My existing condition:
45KVA 480V primary with 60A fused protection and 208/120 secondary with 125A CB protection

maximum load that i measured on secondary:
A= 78Amps
B= 64Amps
C= 64Amps

Under these conditions the 125Amp breaker has tripped twice. my first question why would the 125A trip? worn out/Tired breaker?

if i am interpreting table 450.3B correctly, i can install a secondary OCPD 160A that is at maximum 125% of the transformer secondary rated current? Am i correct?

Actually since 160 is not a standard OCPD size you are permitted to use a 175 amp breaker. However, you must protect the secondary conductors at or below their rated ampacity, so changing the secondary breaker may also require you to change the secondary conductors.

As far as why it is tripping there could be some type of intermittent load that is high enough to cause the breaker to trip. Maybe an intermittent fault. Or if could be a bad breaker.

A 200A frame breaker is tested, by UL, for more than 100 operations, so it is extremely improbable that it is 'worn out'. Poor terminations/connections causing excess heating is a more likely issue.

Did the breaker trip under the same conditions:
Time of day?
Temperature in room?
Same loads turning on/off?

Changing the secondary conductors would be very easy. So i guess i did interpret the table correctly.


Time of the day i don't know, but the load is fairly constant throughout the day. And the temperature of the utility room i would (guess) to be around 70F.

When i was feeling for heat on the 125A breaker i noticed the upper side of the 125Amp breaker (A phase) was very hot compare to the C phase. So it could be a bad connection inside the breaker.

Thank you for your input.

You should think about getting a non-contact thermometer. They are pretty cheap at one of those "China outlet tool stores" like Northern or Harbor Freight. The actual temperature is not as important as the difference when you are looking for loose connections and these work great for that although the $30-$40 one I have seems pretty accurate.
You can quickly scan a panel and locate anything that is running hot with a reasonable degree of accuracy about "how hot".

Conductors to the Secondary C/B follow 'tap' rules.

Hard to believe they are undersized.

Look for OVER TORQUED -- hence stripped lugs on dependent C/B...

As a foreman, I saw that again and again.

Even j-men skip the torque wrench and lay on with a will.

Hence, broken lugs -- loose connections -- hot spots -- trips.

I got to the point that I had to personally intervene, time and again. Clown after clown figures more torque is better. That they've managed to CRACK the lug -- Well, that's a secret best kept from the boss.

Just another reason I started megging everything in sight.

I just purchased a thermometer (per Greg's Suggestion, Thanks Greg) and I will check for temperature on the A phase and compare it with the C side.

I'm with Tesla on the importance of torquing connections. Too much is as bad as too little. Even if you've stopped short of physical damage to the lug, you may have reduced the cross section of the conductor or stressed the connector to the point where thermal cycling will finish it off.

The majority of installers do not get to see the results of their work several years down the line and have no reason to believe their installation technique is a problem.

A recent experiment/study showed that about 80% of electricians and non-electricians could not tighten a connection within +/- 20% of the required value when not using a torque wrench.

I agree with the torquing and I torque all of connections.

I will be checking for temperature today just to see the difference between the phases. However, I have purchased a replacement breaker.

Once you start playing with that thermometer I think you will be surprised how handy it can be.
Knowing how hot something is running can tell you a lot once you see what they run normally.

I made all of my guys use not only torque wrenches, but we had the torque screwdrivers when we installed a new service. I was getting tired of my guys snapping the lugs on the service meter.

After some struggle I was able to take the breaker OFF and it seems like because of over torque the mounting bolt was striped and the bus bar was damaged which in turn created heat and breaker tripped. (at least that is what it looks like).
So for now I took the breaker OFF and need to track down a hardware kit to be able to mount the new breaker.

By reading the code in detail, i got myself confused:

Does a panel board need a main disconnect if it is within 10' of the transformer?
45 KVA 480 primary with 60A fuses, and 208-120 secondary with 1AWG conductors

I was speaking with a fellow contractor and he said "absolutely required" but when read 240.21 and 450.3 i get confused.

Thanks

IIRC...

You can escape the (master) OCPD in such a situation...

BUT...

You're back to a limit of SIX OCPDs in the panel board.

This last little item effectively means that everyone goes with transformer tap protection, and with the size of secondaries that you're running even 25 feet can be allowed.

Study tap rules and Separately Derived Systems.

Normally a 45kVA three phase will feed a bus protected by 125A OCPD -- or less.

Also study up on exception permitting location of said XFMR ABOVE a 'soft lid.'

The panelboard main device is answer by article 408, not 450 or 240.

450 deals with protecting the transformer.
240 deals with protecting the conductor between the transformer and the panel.

Thank you all for your replies.

The more i learn the more i know how much i didn't know.