Calcuate Transformer, Disconnect, & wire form enclosure circuit breaker?

[muckmail]

If I have a single panel that has 24 circuits in it. It is rated for 70A with
a 70A circuit breaker. The incoming line is 480 and the secondary is
240.

Would the transformer rating be 70(240)(1.73)/1.25(1000) = 23.25KVA?
Where 240 is the secondary voltage?
So I need at least a 23.25KVA transformer?

Is the above correct? If not, what am I doing wrong?
If this is correct and I have 480VAC going to the primary of that
transformer thur a disconnect switch. How do I find the size of
the disconnect switch or circuit breaker?

How to a calculate the wire sizes going thru the disc connect switch
to the transform then the secondary wires to the panel?

Thank you,

Thank you,

 

[augie47]

If I have a single panel that has 24 circuits in it. It is rated for 70A with
a 70A circuit breaker. The incoming line is 480 and the secondary is
240.

Would the transformer rating be 70(240)(1.73)/1.25(1000) = 23.25KVA?
Where 240 is the secondary voltage?
So I need at least a 23.25KVA transformer?,

Yes The next normal size would be a 30 kva

Is the above correct? If not, what am I doing wrong?
If this is correct and I have 480VAC going to the primary of that
transformer thur a disconnect switch. How do I find the size of
the disconnect switch or circuit breaker?
,

You size the breaker to protect the conductors

How to a calculate the wire sizes going thru the disc connect switch
to the transform then the secondary wires to the panel?

Thank you,

Thank you,

Determine the load in KVA and select your transformer.
Calculate the primary & secondary current and
Using 450.3(B) determine the size of the primary and secondary OCP
Using 310,16 select your conductors
Check 240.21(C) for appropriate rules,

You already have your secondary selected at 70 amps.
If you used a 30 kva transf. the 480 current would be 36 amps
450,3 would allow you to go to as high as a 90. Select which OCP you want and size wire accordingly
Be sure to check 240.21(C) for tap rukes

 

[muckmail]

If a 25KVA transformer can be found would that be allowable? I think Cutler Hammer
makes one and they are planning on using it. Also, I am new at this so what does OCP mean?

Thank you,

 

[augie47]

You really should calculate your actual load. Your calculation included a 1.25 factor which, from 450.3, allows you to use a 70 amp breaker on a
23 kva load.
There are so many factors involved, it's difficult to say.
Your op calculations indicate 3 phase, but a 3 phase 240/120 transformer has severe neutral limitation (5%).
Is your secondary going to be 3 phase 240/120 or 208Y/120 or is it actually single phase (25 kva is normal for single phase, not 3 phase).
In any event you have to be careful about load distribution (amps per phase)
You really need to start by calculating your load...single phase 120, single phase (240 or 208) and three phase. Look at the load per phase to make sure your transformer is sized properly.

OCP is overcurrent protection.

 

[muckmail]

I think the transformer to be used is a Cutler Hammer G49G1125SS. I have
found a data sheet that has a 248G11S25N but not G49G1125SS. It has
Single-Phase written on the top of the sheet so I guess the transformer has
single phase. Is there another way to idenify a 3 phase transformer from a single phase
transformer? I guess a single phase transformer has the 480VAC on the primary
that is 3 phase and the secondary lines are single phase with 2 hots and a neutral?
A 3 phase transformer would have three phase 480VAC on the primary and 3
hots and a newtral on the secondary side. Is that correct?

On the transformer I have the secondary side have 240/120VAC
I will be able to provide 240VAC from a 2 pole CB or 120VAC for
a 1 pole circuit breaker.


Thank you,

 

[augie47]

I've done my best, but it appears to me you are way over your head.
It appears your load is single phase but your calculations are 3 phase.
I will yield to another poster.