Available Fault Current

N

NoFluxGiven

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Location
Clearwater FL
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Service Manager
We are installing a new service for a generator. The ATS we are trying to use for cost reasons has a current withstanding rating of 18,000 and is not SE rated. When I do the calculation, I show the available fault of 20,000 at the main disconnect. If I use a fused disconnect ahead of the ATS does that drop the available fault going to the ATS?
 
You need to plug in the data, wire sizes and length into a calculator to give you the final number.

I have the Eaton Bussman app.
 
You need to know the available fault current at the ATS, asmentioned by Dsg319. Wire is very good at reducing fault current.

Fuses can not be used to reduce fault current by making selections in the field. However, most ATS manufacturers do have UL tested series ratings between their devices and specific fuses. You can usually find these selection charts in the ATS literature.
 
NoFluxGiven said:
Doing some research I think I found my answer. View attachment 2581728
Click to expand...
This is misleading information.

You as an electrician in the field  cannot select fuses to protect UL Listed ATS, panelboards, disconnects, and other equipment.

The manufacturer can use the above posted information to select fuses prior to submitting the specific combination of equipment to a testing lab for third part testing.

This has been true for at least 40 years, but the fuse manufacturers rarely put this qualifying information in their literature,
 
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Yellow highlighted part still is a factor. Your equipment still needs to be series rated with the device in question. Is some possibility your equipment could already be series rated with "lesser" fuses though. But you need to look into the manufacturer data.

Other options are arranging things to create more impedance ahead your ATS to lower the available fault current at it's terminals. One simple thing can be somehow utilizing longer conductor lengths.
 
Fuses are considered "current-limiting" if they can clear a fault before the first peak of fault current occurs. If the available fault current is high enough the fuse will force the current to zero within a 1/2 cycle (max). Inserting a fuse ahead of something does not automatically reduce the fault current in all situations. If you look at the the TCC for a fuse and for a particular current it takes the fuse 5 cycles to clear the fault, it is not functioning as a current-limiting device at that fault current. That's why it needs to be a tested combination.

In this particular situation, it is likely that the added impedance of the wiring will reduce the fault current quite a bit. It will not take much cabling to get the fault current down from 20 kA to 18 kA at 240 V.
 
This is the important takeaway from that document:

When used as a main (line-side) device, the ECNR400 can allow downstream branch circuit breakers with lower interrupting ratings to be used, provided they are part of an approved series-rated combination.

Ignore the part about them limiting or reducing the available fault current.
 
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