Main Breaker Computation Needed

Depends on what is in the circuit that is downstream. Any Motors? Are all of the circuits and devices individually protected? Transformers?

yes sir the devices are individually protected. In a certain panel, we have motors in it and on the other panels are molding machines (35KVA), press machines etc..

Example:

ckt 1 to ckt 8 are molding machines with 35 KVA at 3P and 200V. the demand factor is 70% . what should be the main breaker to be use?

Thank you for the reply...

In the case that each individual branch coming off the main disconnect is protected, I fuse (CB in your case) for the size of the disconnect. Otherwise I'd have to examine the entire system to see what kind of in-rush current I can expect. Unfortunately, its not as easy as a simple percentage.

thank u sir fro the response.. can u give e a sample for the computationof a certain panel board with 8 circuits containing molding machines with 35KVA,3P,200V with demand factor of 70%?

What should be the main breaker for the panel?

As you seem to be working with an existing system, I would suggest
that you size the Main C.B. to protect the existing circuit.

(Thats the smallest unprotected element of same.)

You appear to have enough diversity to make inrush currents
a negligable factor unless there is one very large load or
a simultaneous start-up situation.

If your load is primarily motors, consider a time delay (K-5 or so) fuse
instead of a breaker. The interrupting current/price of unit is better
and a properly sized main shouldn't open unless you've goofed somewhere.

To properly calculate the size of a new service....

If you have to ask, you shouldn't be doing it but:

Take all your connected loads.
Apply a demand factor to each.
- (Judgement required. Note: Demand Load is NOT Service Factor)
Total the demand loads.
Add 125% of the largest starting current.
Add some room for expansion.
Total.

For interrupting rating required consult your utility for
avalable short circuit current. - Take the next largest
sized breaker rating, (unless its close, you do have motors).
If it's too close for comfort go up a notch.

This is c3p1124...

thank u sir i really appreciate ur reponse. That would help me..

God Bless..