current limiting fuses

[jared albrecht]

I am being told that if i have a panel that has 10k AIC rated breakers and by calculation i have 30kA short circuit at the panel, that i can install current limiting fuses ahead of the panel to effectively lower the calculated short circuit to under 10k aic. i cannot find anything in support of this. Has anyone heard of this? Ideas?

 

[augie47]

See 110.22.
Under engineering supervision or a Listed (mfg) system it is possible.
Most common in by use of factory listings for specific breakers and specific fuses.

 

[jim dungar]

You must contact your breaker manufacturer to see what UL listed series ratings they have. Do not go to a fuse manufacturer.

Most breaker manufacturers have their series ratings on-line.

 

[Carultch]

I am being told that if i have a panel that has 10k AIC rated breakers and by calculation i have 30kA short circuit at the panel, that i can install current limiting fuses ahead of the panel to effectively lower the calculated short circuit to under 10k aic. i cannot find anything in support of this. Has anyone heard of this? Ideas?

Not just any current-limiting fuse, but a class of current-limiting fuse that is a listed series combination with the brand-specific and product-family specific breaker in question, will allow you to take credit for the fuse protecting the breaker, and the breaker can be installed at a location where the fault current would otherwise be in excess of its KAIC rating. The breaker manufacturers will show series combination ratings between models of breakers in their own catalogs, as well as with fuse classes in general. The fuse class can come from any brand, provided it has that particular UL class specified for the breaker. It is rare (if ever) that breakers are series-rated with breakers of other brands, but the fuse class series ratings are fuse-brand agnostic.

An example of a series-rated combination, is Square D QO and QOB breakers (10 KAIC rated), series-rated with class J fuses (200 KA rated).

 

[macmikeman]

Gee, aren't all fuses "current limiting"?

 

[augie47]

Gee, aren't all fuses "current limiting"?

Yep, but you have to be fast to wear the title

 

[Jraef]

See 110.22.
Under engineering supervision or a Listed (mfg) system it is possible.
Most common in by use of factory listings for specific breakers and specific fuses.

When you go to investigate this, you may also discover that you can get a higher rating on the panel by simply replacing the main breaker. Some Square D QO panels for example can get a 42kA Series rating with a QOH main breaker and standard 10kA QO branch breakers.

 

[petersonra]

I am being told that if i have a panel that has 10k AIC rated breakers and by calculation i have 30kA short circuit at the panel, that i can install current limiting fuses ahead of the panel to effectively lower the calculated short circuit to under 10k aic. i cannot find anything in support of this. Has anyone heard of this? Ideas?

It does not work this way. What you sometimes can do is get a series rating of a current limiting fuse in series with a particular breaker that is a higher AIC rating than the breaker would be by itself. It does not change the short-circuit current available at the panel at all.