Fully rating

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Toros

Senior Member
Location
Tujunga, CA
Hi
I have a commercial space, a warehouse under major remodeling and has a 800A, 3 phase 4-wire service.
Available fault by local utility company is 27,000A

we installed four sub panels throughout the space with breakers at 10,000 AIC.

now the inspector wants the AIC of new brkrs in the sub panel to be matched with available fault.

What do think the is the best approch to make a fully rating.... if my short circuit calcs showes 13000-14000 fault at those new breakers?

Thanx
 
bob

bob

Senior Member
Location
Alabama
At what point in the system is the 27 ka? At the transformer terminal? At the weatherhead? The SC at the sub panels is what determines
the requirements for the breakers.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Toros said:
Hi
I have a commercial space, a warehouse under major remodeling and has a 800A, 3 phase 4-wire service.
Available fault by local utility company is 27,000A

we installed four sub panels throughout the space with breakers at 10,000 AIC.

now the inspector wants the AIC of new brkrs in the sub panel to be matched with available fault.

What do think the is the best approch to make a fully rating.... if my short circuit calcs showes 13000-14000 fault at those new breakers?

Thanx
Click to expand...

1. Did you calculate the fault current down the system to the subs using , say, the mike holt calculator, and still have over 10K?
2. Are you sure there isnt a series rating?

If yes to both of the above, I guess you either have to:

1. upgrade the 10k breakers
2. install a main breaker in the sub panels (if currently MLO) with the appropriate interrupting rating and get a series rating to the 10k's with that.
3. Replace some component of the upstream gear to get a series rating.

Note that although you wont get a series rating across different manufacturers, most CB's have series ratings to fuses. Just an FYI in case adding a safety switch somewhere does the trick.
 
T

Toros

Senior Member
Location
Tujunga, CA
electrofelon said:
1. Did you calculate the fault current down the system to the subs using , say, the mike holt calculator, and still have over 10K?
2. Are you sure there isnt a series rating?

If yes to both of the above, I guess you either have to:

1. upgrade the 10k breakers
2. install a main breaker in the sub panels (if currently MLO) with the appropriate interrupting rating and get a series rating to the 10k's with that.
3. Replace some component of the upstream gear to get a series rating.

Note that although you wont get a series rating across different manufacturers, most CB's have series ratings to fuses. Just an FYI in case adding a safety switch somewhere does the trick.
Click to expand...

GOOD
TANX
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
ActionDave

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
GoldDigger said:
Not trying to embarrass you but a Google search for site:mikeholt.com fault current calculator showed it as the first entry.
Knowing that it is there is a lot more than half the battle in doing the search.
Click to expand...
I'm not embarrassed. I'm one of those people that still use the phone book, the one made from trees, and I got a link up before you did. That makes me the winner. Hah! Take that! :p
 
W

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Is the 27kA fault current provided by the utility the actual fault or the fault current from an infinite bus source? You may find that if it is not the actual available fault current, using the actual fault current may result in less than 10kA at the panels.

You can get a feel for what they gave you if you know the transformer kVA and impedance.
 
V

victor.cherkashi

Senior Member
Location
NYC, NY
Fuses in first switch can be replaced with current limiting type, which will reduce requirement for downstream equipment

Sent from my Nexus 5 using Tapatalk
 
kwired

kwired

Electron manager
Location
NE Nebraska
bob said:
At what point in the system is the 27 ka? At the transformer terminal? At the weatherhead? The SC at the sub panels is what determines
the requirements for the breakers.
Click to expand...
That is the first thing one needs to know IMO. Further away from that point you are the lower the available current will be, how much depends on what is between that point and point of interest.

Series ratings of devices also comes into play. If you have breakers but mismatched brands - you may be out of luck - or at least will cost less to just install fused disconnects then to have proper study done. As mentioned most breakers will have series ratings with specific fuse types they just will not have been tested for series with other manufacturer's breakers.

Also consider a main breaker in the sub panels that is higher AIC rated yet series rated with the 10k branch breakers. Most factory installed mains in "loadcenters" are 22 or 25 kA rated and intended to be able to supply 10kA branch breakers.
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
ActionDave said:
It used to be easier to find on the Mike Holt website. I had to dig around for twenty minutes.....yuck. Here is a link

http://www.mikeholt.com/technical-calculations-formulas.php
Click to expand...

There is probably no part of the electrical trade I am less comfortable with than the concept of arc flash. I need to self educate on this. In light of this, which of the formulas on that link is the one you are referring to? The one that references NFPA 70 or the one that refers to "point to point"?
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
Strathead said:
There is probably no part of the electrical trade I am less comfortable with than the concept of arc flash. I need to self educate on this. In light of this, which of the formulas on that link is the one you are referring to? The one that references NFPA 70 or the one that refers to "point to point"?
Click to expand...

There is no arc flash calculator on the site. Just for calculating short circuit current which is used for arc flash calcs.
 
I

Ingenieur

Senior Member
Location
Earth
The 27000 sounds high
800 A service the util is probably using a xfmr with a rated current of perhaps 600
assuming a low pu Z of 0.045 that is only 13.4 kA at the xfmr terminal
assuming an infinite bus

what is the following
xfmr kva and Z
Voltage
utility X/R and avail fault MVA
???

who did the downstream calculations?
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Ingenieur said:
The 27000 sounds high
800 A service the util is probably using a xfmr with a rated current of perhaps 600
assuming a low pu Z of 0.045 that is only 13.4 kA at the xfmr terminal
assuming an infinite bus
Click to expand...

You need to check your math. 27kA sounds reasonable to me.
It would not be unusual for a utility transformer, <300kVA, to be quoted as having an impedance of 1.5%.
The OP said their breakers were rated 10kAIC which is a standard rating for '240V' equipment.

600A / .015 = 40kA

Utilities typically provide available short circuit amps based on the worst case transformer they might ever supply. They want to make sure your equipment does not become overdutied because they had to replace your transformer with 'whatever they had in the yard' during an emergency.
 
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