Fault current

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acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
I looked at a job recently where, the main Fused disconnect needs replacing. There is 10 meters after the disconnect. The utility fault current is 22K, I believe this is why there was fuses, and not a main circuit breaker, current limiting fuses. The EC that is going to do the job wants to put in a main breaker with a 22K rating, all the equipment down stream of the main is old and has a fault current rating of 10K or less. The downstream equipment is fed off a gutter 19ft long.
I say this is not allowed with out an EE signing off on it. Any argument for or against this stand?
 

ron

Senior Member
If he is replacing the main with a fused switch or a circuit breaker, as long as it is properly rated for the available fault current, it is okay. The downstream equipment is a different story. Was it installed with the existing upstream fuse as a series rated assembly?
The new main breaker or fuse may have a series rating with the downstream equipment, the fact that there is there is fused main doesn't mean that the downstream equipment is properly rated today.
The code and equipment listings are very clear. Although an engineer may help you work out the technicalities, an electrician can do it on their own if the ahj permits him to do so for a permit.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
I agree with Ron.

The downstream breakers would either need to be rated for the avaliable fault current or be series rated with the upstream device.

Check out 240.86 for series rating of circuit breakers.

Also make sure that you calculate the avaliable fault current at the downstream equipment, just bnecause the utility avaliable fault current is 22k does not mean that that is what is avaliable at the downstream equipment.

Chris
 

raider1

Senior Member
Staff member
Location
Logan, Utah
240.86(C) seems to spell out your options.

Which edition of the NEC are you using?

240.86(C) for the 2005, 2008 and 2011 NEC deals with motor contribution for series rating.

240.86(A) and (B) are the two options for series rating.

240.86(A) is for engineering supervision for existing installations and 240.86(B) is for tested combinations.

Chris
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
This system has mostly fused disconnects down stream of the main disconnect. It is about 55 years old. The reason the contractor wants to put in a main breaker is because the size of a new fused disconnect will be much larger then the existing fused disconnect and it will not fit.
I don't see the existing equipment working in "series rating" unless there is something I am not seeing that would lower the fault current at the downstream equipment.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Which edition of the NEC are you using?

240.86(C) for the 2005, 2008 and 2011 NEC deals with motor contribution for series rating.

240.86(A) and (B) are the two options for series rating.

240.86(A) is for engineering supervision for existing installations and 240.86(B) is for tested combinations.

Chris

The "C" slipped in somehow :)

This system has mostly fused disconnects down stream of the main disconnect. It is about 55 years old. The reason the contractor wants to put in a main breaker is because the size of a new fused disconnect will be much larger then the existing fused disconnect and it will not fit.
I don't see the existing equipment working in "series rating" unless there is something I am not seeing that would lower the fault current at the downstream equipment.

I agree. The breaker might work with the other breakers (series rated) but possibly not address all issues. That was behind my reference to 240.86, if you change to a breaker it seems engineering would be in order.
I, of course, could be mistaken.
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
I am just concerned that cost and not safety is driving this installation, and that really upsets me. There is no excuse for putting people or property at risk, just to save a buck.
 
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