18,000 amp AIC commercial

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mlnk

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POCO has an 18k AIC requirement. I plan to run wire from the overhead service mast head to a fused disconnect rated 100k AIC. Downstream from the disconnect it OK to use 10K rated meter panels and circuit breakers?
 
The 100k rating on the disconnect is simply the rating of the switch itself and has no bearing on components downstream from the switch.

Assuming your statement indicates POCO has 18k available fault Ideally you would have an engineering study taking into account any motor contribution, etc. but more simply you need (a) breakers rated above 18k, (b) calculations taking into account wire sizes and lengths which reduce the available to below 10kw; or (c) breakers that will series rate with you fuses.
What is the size of the disconnect and the wire sizes and lengths ?
 
mlnk said:
POCO has an 18k AIC requirement. I plan to run wire from the overhead service mast head to a fused disconnect rated 100k AIC. Downstream from the disconnect it OK to use 10K rated meter panels and circuit breakers?
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18K required?

did you mean 18K available?

how much available at any point in the system depends on how much resistance is in conductors between the point in question and the source.

You may have 18k available at transformer terminals, but only 12k available at service disconnect, and only 3k available at some remote panelboard in the facility.
 
Whatever you do the series rating (which you need) must be UL listed by the manuf. Generally the main breaker in a panel may be rated at 22,000 and the branch breakers are rated 10,000. The whole assembly is series rated at 22,000 AIC. Do this all the time.

Also, many manuf's have tables listing given panels in series with given breakers or current limiting fuses. I prefer to have the main OUTSIDE.

Most engineers can help you on this.

RC
 
Generally, the only thing that will reduce the fault current significantly is an imepedance, such as a transformer, line reactor and/or a very long cable run.
 
Fnewman said:
Generally, the only thing that will reduce the fault current significantly is an imepedance, such as a transformer, line reactor and/or a very long cable run.
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I wouldn't say a very long run, just twenty five more feet of conductor often can make a difference on needing a higher AIC rated device. If your service transformer is right next to the building you may have too high fault current, if it is out near property line but every thing else is the same often the fault current level is reduced enough you don't need increased rating on your equipment.
 
mlnk said:
POCO has an 18k AIC requirement. I plan to run wire from the overhead service mast head to a fused disconnect rated 100k AIC. Downstream from the disconnect it OK to use 10K rated meter panels and circuit breakers?
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You need to look at the listed series combinations for the specific fuse you are using and for the specific breakers in the panel. Just because the fuse in the disconnect has a 100,000 AIC rating, does not mean that you can use downstream breakers with an AIC of less than 18,000.
 
don_resqcapt19 said:
Just because the fuse in the disconnect has a 100,000 AIC rating, does not mean that you can use downstream breakers with an AIC of less than 18,000.
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Back when I was wetter behind the ears, I didn't know that. I thought that once I used a 65kA breaker, I could use whatever I wanted in the panel, including 10kA or even smaller fuses. None of them blew up (THANKFULLY), and AFAIK none are still in use (southeast textile market is gone).

We also used fine stranded wire without proper lugs, both in MTW (37 and 61 strand, I think) and something similar to DLO. I'd never pass examination on modern panel design. I was VERY ignorant ... and still am.
 
The main disconnect panel will be 200 A. Then there will be three 100 amp panels with meters on them. I am now planning to make all the panels and breakers 22,000 K AIC.
 
mlnk said:
The main disconnect panel will be 200 A. Then there will be three 100 amp panels with meters on them. I am now planning to make all the panels and breakers 22,000 K AIC.
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A lot of main breaker panels with factory installed main are usually 22k main breakers, and the corresponding 10k branch breakers are series rated with the 22k main.

I bet there is still good chance any 10k breakers you might use are series rated with RK-5 (100k) fuses anyhow.

I also suspect if you have 200 amp service you won't have 18kA available fault current even if the source transformer were right next to the service disconnect. POCO just won't supply that with large enough transformer to have that much fault current if the load isn't there.

50 kVA single phase transformer, rated current 208 A, if it has 1.72% impedance available fault current at the transformer is only about 12kA. Unless you do have high load POCO probably only supplies you with a 25 or 37.5 kVA transformer - fault current will be even less.

75 kVA three phase 208/120 transformer - rated current 208A, if impedance is 1.5% available fault current at the transformer is only about 14 kA. Again unless you have the load, POCO is likely using a smaller transformer.

I have found that you generally don't have to be too concerned about having higher interrupt ratings then what is "usual" until you have about a 100kVA or larger source or if you are very close to the source with that equipment, it can get you in the 50kVA range sometimes, as little as 10-20 feet of conductor makes a huge difference though.
 
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