AIC Calculations for Service Disconnects NEC 110.9

[jar546]

During plan review, we require the AIC calculations for all permit applications that are replacing or installing a service disconnect. This includes Automatic Transfer Switches (ATS) for generators used as a service disconnect. This is covered under NEC 110.9 which we reference when we don't get the calculation and reject the permit.

Who else is doing this? This applies to residential and commercial. The commercial service disconnects go another step further requiring not only the calculation but a permanent label on the service disconnect showing the AIC rating and date of calculation.

 

[steveray]

Does this tie in with 110.24 for available fault current as well?...Sorta

 

[jar546]

Does this tie in with 110.24 for available fault current as well?...Sorta

Absolutely for other than dwelling units.

 

[steveray]

Absolutely for other than dwelling units.

Can you actually get it if they don't know the specifics on the transformer....?

 

[jar546]

Can you actually get it if they don't know the specifics on the transformer....?

Yes, we get a letter from the POCO if the electrician does not know the specifics. If it is a pad-mounted transformer, there is enough information on the label for the electrician to figure out the available fault current at the secondary taps of the transformer. Most pole-mounted transformers have the KVA painted on them and the electrician can use a standard Z% to do the calculation.

The available fault current at the secondary taps fo the POCO transformer is just part 1 of the calculation. Part 2 takes into consideration the length of the service conductors from the secondary taps to the service disconnect, along with the type of wire, the size if it is in conduit, and what type of conduit.

 

[jar546]

Can you actually get it if they don't know the specifics on the transformer....?

We recommend this app that they can use on their phone and send a PDF label right to their permit via email from the app.


It will generate this PDF with the calculation:

 

[jar546]

Can you actually get it if they don't know the specifics on the transformer....?

The letter from the POCO looks like this:

 

[jar546]

bump.
This is too important to ignore.

 

[ICE]

Who else is doing this?

Nobody but you. It happens after the equipment is installed and one in fifty inspectors get the AIC placard per 110.24.

 

[jar546]

Nobody but you. It happens after the equipment is installed and one in fifty inspectors get the AIC placard per 110.24.

In the real world outside of LA, it is part of plan review and marked on the plans for the inspector.

 

[jar546]

Is this part of your plan review since it is a requirement of the NEC?

 

[ICE]

110.9 Interrupting Rating.
Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that is available at the line terminals of the equipment.

Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that must be interrupted.

That second paragraph doesn't get the attention that it deserves.

 

[jar546]

110.9 Interrupting Rating.
Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that is available at the line terminals of the equipment.

Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that must be interrupted.

That second paragraph doesn't get the attention that it deserves.

Agree

 

[Mr. Inspector]

I don't require it at the plan review but do require the service to have the AIC placard by the final inspection. It seemed like I was the only inspector requiring it. They usually can't do it ahead of time because they don't know what transformer the utility company will be using or if the current one will be changed. I was never sure if the engineer of the project is required to do provide a sealed letter for this or if I could just get this from the electrician. I never get clear answers at the electrical seminars.

 

[jar546]

I don't require it at the plan review but do require the service to have the AIC placard by the final inspection. It seemed like I was the only inspector requiring it. They usually can't do it ahead of time because they don't know what transformer the utility company will be using or if the current one will be changed. I was never sure if the engineer of the project is required to do provide a sealed letter for this or if I could just get this from the electrician. I never get clear answers at the electrical seminars.

Do you mean the placard per NEC 110.24? If so, that placard is mandatory for commercial, just not residential one and two-family homes. The subject is compliance with NEC 110.9 & 110.10. How do you know if the equipment you are inspected is equal to or greater than the available fault current unless you have the calculation in hand, and stamped from plan review? I will answer that. You don't.

 

[jar546]

Nobody but you. It happens after the equipment is installed and one in fifty inspectors get the AIC placard per 110.24.

Inspectors don't 'get' a placard. It is permanently affixed to the service disconnect, otherwise the inspection fails.

 

[Paul Sweet]

I have always asked the power company for the short circuit current.

 

[jar546]

I have always asked the power company for the short circuit current.

Our FPL provides a letter with the available fault current at the secondary taps of their transformer. From that point to the service disconnect, the electrician is responsible for the calculation, otherwise we would have some pretty big numbers. For example, the available fault current at the secondary taps of the transformer can vary from 23,000A to 64,000A, but most often in the 35,000A range. By the time you take into consideration the size and type of the wire along with length from the secondary taps to the service disconnect, etc. it can be as low as 7,000A L-L or as high as 19,000A in residential areas. Commercial is a whole other ball game.

 

[ICE]

I am convinced that electrical engineers will specify a particular conductor and a length for that conductor with no knowledge of the physical layout. You end up with a design that has 80' of conductor for a service disconnect that is 15' from the transformer. It can be the same with series rating.

 

[jar546]

I am convinced that electrical engineers will specify a particular conductor and a length for that conductor with no knowledge of the physical layout. You end up with a design that has 80' of conductor for a service disconnect that is 15' from the transformer. It can be the same with series rating.

That is why we have inspectors in the field. To verify this stuff.