short circuit capacity of MCB

[incognito1]

how to choose a MCB from 3kA, 4.5kA MCB, 6kA MCB and 10kA MCB for domestic application? i have seen 10kA MCB are used for industrial applications. but why industrial applications need 10kA MCB instead of 3kA, 4.5kA MCB, 6kA MCB

what is the thumb rule of selecting short circuit capacity of a MCB.

 

[davidbeach]

Going way out on a limb here, but my guess is that if you look at manufacturer A who makes 3, 4.5, 6, and 10kA breakers and manufacturer B who only makes a 10kA breaker that manufacturer A's 10kA breaker is going to be more expensive than manufacturer B's 10kA breaker. A's 3 or 4.5 will probably be less than B's 10, but the cost of making multiple versions will push all of A's costs up.

I've seen, and continue to deal with, the results of people optimizing designs in the past to use just the bare minimum; in fact we paid contractors bonuses 20+ years ago to spend as little as possible on as tight a design as possible. The added O&M costs since then probably exceeded the first cost savings within a few years and we keep paying and paying. Useful projects get put off simply because a $10k project will require many times that in infrastructure upgrades simply to make the necessary space available.

If you're designing a manufactured product that's going to be produced thousands or millions of times then by all means make it as cost efficient as possible, optimize the heck out of it. Particularly if you aren't going to be responsible for ongoing maintenance of it. But if you're building something for the ages, the guys/gals that come after you to maintain it will curse your name for years to come if you optimize it to the nth degree and leave no wiggle room.

If you can use a 6kA today because you calculate 5.8kA and then the utility upgrades their system and then the value calculates out to 6.1kA, how much have you saved in the long run by installing 6kA breakers instead of 10kA? Not going to happen you say, the utility won't need to make that change; well something will change. Maybe the laws will change and indoor grow operations for personal medical or recreational use will become legal and the neighborhood power consumption goes up by 50% and the transformers all become overloaded and get replaced with larger transformers. Things happen and overly optimized designs become obsolete much quicker.

Having too many times come after the guy that picked just the "right" size/rating/layout over the years, I get a bit testy; "oh, not again". The original question, and some of the follow ups, remind me of the design "logic" that ended up with a single section 600A MCC (lowest vertical bus rating is 600A) with two size 1 starters and a 60A feed to the MCC. The other four cells in the MCC were forever useless because that 60A feed couldn't carry anything else. The owner then paid much more to get a couple of motors added than they would have if the feed to the MCC had been 200A originally. But 60A was a lower first cost. Bah.

 

[Mbrooke]

I agree, an excellent perspective (especially on cost being driven up on the high end to save on the low end), but the same could be said about the US. What happens when tankless heaters become all the rage? Soon that 25kva pole pig needs to be upgraded to 75kva and the LV secondaries following. Granted most residential service drops fed from typical over head lines do no come near 10ka so there is a margin that can be taken advantage of, but still in some places those 10ka breakers will become obsolete as well. Also keep in mind that a 5ka 240 volt breaker is about the same as a 10ka 240 volt breaker in extremely simplified terms. The voltage may be double, but the current will be half during a short circuit on 240/416Y transformer when compared to a 120/208Y of same impedance and source impedance.

This is a really handy paper imo:

http://www.alabamapower.com/busines...df/A E Fault Currents Tables FINAL 8 2003.pdf

 

[cuky2000]

The SC at the transformer bushing is significant damped by the LV service entrance feeder impedance. The standard practice of utility in the US is not to oversize distribution transformers for economic reason. Therefore, exceeding the SC in the US residential service at the interconnecting point it is an unusual event.
The majority of the residential services are single phase (split-phase) 120/240 volt system. The 240 V secondary winding is center-tapped and the center neutral wire grounded. Most service transformers are pole mounted a less quantity are pad mounted unit.
See below a typical SC damping effect on the service entrance feeder.

 

[Mbrooke]

IS the wire shown CU or AL? My apologies.

 

[cuky2000]

The author used Cu to develop the graphic above.
Below is another approximate curve for a single phase residential service 120-240 Volts based in 25kVA & 50 kVA with 1.4% and 1.8% impedances respectively.
The SC provided by pole mounted transformers is usually lower for the same kVA capacity since those transformers have larger impedances than the pad mounted units.

 

[cuky2000]

CORRECTION: The solid lines on the graphic above are for copper conductors.

 

[Mbrooke]

Thank you! This graph is worthy of a "sticky"

 

[waross]

Cucky, have you tried the new edit feature?
You may be able to correct the graph.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[incognito1]

can someone clarify why short circuit current rating getting decreased like this?

 

[waross]

At the transformer the transformer impedance limits the fault current.
At the service the impedance of the service conductors plus the transformer impedance limits the fault current.
At the branch panel the impedance of the branch circuit conductors plus the impedance of the service conductors plus the transformer impedance limits the fault current.
etc.

Bill
--------------------
"Why not the best?"
Jimmy Carter