Transformer Mcb

Yes, you have to watch for inrush current. You have a total of 4800 VA worth of transformers. I don't know your Voltage, but let's say you have 240VAC. The maximum operating current would be 4800 VA/240V = 20 A. and you might think that, for example a 30 A breaker and wire would be OK. But, if you turn on all the transformers at once (or, worse, if you use the breaker as an on/off switch), the inrush might easily go to 60 A or more. Breakers used as high current switches can develop rough arc spots and later malfunction.

I recommend getting a licensed electrician to see what your local code says about transformer loads and derating. But, I'd above all put on/off switches on the transformers.

As TVP45 says, you do need to accommodate the inrush current.

Here in New Zealand, we would normally use a D-curve MCB for an application such as this. D-curve breakers are designed for applications with significant inrush current, and the magnetic (instantaneous) trip is normally set to 14 x In, where the more common C-curve trips magnetically at 8.5 x In.

When we acquired a variac for our test bench, it would trip the MCB protecting the bench most times when plugged in. A D-curve MCB eliminated the problem with no loss of protection.

The Breaker for 400VA for each segment would be 400VA+25% of total rating which would be 500VA.But the next disconnect would be 500VA+400VA+25%of power to give the next rating of the next disconnect box 1000VA. Next disconnect =1000 VA + 400VA + 25%=1500Va for next disconnect, with a rating +25% more power + 400VA+25% of that .It multiplies, but as long as you have the following disconnect rated for the added power increase + 25% of box rating, there should be no problems.Just make sure your wire rating for such power is increased along with the rating of the disconect boxes increased by 25%.Hope this helps in your delima? Keith_moc, Industrial Electrician/Retired P.S. Also check on the updated codebook for this year.It's updated every 3 years.As always BE CAREFUL WITH VERRY HIGH VOLTAGE!!! It will instantly kill, you as you will be the ground.Keep power off untill you get an Electrical Inspection done, but you dont need power untill work is finished....later..Keith_moc@yahoo.com

One important issue is you must also have to select the breaking capacity of a circuit breaker according to short circuit current values of all transformers.

HI,

Use the following formula "Breaker I= (VA / V) *1.25" , this will put you in the ballpark buy the closest one off the shelf.

Because the transformer is a inductive device with inrush you will need a breaker with a transformer time delay (slow trip characteristic). The breaker MFG can recommend the proper line.

simple take a thumb rule of finding out of breakers current rating

1.5 times the kva

i.e 400 x 1.5 = 600 aproximately equals to 630 amps

so for 400 VA transformer we required 630 amp of a breaker

moreover for inrush of currents you have to select the maximum setting of the thermal so as to overcome the instatanious tripping of the breaker.