Let thru current

[hhsting]

I have available fault current of 60kA. I have engineer that tells me he will provide 100kA fuse but the downstream panelboards 10 feet away he will provide 22kA because manufacturer install instruction says let through current of the fuse is 14kA. See attached let thru current. The fuse is 600A 3 poles JM60600-3CR.

Would 22kA be sufficient or he should be providing 100kA?

 

[jim dungar]

Your engineer is wrong. Even the manufacturer says this chart is not to be used to protect circuit breakers with fuses. They have been saying this for more than 40 years.

What your engineer is trying to accomplish is to Series Rate a fuse and a circuit breaker. The NEC requires series ratings to be tested and not field derived.

You need to select your fuse-breaker combination using charts from the breaker manufacturer.

 

[hhsting]

Your engineer is wrong. Even the manufacturer says this chart is not to be used to protect circuit breakers with fuses. They have been saying this for more than 40 years.

What your engineer is trying to accomplish is to Series Rate a fuse and a circuit breaker. The NEC requires series ratings to be tested and not field derived.

You need to select your fuse-breaker combination using charts from the breaker manufacturer.

I see so it has to say so and so fuse with downstream so an so breaker is series rated. I have fuse then downstream of that panel 1 and downstream of that panel 2.

If manufacturer data indicates fuse is series rated with breaker used in panel 2 would panel 2 be series rated or not? Its down downstream of the fuse?

 

[augie47]

Yes.

Here is a link to SquareD Series Rating charts you might find helpful

Short-circuit current rating (SCCR) and series rating information for panelboards and load centers | Schneider Electric USA

 

[hhsting]

Yes.

Here is a link to SquareD Series Rating charts you might find helpful

Short-circuit current rating (SCCR) and series rating information for panelboards and load centers | Schneider Electric USA

But I dont see where manufacturer install instruction it says not to you fuse let thru current chart posted post #1?

 

[hhsting]

Your engineer is wrong. Even the manufacturer says this chart is not to be used to protect circuit breakers with fuses. They have been saying this for more than 40 years.

What your engineer is trying to accomplish is to Series Rate a fuse and a circuit breaker. The NEC requires series ratings to be tested and not field derived.

You need to select your fuse-breaker combination using charts from the breaker manufacturer.

Engineer is saying if the fuse lets through 14kA fault current then series rating is not required. Max fault current downstream would be 14kA. I just dont see manufacturer instruction it cannot be used as what the engineer says??

 

[jim dungar]

Engineer is saying if the fuse lets through 14kA fault current then series rating is not required. Max fault current downstream would be 14kA. I just dont see manufacturer instruction it cannot be used as what the engineer says??

The NEC prohibits it, in 240.86
UL listing prohibits it.

I don't know the exact literature your manufacturers chart is from, by my 2008 version of Bussman's SPD catalog said fuse-breaker series combinations needed to be tested. Try searching your for that term 'series combination'.

 

[don_resqcapt19]

But I dont see where manufacturer install instruction it says not to you fuse let thru current chart posted post #1?

The chart tells you what fuses can be used with what breaker to have a listed series rating. You can only use the listed combinations.

For example in Table 1, page 4 a 3 pole Type FA breaker has a kAIC of 18, but when combined with an upstream Type J fuse, the series rating for the breaker is 200 kAIC. Table 4, page 8.
Every breaker manufacturer has these types of tables so you can find the listed fuse breaker series rated combinations.

 

[ron]

240.86 is your only opportunity to exceed the rating of the downstream circuit breakers.
The requirements in 110.9 and 110.10 say to only use equipment within it ratings. If you can't comply with 240.86 (tested combinations), then you must be sure the breakers are rated for the available fault current, not some hypothetical reduction of short circuit current from a CLF.
This is even for overcurrent >1000V, as there is no leniency for a fault limiter of some sort.

 

[Jraef]

But I dont see where manufacturer install instruction it says not to you fuse let thru current chart posted post #1?

It doesn't need to.

IF the circuit breakers have been TESTED AND LISTED by the breaker manufacturer to have a Series Rating with specific fuses, then that's all you need to know. If it has not, then the let-through of the fuse is irrelevant as far as trying to "create" your own series combination. That chart is useful to the BREAKER MANUFACTURER in pre-determining the likelihood of a series listing test to pass, which is helpful because that is VERY expensive. But that chart is basically useless for this purpose in the field.