Increasing Utility service - Higher available fault current impacts existing equip.

[Paul the BIG Finn]

Our client increased the electrical service to his light industrial building from 800A, 120/208V, 3 phase 4 wire to 1,600A to power additional equipment. He installed a 1,600 switchboard and back feed the existing 800A panelboard from the switchboard with an 800A circuit breaker. My company was brought in to perform an arc flash analysis and provide warning labels. We contacted the power company and found the available fault current at the dedicated pad mounted transformer was just under 50 kA. The new switchboard is rated 65 kAIC, but the circuit breakers in the original 800A panelboard are rated 30 kAIC.

The circuit breaker in the switchboard supplying the existing service is rated 65kAIC, but is not current limiting.

I have found a current limiting fuse we could install in the feeder to the 800A panelboard, but the client would prefer a current limiting circuit breaker. Is there another solution short of replacing the panelboard? Will a current limiting fuse or circuit breaker satisfy the NEC?

I appreciate your opinions and experience.

 

[don_resqcapt19]

You are looking at a "series rated" combination, if you want to use an OCPD to make the installation code compliant, and the combination of devices used must be listed.
How far away is the service equipment from the utility transformer?
How far from the service equipment is the old 800 amp panel?
There will be a reduction on the available fault current based on the size and length of the service and feeder conductors. There may be enough length to reduce the available fault current enough so you don't have to add anything.

 

[topgone]

Our client increased the electrical service to his light industrial building from 800A, 120/208V, 3 phase 4 wire to 1,600A to power additional equipment. He installed a 1,600 switchboard and back feed the existing 800A panelboard from the switchboard with an 800A circuit breaker. My company was brought in to perform an arc flash analysis and provide warning labels. We contacted the power company and found the available fault current at the dedicated pad mounted transformer was just under 50 kA. The new switchboard is rated 65 kAIC, but the circuit breakers in the original 800A panelboard are rated 30 kAIC.

The circuit breaker in the switchboard supplying the existing service is rated 65kAIC, but is not current limiting.

I have found a current limiting fuse we could install in the feeder to the 800A panelboard, but the client would prefer a current limiting circuit breaker. Is there another solution short of replacing the panelboard? Will a current limiting fuse or circuit breaker satisfy the NEC?

I appreciate your opinions and experience.

The wires supplying power from the new switchboard to the 800A panelboard have its impedance. That said, a 50 kA Isc at the main switchboard will drop at the point where your 800A panelboards main breaker receives power! The usual way we do is to calculate prospective short circuit current at the point where the breaker is connected-->take the length of the wires and lookup for the corresponding impedance and recompute the short-circuit amps at the 800A breaker. If the results tell you that the prospective short circuit current is above 30kA, try and recompute the Isc at the 800A breaker using a longer set of wires.

If nothing else fits, you need to consider "series-rated" breakers. Check the feeder breaker at the new switchboard if it is listed as "series-rated" with the 800A breaker in your panelboard.

 

[Ingenieur]

Is the 50 ka at the xfmr lugs?
how long are the service conductors?
it will be <50 ka at the new 1600 sb
then additional drop from the 1600 to the existing 800

your xfmr z in ohms ~ 206/50000=0.004 or so

if your conductors from xfmr to sb are only 1/2 of that your new i fault ~ 33 ka
makes a big difference

 

[mivey]

It doesn't take too long to get from 50k down to 30k. Hopefully you have enough cable.

 

[Ingenieur]

100' xfmr to sb
4 sets 500 kcmil
should be close to 30k

 

[don_resqcapt19]

100' xfmr to sb
4 sets 500 kcmil
should be close to 30k

The Bussman app says that will get you from 50,000 to 36,000 if you run it in non-magnetic conduit and 34,000 if you use a ferrous conduit.

 

[GoldDigger]

Just out of curiosity, some members have described resorting to coiling up large sections of extra wire/cable in series to drop the fault current to the needed level.
What kind of wiring and wiring method did you end up using for that purpose, since just coiling NM or SER or whatever on the floor or hanging it in loops on the wall seems problematic under the Code?
Looping TC back and forth along a tray? Using DB cable in loops underground?

 

[topgone]

Just out of curiosity, some members have described resorting to coiling up large sections of extra wire/cable in series to drop the fault current to the needed level.
What kind of wiring and wiring method did you end up using for that purpose, since just coiling NM or SER or whatever on the floor or hanging it in loops on the wall seems problematic under the Code?
Looping TC back and forth along a tray? Using DB cable in loops underground?

I think the whole idea is to give the OP the facts re kA reduction considering the length of the supply wires. Others will opt for "current limiting" devices upon seeing the big difference between POCC available short-circuit amps and their OCPD kA ratings, not knowing that a small interposing impedance in between the high kA source and the breaker will do the job with no additional cost. How the fallback options will be done is left to the OP.

 

[Ingenieur]

The Bussman app says that will get you from 50,000 to 36,000 if you run it in non-magnetic conduit and 34,000 if you use a ferrous conduit.

thanks
I used rough code Z numbers in pvc figuring duct bank
and he said less that 50 k

op more info please

My WAG
208 / ((208 / 48000) + (200 / 1000) x 0.043 / 4) = 32 ka

 

[wbdvt]

Our client increased the electrical service to his light industrial building from 800A, 120/208V, 3 phase 4 wire to 1,600A to power additional equipment. He installed a 1,600 switchboard and back feed the existing 800A panelboard from the switchboard with an 800A circuit breaker. My company was brought in to perform an arc flash analysis and provide warning labels. We contacted the power company and found the available fault current at the dedicated pad mounted transformer was just under 50 kA. The new switchboard is rated 65 kAIC, but the circuit breakers in the original 800A panelboard are rated 30 kAIC.

The circuit breaker in the switchboard supplying the existing service is rated 65kAIC, but is not current limiting.

I have found a current limiting fuse we could install in the feeder to the 800A panelboard, but the client would prefer a current limiting circuit breaker. Is there another solution short of replacing the panelboard? Will a current limiting fuse or circuit breaker satisfy the NEC?

I appreciate your opinions and experience.

From your description it seems like the utility probably installed a 500kVA padmount transformer and I would question whether the power company provided the actual available fault current or used an infinite bus approach. Using the infinite bus approach and assuming a transformer impedance of 2.8% will result in a short circuit value of ~49.5kA. Using actual available fault current, it is possible that the short circuit current could be as low as 15kA depending on how stiff the utility system is or is not.

I have run into this issue when asking for the available fault current from utilities and I will usually get a value based on an infinite bus usually from the Customer Service people. You really need to get the value from the Engineering department. Of course it is most prudent to size to maximum fault current but that may be a value that can never be delivered by the utility system especially if your facility is off of a distribution circuit versus being large enough to be fed from a sub transmission system.