ECN Forum Forums General Topics of the Trade General Discussion Area New Transformers/New Short Circuit Current

The utility has already said that they will not put in different transformers. Mostly because if they have to replace, the low impedance transformers are what they will have availible "on the shelf".
As for extra service entrance length - this is only feasible if we install a pad mount transformer outside instead of replacing the inside transformers which is not that bad of an option - BUT what would be the cost of 200 feet of a 2500 amp (or so) secondary. And we would still have a 50 year old switchgear.

Replace the switchgear.

Ron
EXACTLY!

With the parameters set by the OP, it really seems like a new gear is going to be installed.

I see a few options from an installers view.

1) Replace the gear, continue as planed.

2)Do not touch the gear or the transformers, have the power co bring in a second service with it's own transformer to power the addition. (AHJ can allow it)

3)Replace the old 1600 amp gear with a new 2500 amp gear and feed both buildings from this new gear.

4)Bring the service to the addition and feed the original 50 year old gear from the additions service. This would add impedance to the circuit ahead of the old gear.

Anyway you want to go we would be happy to do the work, lots of gear and copper.

Bob

Changing the switchgear has always been the number one option. I am just looking for any lower cost option so I can tell the owner that we looked for lower cost options. The switchgear has the 1600A MCB plus about 12 200A CBs and a few smaller CBs.
Any option that cost nearly the cost of replacing the switchgear is (IMO) not really an option as it they would still have the 50 year old switch gear.
So far the options are:
1. Replace the switchgear.
2. Current limiting fuses before the switchgear if it can be shown that the fuses will limit the let through current to less than 45K at any fault current over 45K up to the available. (I am still not sure this is an option)
3. As mentioned above, a separate service for the addition. I wonder if the utility would install a second set of transformers for the addition in the same location instead of replacing the existing with larger?

I apologize beforehand if my question is awfully stupid, but is it not true that the circuit let through current limit will be 50000 amps (the one of the main breaker, as stated by “cgw”)?

Why is important to consider the transformer short circuit current if it is higher than the breaker one?

Regards,

Joe.-

Joe,

The deal is that the "50,000 Amp" short-circuit rating of the breaker does not mean that it will limit the current to 50,000 Amps. Rather, it means that the breaker is designed to reliably open and quench the resulting arc when up to 50,000 amps is flowing through it. If there is more than 50,000 amps, the breaker may fail to open. It might never be able to quench the arc, or it might fuse solid in the closed position, or might fail through any number of other failure mechanisms.



[This message has been edited by SolarPowered (edited 04-17-2006).]

Which is a very gentle way of saying it could flame out big time or more likely explode, damaging the buss work and causing further chaos!!

I've seen in person the end result of this, it's not pretty!!

Somewhere here on ECN (I think it was the Photos or Violation forums..) there was a picture of what that end result looked like.
(IIRC the thread was titled "I wonder what he's thinking?")