22k AIC requirement from inspector on resi???

[LocalNowhere]

Here is the breakdown: A new 400Amp service on a house. Feeding a 200 amp sub-panel via a 22k main breaker. Utility Fault Current= 22k. Utility transformer is about 300' away. The inspector is wanting me to use 22k breakers in the sub panel. I did the available fault current calculations and it is nowhere near even 10k. Am I missing something??? I am aware of 240.86, 110.10 etc.

Additional info: Meter panel (400 amp) and sub panel (200 amp) are different manufacturers. And I didn't have Anything to do with deciding or recommending a 400 amp service. I inherited this job from another contractor who had to drop it half way thru.

 

[bob]

If the transformer is 300 ft away you won't have 22 ka. In fact you may not have 10 ka. What you will likely have is low voltage problems. You need to get the utility to install a new transformer at the point of service to the house.

A new 400Amp service on a house. Feeding a 200 amp sub-panel via a 22k main breaker. Utility Fault Current= 22k.

If the fault is less than 10 ka, why did you post 22 ka? Depending on the load
it appears you need a 25 or 37.5 kva transformer. The est fault for the 37.5 kva is 6250 amps.

 

[LocalNowhere]

If the fault is less than 10 ka, why did you post 22 ka?

The utility max. fault current is 22ka. But my understanding is that this it at the transformer secondary. I did a point to point calculation (based on what I had read elsewhere on this thread) and that is where I got the <10ka.

I have no say in the size of the poco's transformer.

No problems with voltage drop.

 

[LocalNowhere]

I did a point to point calculation (based on what I had read elsewhere on this thread)

I meant this forum, not this thread. It was the Available Fault Current Calculation. Which I understand to give me the available fault current at the meter panel essentially. Is this correct? And is it relevant? I don't want to have to spend hundreds on 22k breakers. Thanks in advance for any help.

 

[Ragin Cajun]

What is the size of the service transformer? As posted by bob, for a 37.5 KVA transformer fault current at the transformer is less than 7000A. Get the POCO to verify the 22K SC.

RC

 

[flatlander]

From how our POCO's work you must go by what they state the Fault current is to be. If they claim 22k then that is what it will be.


Then there is the issue of sieries rating of equipement. Your sub panel may not be rated in series from the main panel.

 

[bob]

The utility max. fault current is 22ka. But my understanding is that this it at the transformer secondary. I did a point to point calculation (based on what I had read elsewhere on this thread) and that is where I got the <10ka.

I have no say in the size of the poco's transformer.

No problems with voltage drop.

Where are you located? 22 ka is close to impossible regardless of what they say. It depends on the % impedance of the transformer and the size of the transformer. Do you know what size the secondary conductor is? They may be giving you the hot leg to neutral fault. It will be higher but 22 ka is a streach.

 

[LocalNowhere]

This is in Southern California and is dealing with SDG&E. They did confirm the 22k rating at the transformer. I don't know the transformer size or the secondary conductor size. But I did the calculations with a wide range of wire sizes and they all seem to support not needing 22k breakers in the sub panel. Not even close.

I spoke with SDG&E and they said they had never heard of an inspector requiring 22k breakers in this situation.

Is my reasoning correct in using the Available Fault Current calculation? And does that negate the need for using 22k breakers in the sub panel?

 

[Sierrasparky]

I am a bit baffled here. You say the POCO confirmed 22k , so what's the issue.

I have heard that in some areas you don't need to use equpment rated for the fault current and that it is an extra measure and not required. However I have seen inspectors require the installation to meet the fault current.

 

[david luchini]

Where are you located? 22 ka is close to impossible regardless of what they say. It depends on the % impedance of the transformer and the size of the transformer. Do you know what size the secondary conductor is? They may be giving you the hot leg to neutral fault. It will be higher but 22 ka is a streach.

Not sure how you're arriving at this, but a 100kVA, 240/120V (secondary) transformer with a 1.9% impedance would give 22kA available fault current. That seems reasonable.

Is my reasoning correct in using the Available Fault Current calculation? And does that negate the need for using 22k breakers in the sub panel?

Yes, your reasoning is correct, if your calculation shows under 10kA available fault current at the subpanel, then the c/b's in the panel wouldn't need an interrupting rating over 10kA per 110.9.

 

[Dennis Alwon]

Show the inspector your fault current calculation and he/she will probably be very impressed.

 

[mcclary's electrical]

This is in Southern California and is dealing with SDG&E. They did confirm the 22k rating at the transformer. I don't know the transformer size or the secondary conductor size. But I did the calculations with a wide range of wire sizes and they all seem to support not needing 22k breakers in the sub panel. Not even close.

I spoke with SDG&E and they said they had never heard of an inspector requiring 22k breakers in this situation.

Is my reasoning correct in using the Available Fault Current calculation? And does that negate the need for using 22k breakers in the sub panel?

Don't confirm the fault current at the transformer, ask them for a fault current letter for the meter location.
It will be much less. Dominion guarantees SFD to be less than 10,000AIC so it's a nonissue for residential

 

[bob]

Not sure how you're arriving at this, but a 100kVA, 240/120V (secondary) transformer with a 1.9% impedance would give 22kA available fault current. That seems reasonable.

I've never seen a 100 kva serving a house with a 400 amp service. If the transformer is a 100 kva then you may be correct.

 

[Sierrasparky]

I doub't there is a 100kva trans supplying one home.
Is that they way they do it on the other coast.
In my neck of the woods 4-6 homes can share a 75 or a 100kva trans
Don't know haw they calc it but thats what they do.

 

[don_resqcapt19]

I doub't there is a 100kva trans supplying one home.
Is that they way they do it on the other coast.
In my neck of the woods 4-6 homes can share a 75 or a 100kva trans
Don't know haw they calc it but thats what they do.

That is huge for around here. My house is one of 7 fed from a 50kva.

 

[don_resqcapt19]

I am a bit baffled here. You say the POCO confirmed 22k , so what's the issue.

I have heard that in some areas you don't need to use equpment rated for the fault current and that it is an extra measure and not required. However I have seen inspectors require the installation to meet the fault current.

That current is at the secondary lugs of their transfomer. The impedance of the conductors between the lugs and the serivce equipment will reduce the available fault current at the service equipment by a substantial amount.

 

[Sierrasparky]

That current is at the secondary lugs of their transfomer. The impedance of the conductors between the lugs and the serivce equipment will reduce the available fault current at the service equipment by a substantial amount.

Ok , but I am use to the POCO engineer quoting the fault current at my installed equipement after they calculate the impedance due to wire size and distance.
The issue is 22k at the trans?
or 22 at the metering equip?
The OP needs a letter from the POCO telling what the Fault current is at the meter and then install the properly rated equip downstream. I doub't the city would allow the rating be done by the EC unless he is a EE.

The letter from the POCO should suffice.

 

[LocalNowhere]

The issue is 22k at the trans?
or 22 at the metering equip?

Thank you for the responses. This was my first post, and looking back I could have made my concern more clear in the original post. The issue for me was the inspector saying I had to use 22k breakers at the sub panel. I knew there was a 22ka rating at the transformer secondary (about 300' away), but I also knew that by the time it got to my meter it was much much lower.

So I need to contact the poco and request a letter (or some kind of written form) with the Available Fault Current at the meter, so as to satisfy the inspector that I do not need to use (and spend the money on) 22k rated breakers on my sub panel. Assuming of course that a verbal explanation to the inspector does not suffice.

Re: transformer load- as I mentioned before, I don't know the transformer size. But it is feeding multiple houses all with underground feeds. At least 5 2000+ sf houses.....One of things I am really shaking my head about tho, is that this property has a solar array which has kept the meter at zero for over a month now. And yet he got roped into a 400 amp service upgrade (not by me).

 

[Sierrasparky]

How much solar is installed.

rememer the rule for max backfeed for net metering/solar/wind ect.
That would be no more than 120 % of the buss or conductor.

 

[mxslick]

If the transformer is 300 ft away you won't have 22 ka. In fact you may not have 10 ka. What you will likely have is low voltage problems. You need to get the utility to install a new transformer at the point of service to the house.

On what fact do you base that statement? If the feeder/service drop is sized correctly there will not be voltage drop issues.

Just wondering.