Available Fault Current

[ohmti787]

Got this inspector arguing that the MCB's AIC rating at a house should not be less than the AFC at the Xfmr's secondary terminal. But that doesn't make much sense to me. I ran calculations, with some assumptions of course (since the utility doesn't provide all the information you need), and the AFC value drops significantly at the house compared to its value at the Xfmr's terminals.

Am I missing something here? Wouldn't the AFC at the house be a more realistic number to determine the AIC rating of the MCB? Please advise.

Thanks,

 

[texie]

Of course.

 

[Dsg319]

It’s a crying shame people like this are in these positions.

Also I thought that showing the AFC calcs were only required on services other than residential, I suppose it’s a local utility requirement?

 

[augie47]

Local inspectors will ask for verification on services > 400 amp resi & commercial and even for smaller ones if they are fed from large transformers but always at the over-current device.

 

[jim dungar]

The NEC does not address how to calculate the AFC, as this is a design issue, there is also no industry standard for performing these calculations. However, there is no calculation methodology that does not take into account easily determined impedances, like those from service drops and other conductors.

 

[ohmti787]

It’s a crying shame people like this are in these positions.

Also I thought that showing the AFC calcs were only required on services other than residential, I suppose it’s a local utility requirement?

this is straight up, 100% AHJ requirement only. the utility will provide an AFC letter for every house fed by the Xfmr, but the letter specifically says "at the secondary terminals". inspector is having me run calculations at the house which, even in the worst case scenario, don't even come close to the AFC at the terminals. yet, he want's us to put a 65kAIC rated MCB breaker in this house

 

[jim dungar]

...he want's us to put a 65kAIC rated MCB breaker in this house

Does he want all of the other devices to series rate with this 65kAIC main?

 

[Tulsa Electrician]

Just curious, what number did utility provide you with.

For a single family dwelling residential area that seams high.

 

[electrofelon]

this is straight up, 100% AHJ requirement only. the utility will provide an AFC letter for every house fed by the Xfmr, but the letter specifically says "at the secondary terminals". inspector is having me run calculations at the house which, even in the worst case scenario, don't even come close to the AFC at the terminals. yet, he want's us to put a 65kAIC rated MCB breaker in this house

Sounds right to me . Take your AFC at the transformer secondary and run it thru a calculator to find the reduction from the conductor impedance to get the AFC at the breaker.

 

[Hv&Lv]

this is straight up, 100% AHJ requirement only. the utility will provide an AFC letter for every house fed by the Xfmr, but the letter specifically says "at the secondary terminals". inspector is having me run calculations at the house which, even in the worst case scenario, don't even come close to the AFC at the terminals. yet, he want's us to put a 65kAIC rated MCB breaker in this house

Do you have a fault current calculator?
Here is one.

Fault current calculator

A mobile app and web-based application for determining the fault current of single- and three phase systems with fuse selection and conductor sizing guides.

www.eaton.com

Just for giggles I put in 72,000A on the transformer and 150’ of sweetbriar 4/0.

 

[ohmti787]

Does he want all of the other devices to series rate with this 65kAIC main?

so far, he hasn't. but i wouldn't be surprised if he changes his mind later

 

[ohmti787]

Just curious, what number did utility provide you with.

For a single family dwelling residential area that seams high.

this one came up to 35,750A sym. the letter doesn't specify, but i'm assuming this is L-L, correct? it says at the "secondary terminals"

 

[jim dungar]

so far, he hasn't. but i wouldn't be surprised if he changes his mind later

Then he really doesn't understand the code.
 All protective devices must be fully or series rated. This has been explicit in the NEC for some almost 40 years, before that we just had 110.9 to rely on.

 

[ohmti787]

Do you have a fault current calculator?
Here is one.

Fault current calculator

A mobile app and web-based application for determining the fault current of single- and three phase systems with fuse selection and conductor sizing guides.

www.eaton.com

Just for giggles I put in 72,000A on the transformer and 150’ of sweetbriar 4/0.

i've been using that one. you even used more conservative numbers than i did and still.......i'd say a 10kAIC rated breaker would work here. but this guy is just not having it

 

[Charged]

Check out the utility APS service guide available online. It’s Arizona . There one of the utilities that publishes data due to fault current calculations being standard requirement by AHJs in the area. …….whats unique about APS from others that do this is they direct the value to be used based on service equipment amp rating. ……it’s super conservative because it’s similar values you would see published at the secondaries of utility transformer. …..may be of interest to you, I always thought it was curious but I landed on if you do it this way they can limit there liability to be held accountable for changes to there system that’s why they have it based on the customers equipment , just my guess though

 

[augie47]

If I were wearing my inspector hat and was advised the SCA was 35750 I might take a look at the service drop/lateral to see if it were significantly short if your main was only rated 10k. 50 ft or more of drop I would not expect a problem

 

[Hv&Lv]

i've been using that one. you even used more conservative numbers than i did and still.......i'd say a 10kAIC rated breaker would work here. but this guy is just not having it

I agree. Would it be worth it to go over his head?

 

[ohmti787]

If I were wearing my inspector hat and was advised the SCA was 35750 I might take a look at the service drop/lateral to see if it were significantly short if your main was only rated 10k. 50 ft or more of drop I would not expect a problem

in this particular case, the service is underground and the MSP is roughly 125' from the transformer

 

[ohmti787]

I agree. Would it be worth it to go over his head?

we already did once


it was for another deal though, but i believe that's why he's being so stingy

 

[Jraef]

Do you have a fault current calculator?
Here is one.

Fault current calculator

A mobile app and web-based application for determining the fault current of single- and three phase systems with fuse selection and conductor sizing guides.

www.eaton.com

Just for giggles I put in 72,000A on the transformer and 150’ of sweetbriar 4/0.

Nice find. I like how it can print out the labels too!