AIC Calculations & It's Significance

[mivey]

Certainly possible. However I cant remember the last time I saw a commercial service fed from a non-dedicated bank. Dwellings sure, they do that all the time.

Look in urban setting or other dense areas. Also look in an industrial complex with multiple buildings in close proximity.

 

[mbrooke]

not according to utilities
on many systems the source is not the limit, the xfmr is

in the op's case it is 20 mva or 870 A at 13.2
even if we assume it is off by a factor of 2 it is >430 A

Correct- but the source can (and does) play a role.

 

[mbrooke]

Certainly possible. However I cant remember the last time I saw a commercial service fed from a non-dedicated bank. Dwellings sure, they do that all the time.

Outside of secondary city networks- very true. Even then spot networks are common for some 277/480 volt customers.

 

[mivey]

Correct- but the source can (and does) play a role.

Yes. Utilities will try to keep the source relatively stiff compared to the load.

A softer source may require an over-sized transformer to help compensate for volt drop during motor start, especially for very large motors of comparable size to the normal transformer kVA. A softer source will further limit the size of smaller across the line start motors and/or their re-start frequency.

 

[mbrooke]

Yes. Utilities will try to keep the source relatively stiff compared to the load.

A softer source may require an over-sized transformer to help compensate for volt drop during motor start, especially for very large motors of comparable size to the normal transformer kVA. A softer source will further limit the size of smaller across the line start motors and/or their re-start frequency.

Yup- and well said

 

[m sleem]

Can you run the 200A feeder a little longer and drop the short circuit current down to 10kA?

The 200A panel short circuit rating must be equal to or greater than the calculated available fault current at the end of that feeder terminating in the panel.

Nice idea!:thumbsup:

 

[ekbrunn]

Nice idea!:thumbsup:

The only thing better than the NEC is having a group discuss it the particulars of a requirement. I love this profession!!! The discussion has gone all over, and the information has been very valuable, but I am not sure the OP has gotten a suggestion he wants.
Yes, "it is what it is," but looking at the load of 6.5 amps, I would ask if the load on the panel warranted a 200 amp feeder. I would look at the possibility of reducing the feeder size and increasing the impedance that way. I have seen drawings where the EOR has required a minimum circuit length to add impedance. This could be done if the reduction in conductor size was not sufficient.
Going from a 4/0 AWG to a #3 AWG (as a 100 amp feeder/circuit breaker) adds a lot more impedance to reduce the available fault current, and increasing the length to approximately 55 feet brings the level below the 10k threshold he probably needs. If there needs to be more capacity for growth, since this seems to be a house panel, provide a second available space for a 200 amp circuit breaker for an additional house panel.
This mitigates the upfront expenses and allows for future loads.
Alternatively, install a one-to-one Y/Y transformer.
Just a suggestion...