How do Utility Departments Size Transformers?

[gobblerhuntr]

w/sf is watts per square footage. That is a number that we typically express for cosumption or demand of a building, we use this in the beginning phases of a project when someone needs a quick size or if we review an existing building for an owner. Typically a DESIGN load is 12-15 w/sf for commercial space but the actual DEMAND load is 6w/sf.

Never used w/sf, have known others that did, I guess I just stick to the method of what has worked in the past. But always open to new ideas. I may do some comparisions of some future jobs and see how the w/sf compare.

 

[lile001]

Magic Number

Magic Number

gobbler,

is there any particular "magic number" such as a w/sf that you would use for a commercial property to size the transformer or is it typically a percentage number of the NEC calculated load, which we all know to be oversized?

Thanks

The local POCO engineer told me he takes whatever i give him for NEC connected load calcs and multiplies it by 2/3 for a standard commercial building. For industrial, he might treat it differently, up to 100%. His transformers have a big overload capability, 125% IIRC. You would only see the full connected load if all of the HVAC was going at once, all of the outlets had things plugged into them, all of the kitchen appliances were going full blast and all of the lights and motors were on. This might happen for a few minutes one day but lots of that stuff cycles on thermostats, is turned off, or doesn't have anything plugged into it. Transformer overload capacity can handle this for a short time. That's how he gets away with 2/3.

Also, if he has a demand meter, he'll know pretty quick if he's made it too small, if he bothers to check the bills.

 

[bwyllie]

gobbler,

please keep us updated on your findings.