Fused or non-fused disconnect and fault currents

[Platinum]

I have a 480V 3phase ckt feeding a 20hp AHU with a VFD. Both the panel and the AHU are in the same mechanical room. On my drawing, I show a non-fused disconnect feeding the VFD so that there is a local disconnect available. Contractor comes back and says the VFD they purchased has an integral disconnect, so that is covered.

My question is: should i have placed a fused disconnect in front of the VFD instead of non-fused? My concern is fault current protection. I know some of these VFD's I've seen have only a 5kAIC rating.

I know we do the fault current calculations to provide panels and breakers with adequate AIC ratings....but what about end devices and their AIC ratings? They require protection too?

 

[scotteng]

If the panel is within sight of the equipment, the breaker in the panel can be used as the disconnecting means and thus the disconnect can be eliminated all together.

Using fusing in a disconnect has no recognized effect on the fault current that can be delivered to the connected equipment. The NEC does not recognize the current limiting effects of fuses (up over and down).

IMHO, your VFD should have a withstand rating in excess of the available fault current anticipated, with the calculation bases solely on the conductors and type of conduit used.

 

[cpal]

Using fusing in a disconnect has no recognized effect on the fault current that can be delivered to the connected equipment. The NEC does not recognize the current limiting effects of fuses (up over and down).

QUOTE]


I am very interested in this comment, but I am not a EE
So my question is if the ocpd is not recognized by the NEC how does one deal with 110.10??

 

[jim dungar]

I know we do the fault current calculations to provide panels and breakers with adequate AIC ratings....but what about end devices and their AIC ratings? They require protection too?

Amps Interrupting Capacity (AIC) is only applicable to protective devices.
Every thing else gets an SCCR or a withstand rating.

Yes, per NEC 110.10 everything must be applied correctly based on its available fault current.

 

[scotteng]

CPAL, I meant to say that the NEC does not recognize the current limiting effects of fuses (or other OCPDs) to be considered as a means for reducing available fault current let-thru those OCPDs. The available fault current in a system is a function of the starting fault current, the conductor type/size/length, etc, and the raceway in which they are installed. The characteristics of overcurrent devices along the way have no bearing in the calculations.

The NEC does of course recognize series combination ratings for protective devices, but this would have no affect on the end piece of connected equipment, the VFD in this case. So the available fault current at the VFD will be identical regardless of whether a disconnect if fused or not.

 

[cpal]

CPAL, I meant to say that the NEC does not recognize the current limiting effects of fuses (or other OCPDs) to be considered as a means for reducing available fault current let-thru those OCPDs. The available fault current in a system is a function of the starting fault current, the conductor type/size/length, etc, and the raceway in which they are installed. The characteristics of overcurrent devices along the way have no bearing in the calculations.

The NEC does of course recognize series combination ratings for protective devices, but this would have no affect on the end piece of connected equipment, the VFD in this case. So the available fault current at the VFD will be identical regardless of whether a disconnect if fused or not.

I'm am not aware of a method to calculate fault current within the NEC. That is a engineering requirment. As I understand it the fault current to the furthest point of a distribution system will decrease for a given conductor length, configuration,and wiring method. Are you saying that the peak let thru is not assesed in determining a point to point sccr evaluation ( is the entire prmisi wiring system requyired to be fully braced for the available fault at the service point??).

 

[jim dungar]

Are you saying that the peak let thru is not assesed in determining a point to point sccr evaluation ( is the entire prmisi wiring system requyired to be fully braced for the available fault at the service point??).

Fault current is reduced by the impedance of the system. The fault current available at the end of a 250' run of #12 conductor will be lower than at the beginning.

The term 'peak let-thru' is associated with a protective device operating, not with the reduction in fault current due to conductor/equipment impedance. Your use of this term maybe part of the confusion.

 

[cpal]

Fault current is reduced by the impedance of the system. The fault current available at the end of a 250' run of #12 conductor will be lower than at the beginning.

The term 'peak let-thru' is associated with a protective device operating, not with the reduction in fault current due to conductor/equipment impedance. Your use of this term maybe part of the confusion.

Thanks Jim
if you get a chance to answer this I will check when I get home

I guess the question is, does the selection of a current limiting ocpd reduce the fault current thru out a structure or is it only reduced by the impedence of the system.

 

[jim dungar]

I guess the question is, does the selection of a current limiting ocpd reduce the fault current thru out a structure or is it only reduced by the impedence of the system.

A protective device only limits current if it opens the circuit in the current liming range of its TCC.

In general, short circuit fault calculations ignore the effects of current limiting devices.

Normally, 'let-thru' currents are only used when evaluating two devices in series (i.e. a fuse feeding a VFD). I cannot think of a case where they could be used "thru out a structure".

 

[cpal]

If
Using fusing in a disconnect has no recognized effect on the fault current that can be delivered to the connected equipment. The NEC does not recognize the current limiting effects of fuses (up over and down).

.

Jim
I guess this is the post that threw me
thanks for the info

 

[jim dungar]

Jim
I guess this is the post that threw me

That post referenced a method for determining the short circuit current downstream of a current liming device feeding a static device (i.e. non-overcurrent) like busbars. This method has very limited applications although is routinely taught.

The NEC requires devices to be rated to interrupt or carry the fault current to which they are exposed. How you determine the fault current is your choice.

But, the NEC does have restrictions on the use of fuse 'let-thru' currents and circuit breaker (these are called series ratings)

 

[augie47]

Two comments if I may.
I might suggest that you check the manufacturer's requirements for the VFD. Some I have seen require a current-limiting fuse ahead.
I don't know what AIC you are dealing with but most NF disconnects have a 10k rating.

 

[Jraef]

Two comments if I may.
...
I don't know what AIC you are dealing with but most NF disconnects have a 10k rating.

This is a little known fact and has been largely ignored until recently. I know a number of people who have been burned by this issue. Some of the equipment manufacturers are dealing with it now by getting series ratings for their NF switches, but be warned that if the upstream OCPD is a circuit breaker, they will only be getting the series rating with their own brand.