LT panel ratings

It can not be assumed that the outgoing circuits will be using 100% of the amp rating of the CB's.

What is used to determine the size of the fusing or circuit breaker is the wire size of the circuit, not the amp load. It is the amp load that is used to determine the wire size, which is usually not expected to carry 100% of that capability either.

How many out going circuits will be determined by the loads and their locations.

The main issue is that the expected loads in amps, when added together, do not exceed 80% of the 400 amps that the panel can provide safely. This is not an addition of the circuit breaker values, but the addition of the actual measured (or estimated in advance for design purposes) loads that are expected to run at the same time.

As an example; if one man has a workshop with 100 machines with 20 amp circuits each that collectively could draw 2000 amps of connected load, but being only one man he could run only one at a time, then he would only draw around an average of 10 amps at a time.

If then this same shop regularly had 10 persons working machines, the usual expected amp load could be 1000 amps or at times higher.

This is the type of information that determines the size of the main panel and how many feeders will be safe, with the goal of using only 80% of the available power at any one time.

The percentages stated will vary by location on the globe, local code restrictions etc, but the principle will remain the same. It is the actual load that matters, not the numbers on the outgoing CB's.

it dosent matter how much feeder you add but the problem starts when u try to run all those loads connected on same feeder at atime

as u told u have a 400 A incomer with 10 nos of 63 A outers , but if u are not going to use all the feeders at atime then there is no problem with your feederprovided that ur load must not exceed than 85% of ur incomer rating

and current loading/carring capacity is not based on the incomer rating but on the size of busbars and cables u used in ur pannel

try to give 125 A incomeres for each 3 nos of 63 A outers if u can ,that will be more realible.

if u wanna thanka pray for megoodby

Hi Guest

As per your view since there are 10 numbers of 63 A out going feeders the incomer should be minimum 630 amps. Theoretically it is correct. but in actual design due to economic reasons you will consider the diversity factor. All the 63 amps feeders are not loaded to the maximum and all load will not be functional all the time. You consider the peak load ,average load and connected load for each feeder to arrive at the actual loading on each 63 A outgoing feeder and actual load requirement of the incomer. So after calculating total connected load you can decide weather 400 A incomer is sufficient . If the LT panel is functional check the peak current with the help of tong tester for all outgoing feeders and incomer.

The same process is applicable for panel 2 also..

Dear sir,

Thank you for the reply. I was eagerly waiting for your reply. You have always given good replies in almost all the posts.

If you can elaborate more on the 'diversity factor' , it will be helpful

Regards

Prashanth

Hi Prashanth,

Thanks ,I feel very happy when the members or Guests who initiates the thread reads the reply and respond. It gives me satisfaction that the time I spent in replying has not gone waste.

While designing the capacity of LT panel it is essential to study the type of the load to be connected to the panel through various outgoing circuits. There are furnace load and lighting load which is uniform, the load of press or shearing machine will be non-uniform. The normal shop floor machines like lathe, milling, grinding machines will be normally uniform except while starting. Some machines will be regularly used some may be idle for long time depending on functional requirement. All the machines will not be operating simultaneously.

So the capacity of outgoing cable and switches fuses and circuit breakers are designed depending on above considerations and factor of safety. While selecting the capacity of bus bar and incoming breakers the total functional average load , peak load and connected load are considered. The functional requirement of the incomer or outgoing breakers are to"switch off" the system before dangerous load more than rated capacity is drawn. In case if such over current is allowed there will be overheating of bus bar and cable. So the rating will depend on rated load and short circuit rating. The panel design is to be done with clear understanding of load.

Thank you sir for quenching the thirst of knowledge.

Regards

Prashanth

I am in total agreement.Diversity factor that is the most important aspect.

Incoming rating of MCC Switch design baisis is connected load.

1.How much your connecting load.

2.How many feeders & rating of each feeders .

PL tell these answer than we decide the incomning rating of switch.

Regards