Meet or exceed MAFC

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billybandit

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Location
Greensboro NC
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electrical contractor
Due to missing dead fronts, I am replacing (fire marshal mandated) Zinsco combo meter panels installed during the original buildout of this apartment campus in 1975.

The existing Zinsco main breaker is 200 amps and the breaker feeding into individual sub panel units is 150 amps. I cannot find any information indicating Zinsco kaic greater than 10k.

The local utility has calculated MAFC for some of the laterals at 28k and 32k respectively (all others at 11k)

I am looking for a meter combo (minimum 6-8 spaces) or other service configuration to handle the 28k and 32k fault currents.

Pictures of the existing Zinsco configuration and 22k replacement configuration are attached.after.jpgbefore.jpg
 
Series ratings were not very prevalent prior to 1985ish, so it is likely your gear from 1975 was only rated for 10kA.
 
POCO came to site and gauged wire at 350 and lateral lengths from 167kva transformer terminals to be 15ft and 20ft yielding 32MAFC and 26 MAFC respectively.
The previously mentioned 11MAFC laterals were distanced at 130ft.
 
FYI-- following are options presented to me from POCO based on their corporate policies.
  1. A full cost estimate to move the transformers further away and possibly go with a smaller KVA size if loading allows which could lower the fault current.
  2. Size your breakers to match the fault current and have the proper rating
  3. Customer to install CL fuses on customer owned equipment.
 
billybandit said:
POCO came to site and gauged wire at 350 and lateral lengths from 167kva transformer terminals to be 15ft and 20ft yielding 32MAFC and 26 MAFC respectively.
The previously mentioned 11MAFC laterals were distanced at 130ft.
Click to expand...
IF I did it correctly you have about 10451 available at the gutter. Now what are the details from there on.

eta: Assumed to be 1ph 120/240
 
billybandit said:
MAFC= Maximum Available Fault Current
Click to expand...
Which for many utilities means the worst case they may be able to deliver over the life of the equipment. It is common their supplied value is not the currently available value you might calculate yourself.
 
Last edited:
Thanks for the comments.
My original question is yet to be answered--

I am looking for a meter combo (minimum 6-8 spaces) or other service configuration to handle the 28k and 32k fault currents.

All responses so far suggest that the POCO could be wrong in their calculations.
I am stuck between the POCO and the local AHJ.

I am looking for references to specific manufacturers service equipment to resolve the issue; or suggestion for configurations such as --- current-limiting-fused disconnect between the trough and the meter base, followed by a main breaker/disconnect.
If I am out of line, please let me know.
I value your opinions/suggestions.
 
billybandit said:
Thanks for the comments.
My original question is yet to be answered--

I am looking for a meter combo (minimum 6-8 spaces) or other service configuration to handle the 28k and 32k fault currents.

All responses so far suggest that the POCO could be wrong in their calculations.
I am stuck between the POCO and the local AHJ.

I am looking for references to specific manufacturers service equipment to resolve the issue; or suggestion for configurations such as --- current-limiting-fused disconnect between the trough and the meter base, followed by a main breaker/disconnect.
If I am out of line, please let me know.
I value your opinions/suggestions.
Click to expand...
If you are on the 2020 or 2023 code, there is no equipment on the market that is code complaint for that application.
 
billybandit said:
Thanks for the comments.
My original question is yet to be answered--

I am looking for a meter combo (minimum 6-8 spaces) or other service configuration to handle the 28k and 32k fault currents.

All responses so far suggest that the POCO could be wrong in their calculations.
I am stuck between the POCO and the local AHJ.

I am looking for references to specific manufacturers service equipment to resolve the issue; or suggestion for configurations such as --- current-limiting-fused disconnect between the trough and the meter base, followed by a main breaker/disconnect.
If I am out of line, please let me know.
I value your opinions/suggestions.
Click to expand...
The POCO is never wrong. They are the source so they get to set the rules. They often want to use what looks like an absurdly high fault current so that the equipment, you install, is always adequate. They don't like being put in the position of telling customers to change what is existing. You are free to make additional calculations, if the AHJ accepts them, but the POCO values need to be your starting point.

As Don said, a meter-main may not be the way to go. Separate equipment is likely the only choice.
 
billybandit said:
Let me change my question---
How would you do this using information given so far?
Click to expand...
I would ask the AHJ if they would accept calculations based on as installed conductor lengths.
Otherwise your only solution is individual components using series ratings.
 
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