# what is a commitment letter?



## TOU (Jan 30, 2009)

I recently designed a 600amp, 3phse, 120/208v main service for a building. first time around, the plan checker asked for a "commitment letter." i thought this was a "will serve" letter and contacted the utility company and they provided me with a will sere letter.

2nd time around, i got the comment back on the same issue saying that the county didn't want a "will serve" letter..but instead, they just want to "verify if this 600amp main service is able to withstand the available current from the utility transformer (installed by PGE) under potential fault conditions."

can anyone tell me what the plan checker is asking for and how to go about in getting what the planchecker want? what is a commitment letter anyway?

tou


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## paul d. (Jul 13, 2008)

call your CPA.


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## 480sparky (Sep 20, 2007)

Why not just ask the plan checker?

Sounds like he's asking for verification that the switchgear is rated for the transformer's available fault current.


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## micromind (Aug 11, 2007)

They do that around here too. Usually on a large service, the building department wants some sort of verification that the electrical installation can safely interrupt a bolted fault at any point in the installation. 

We ask the POCO for a fault-current study, and they'll give us a letter stating the maximum current they expect to see at the switchboard incoming line terminals. 

As long as the breakers/bussing, etc. can interrupt this level of current, they're satisfied, but they want it in writing. 

If, for example, the POCO can supply 17,000 amps of fault current, and the breaker is rated for 25,000 AIC, this is acceptable. If the main is rated for 14,000 AIC, a bit of re-design is in order. 

If you've ever seen the results of a breaker (especially 480 volt) that has interrupted more current than it can handle......

Rob


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## goose134 (Nov 12, 2007)

Since you are designing the services, it will be your responsibility to get a study done by the POCO on fault current. Usually and engineering firm will do them, but the end result is the same, utility company will tell them what they can and can't have.


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