# Determining OCPD KAIC rating



## SWDweller (Dec 9, 2020)

What is the fault at the panel where your going to place the breaker?


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## WronGun (Oct 18, 2013)

SWDweller said:


> What is the fault at the panel where your going to place the breaker?


The panel has mixed 14 and 65 KAIC rated breakers installed. 












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## 99cents (Aug 20, 2012)

Can the utility tell you what the available fault current is?


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## SWDweller (Dec 9, 2020)

The panel name plate is not needed for this situation. 
Since the previous people are not paying attention to AIC's (KUDOS to you). It will make it harder.
Call your POCO and ask them what the AIC is at the service. 

You can always spend the bucks for the 65K and not go through the learning process.

Arc Fault studies are complicated, and require the AIC level at the meter and all of the the electrical equipment from there to the last panel. a bit of reading for you.








The 7 Steps to Complete an Arc Flash Analysis


A list of the 7 steps to complete an Arc Flash Study. An Arc Flash Analysis and Electrical Safety Training can greatly reduce the risk of incidents.




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Most insurance carriers REQUIRE arc fault studies and the stickers on every panel. So you can keep your insurance.

The biggest single modifier of arc fault is many feet of large diameter wire.


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## WronGun (Oct 18, 2013)

I’m sure they can, I had to call them to find out transformer configuration. They should have this info. What would be the downside to going with 14kaic like half of the breakers already in the panel ?


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## SWDweller (Dec 9, 2020)

downside, explosions, letting all of the magic out in a bang. 
Or nothing as long as the panel never sees a fault. 

I found in my half century of the electrical trade, guessing was the worst thing you can do most of the time. Knowing like perfection is what keep you and the people you work around/for safe. 
I do not like taking chances with someone else's life, limb and property.


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## WronGun (Oct 18, 2013)

I may just go 65kaic


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## micromind (Aug 11, 2007)

Getting the available fault current from the PUCO is the first step, if it's less than 14K, you can use 14K breakers anywhere in the system. 

If it's higher than 14K then you need to consider the extra cost of a higher rated breaker vs. the cost of a study to determine exactly how much fault current is available in the switchboard. 

Are 14K and 65K the only ratings available? Often these breakers come in 25K, 35K and 42K. Less $$$ than 65K.........


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## power (Feb 27, 2012)

Where I am from, it is usually the main service only that meets/exceeds the available fault current. After that, often by way of current limiting fuses, the available fault current is reduced to that of radial distribution, which is usually somewhere around 10-15kA depending on your locality.

Our network area services are 200kA (HRCI-J), with only 10kA downstream required. Our radial area is 10kA.

Your probably ok with the 14K, but like earlier said, the safe/easy route is go 65K.


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## TEO (Jan 17, 2020)

If it's T&M just get the 65K and your covered no need to worry about if it's needed, if it's a quoted job then spend the time to find out.


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## backstay (Feb 3, 2011)

I would learn how to do the calculations. It’s not hard, just requires inputting the wires and footage. Then the PoCo’s number. There is usually a option to not use the utility because some, like here won’t do that.


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## WronGun (Oct 18, 2013)

TEO said:


> If it's T&M just get the 65K and your covered no need to worry about if it's needed, if it's a quoted job then spend the time to find out.


It’s definitely something I’m going to read up on. I quoted the job based on the most expensive equipment. I even worked in a new Itoolco 6k puller package. It took them 8 weeks to approve the PO. Now the disconnect and breaker equip I need are on back order. So I ended up pulling some of the material as new open- box tested/certified off of eBay in order to get it on time. Saved a total of $4500 so far. The 65KAIC OCPD’s were cheap enough this way. The only thing I’m getting from supply now is pipe, wire, and hardware. 

On big money jobs I sometimes do this if I can find new equipment for sale and cut thousands off the back end. 

Now my only worry is flipping the 1600A lever to shut down the gear for breaker installation and the wire pull.


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## Signal1 (Feb 10, 2016)

WronGun said:


> It’s definitely something I’m going to read up on. I quoted the job based on the most expensive equipment. I even worked in a new Itoolco 6k puller package. It took them 8 weeks to approve the PO. Now the disconnect and breaker equip I need are on back order. So I ended up pulling some of the material as new open- box tested/certified off of eBay in order to get it on time. Saved a total of $4500 so far. The 65KAIC OCPD’s were cheap enough this way. The only thing I’m getting from supply now is pipe, wire, and hardware.
> 
> On big money jobs I sometimes do this if I can find new equipment for sale and cut thousands off the back end.
> 
> Now my only worry is flipping the 1600A lever to shut down the gear for breaker installation and the wire pull.


Yes, go with the higher AIC rating if you can.

As far as flipping the switch, make sure ALL of the covers are on and bolted, maybe gear up as well.


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## WronGun (Oct 18, 2013)

Signal1 said:


> Yes, go with the higher AIC rating if you can.
> 
> As far as flipping the switch, make sure ALL of the covers are on and bolted, maybe gear up as well.


Yes definitely, have our 40 cal Oberons ready. 


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## WronGun (Oct 18, 2013)

Signal1 said:


> Yes, go with the higher AIC rating if you can.
> 
> As far as flipping the switch, make sure ALL of the covers are on and bolted, maybe gear up as well.


Yes definitely, have our 40 cal Oberons ready. 


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## paulengr (Oct 8, 2017)

The easiest way if you have transformer data to get to an approximation us the infinite bus assumption. You just assume that the utility has a super stuff bus (no voltage drop) and that voltage drop through the conductors is minuscule. And no huge transformer or motor loads. At that point all the restriction is in the transformer.

So first calculate normal rates current. If we have a 100 kVA, 3 phase transformer at 480 V the rated current is:

100 kVA x 1000 (VA/kVA) / 480 V / 1.732 = 120 A.

Now short circuit would be normal current divided by %Z assuming all the impedance is reactive. So if %Z is 2% then

120 / 0.02 = 6 kA.

If you need something more accurate but don’t need/want to be an engineer or spend thousands on software you can use the Cooper point by point method.

So starting from where we know the short circuit current we want to calculate at a new point after a length of cable. So we calculate the F factor.

F = P x L x Isc / ( C x n x V) where:
P is 2 for single phase, 1.732 for 3 phase
L is cable length
Isc is the upstream (known) short circuit current
n is number of parallel conductors
V is line to line voltage (or line to neutral if single phase circuit is say 277).
C is conductor factor. See:



http://2.bp.blogspot.com/-mZnwibJL76I/UdGT8h48T6I/AAAAAAAAAhY/e7K9PTQskow/s585/C-values+for+conductor+short+ckt+calc.png



Now multiply Isc by 1/(1+F) to get the downstream short circuit current.

Here are some examples:









How to Solve Short Circuit Using Point to Point Method


A blog about electrical design, electrical installation, earthing system, power system analysis, fault calculation, industrial automation




www.electricalaxis.com





That’s it! This gets you past all concerns except large motors and transformers.

Here is a much bigger list of C factors and examples:



https://www.bu.edu.eg/portal/uploads/Engineering,%20Shoubra/Electrical%20Engineering/2904/crs-10562/Files/mohamed%20abd%20elwahab%20ali_Lecture%2007.%20Short%20Circuit%20Calculations.pdf



This method gets you answers about 95% of the time. It is so accurate that the previous version if the IEEE arc flash standard came with a spreadsheet that used it. It is the same as one of the ANSI/IEEE methods except the conductor factors are already worked out for you.

As to the guy commenting that short circuit is mostly only a “service” thing, not true. Every time you go through a step down transformer short circuit current goes up a lot. Granted it’s not “infinite bus” but it’s often pretty close. But as soon as you use the point by point method current drops quickly.

Let’s say we have a dry transformer directly feeding switchgear with about 20 feet of copper bus bar in the compartment. Let’s say the transformer short circuit current is 40 kA and it’s a 1000 A circuit at 480 V with copper bus.

So F = 1.732 x 20 x 40000 / (62900 x 1 x 480) = 0.04589.

Short circuit at switchgear is:
40 x 1/(1+F) = 38 kA.

Should be no surprise…bus bar is pretty lossless.

Now how about 1000 feet away on 350 MCM in steel conduit? Using 75 C we need 4 conductors per phase.

F = 1.732 x 1000 x 40000 / (154&4 x 4 x 480) = 2.330.

New short circuit current is 12 kA.

So with the statement that short circuit only matters at the service it depends on the service and the feeders. This CAN be a true statement. Don’t forget the short circuit current goes way up downstream if any step down transformer.

So I’m not a believer in the idea that only engineers can do these simple calculations or that you need a $5,000 software program and 2 weeks of training costing even more to use it. The ANSI and IEC standards for calculating short circuit current were written at a time where we used slide rules and computers were the size of a house. Short circuit calculations still got done and we did not see breakers exploding because of math errors.

Also I’d encourage you to do point by point because high fault breakers are considerably more expensive.

Over time what tends to happen is maintenance is faced with availability or price differences or lack of knowledge. So their options are say a 15 kA or a 65 kA rated breaker. They will be looking at the big “100 A” on the handle not the mouse print on the side and put in the cheaper/more common 15 kA breaker.

As a drive/starter guy I get to look at the blown drive that failed when the motor shorted. So 5 minutes of checking name plates increases the price as customer has to choose between adding a line reactor or expensive/rare semiconductor fuses, a better VFD, or using the VFD as a fuse.


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## WronGun (Oct 18, 2013)

paulengr said:


> The easiest way if you have transformer data to get to an approximation us the infinite bus assumption. You just assume that the utility has a super stuff bus (no voltage drop) and that voltage drop through the conductors is minuscule. And no huge transformer or motor loads. At that point all the restriction is in the transformer.
> 
> So first calculate normal rates current. If we have a 100 kVA, 3 phase transformer at 480 V the rated current is:
> 
> ...


Looks like a-lot of work. Although, learning this might land me some extra work into getting them to swap out all their 14KAIC breakers if they aren’t supposed to be there. 


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## backstay (Feb 3, 2011)

WronGun said:


> Looks like a-lot of work. Although, learning this might land me some extra work into getting them to swap out all their 14KAIC breakers if they aren’t supposed to be there.
> 
> 
> Sent from my iPhone using Tapatalk


Just go here. 



Fault Current Calculator


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## WronGun (Oct 18, 2013)

Signal1 said:


> Yes, go with the higher AIC rating if you can.
> 
> As far as flipping the switch, make sure ALL of the covers are on and bolted, maybe gear up as well.


I’m a little worried about the liability in shutting down a massive factory. At some point a 1600amp main breaker is bound to fail. 


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## paulengr (Oct 8, 2017)

Signal1 said:


> Yes, go with the higher AIC rating if you can.
> 
> As far as flipping the switch, make sure ALL of the covers are on and bolted, maybe gear up as well.


It takes about 1 PSI to blow a door off. Think about it. 1 PSI on a 20 x 20 “size 3 starter” has 400 pounds on it. If arc flash is a “thing” the four can and will launch in under 1 cycle. So worrying about latching and bolting down just makes it a projectile. It does NOT provide any arc flash protection unless it’s arc rated gear or XP rated.

As far as gear you either wear it at the needed rating or you don’t. No half ways. However in many cases if you are wearing FR the FR itself gives most of the value.


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## MikeFL (Apr 16, 2016)

If you call POCO to get the AIC rating have the meter number handy along with the site address. At least here we call the POCO construction division and give them a meter number and we can ask anything we want. That's how their database is set up.


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## WronGun (Oct 18, 2013)

paulengr said:


> It takes about 1 PSI to blow a door off. Think about it. 1 PSI on a 20 x 20 “size 3 starter” has 400 pounds on it. If arc flash is a “thing” the four can and will launch in under 1 cycle. So worrying about latching and bolting down just makes it a projectile. It does NOT provide any arc flash protection unless it’s arc rated gear or XP rated.
> 
> As far as gear you either wear it at the needed rating or you don’t. No half ways. However in many cases if you are wearing FR the FR itself gives most of the value.


I’m wearing an Oberon 40cal full suit. There isn’t a study on the gear so is 40cal enough im really not sure. That’s the switch to the left and my new breakers go in on the right and we run 400’ of 2” from the top, I’ve worked in this gear before but the work wasn’t as large, and we weren’t pulling new feeds from the top from 400’ away.










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## SWDweller (Dec 9, 2020)

Shed the load, shed the load. What kind of 1600 amp switch? Be especially careful of "bolted pressure switches. They hang up with time if maintenance is not done on them.

FR clothing for sure, SHED THE LOAD


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## WronGun (Oct 18, 2013)

SWDweller said:


> Shed the load, shed the load. What kind of 1600 amp switch? Be especially careful of "bolted pressure switches. They hang up with time if maintenance is not done on them.
> 
> FR clothing for sure, SHED THE LOAD


I’ll be shutting down all loads separately before hitting the switch, for sure. 


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## Signal1 (Feb 10, 2016)

WronGun said:


> I’m wearing an Oberon 40cal full suit. There isn’t a study on the gear so is 40cal enough im really not sure. That’s the switch to the left and my new breakers go in on the right and we run 400’ of 2” from the top, I’ve worked in this gear before but the work wasn’t as large, and we weren’t pulling new feeds from the top from 400’ away.
> 
> 
> 
> ...


At least it not a Pringle switch or a bolted pressure, those make me nervous.

But still, load shed and use the suit. If for anything it's solid practice.


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## Signal1 (Feb 10, 2016)

paulengr said:


> It takes about 1 PSI to blow a door off. Think about it. 1 PSI on a 20 x 20 “size 3 starter” has 400 pounds on it. If arc flash is a “thing” the four can and will launch in under 1 cycle. So worrying about latching and bolting down just makes it a projectile. It does NOT provide any arc flash protection unless it’s arc rated gear or XP rated.
> 
> As far as gear you either wear it at the needed rating or you don’t. *No half ways.* However in many cases if you are wearing FR the FR itself gives most of the value.


I agree with you on the gear.

But you think it's safer to have the covers OFF?


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