Calculation for 400A all-in-one service

[hmspe]

"Standard procedure" for calculating a residential 400A all-in-one service (internal 200A 40 space panel plus a 200/2 breaker for a remote panel), at least in the engineering offices in the area I've polled, has been to do a dwelling calc per 220-30 (still on 1999 NEC here), show panel schedules, but not do calcs for the individual panels. Recently a plans reviewer rejected a plan set on the basis that a complete calc was required for each panel feeder in addition to the whole dwelling calc (ie, . Since the demand factors that are available for a panel calc are not the same as for the entire dwelling, I'm seeing a number of instances where the total dwelling calc may be only 300A but there's no way to distribute the loads and calc the panels so that each panel is under 200A. I'm getting lots of irate calls from contractors because the equipment costs are significantly higher for an installation with a 400A service with mains only .

How is this handled in other jurisdictions? All comments welcome.

 

[charlie b]

Re: Calculation for 400A all-in-one service

Speaking as a Design Engineer, I agree with the Plans Reviewer. Separate calculations are needed for the service and for any individual panel feeders. Each panel must have a rating that is sufficient to carry its load, and its feeder must be able to handle that same load. I handle this task by using a spreadsheet to create a "Panel Schedule" for each panel, with the calculation built into the spreadsheet.

The fundamental mathematics behind the demand factors lies in the realm of probabilities. The higher the number of loads (here, I?m not talking about the current drawn, I?m just counting the number of things that might draw current), the lower the probability that all of them (or even most of them) will be on at the same time. A service has a greater number of connected loads than does a sub-panel. Therefore, we get to use a smaller demand factor for a service. For example, the home might have five appliances that are fastened in place: three on one panel (serving the kitchen) and two on another panel (serving a wet bar). 220.17 gives us a 75% demand factor for the service, but that factor cannot be used on the feeder to either panel.

 

[hmspe]

Re: Calculation for 400A all-in-one service

Charlie,

I agree with your assessment, and my preference is to do all the calcs. What I'm running into is three issues: (1) electrical contractors who complain about not being able to use the all-in-ones that they've been using on similar houses for years and about the added cost. (2) electrical contractors who don't follow the plans, do what they've been allowed to do for years, then expect me to change my design after the job gets redtagged. (3) complaints from clients because the PE down the street still uses the "old method", so my designs are thousands of dollars more to construct than his.

If all the local jurisdictions could agree on the acceptable calculation method....