i use a 3p transformer for the primary line, on the secondary will be single phase?

...so, I heard that i was able to use a 3p transformer to 240/120v is this true? have tree wires on the secondary one neutral and balance two legs at 120v 104amps.
is this right?

A 3? transformer with a 480V primary and a 240/120 secondary has a 3? secondary and known as a delta-delta transformation. A-B, B-C, and A-C line to line voltages would all be 240. Only A-N and C-N would be 120V. If all your loads are 120V, you'd not be utilizing two-thirds of the transformer to it's full potential (i.e. wasted $$$).

What you are after, seemingly, is a transformer with a 480V 1? primary and a 120/240 1? 3W secondary. Though this would provide what you are looking for, it will lightly unbalance your 480V distribution lines somewhere between 86.6% and 100% of the 1? load. The unbalance will occur on two of the three 480V lines.

133.8A ? 120V ? 480V x .866 to 1.000 = 28.9 to 33.5A​

This amount of current is in addition to the 480V loads current on those two lines, of which we have no idea what that may be (i.e. you have not told us the level of 480V 3? loading on the distribution panel).

Additionally, your secondary line current will be higher:

133.8A ? 120V ? 240V = 66.9A/line, if ideally balanced​

While this is not an entirely unworkable scenario, IMO the 208Y/120 3? 4W solution is preferred in an industrial environment (unless one can foresee adding 240V loads in the future).

 

[mav01]

Thank you all for your input I am very impress of nice people in this forum.

I am going to follow your advice smart $. I explain to my boss that using 3p 480v to 240v is a wasted money for the reazon you explained, and he agreed with that, so my transformer will be a 30kva 3p 208y/120v 4w.

just for the record that was one my original proposed just single phase 480v to 120v.

thank you smart $.

best regards.