jd77

I have a hotel I'm bidding on with 60 rooms, each room has a FLA wall heater of 25 amps 240 volt (single phase). I figured 6000VA for each unit
6000VA x 60 units = 360,000VA

I= VA / E 360,000 / 240 = 1500 amps
1500 amps towards the total service calc from heat (larger load than the AC)
I beleive this 1500 amps would be at 100%, but in the NEC 250.51 has an exception that gives the local AHJ authority to change the %.

How can the AHJ even think that all 60 rooms in the middle of winter couldn't be sold out and all using there heat.

Has anyone ran across this before??

MDShunk

I will only calculate the heat at less than 100% if I've personally done a ACCA Manual 'J' calculation such that I know the heat is oversized. If the heat is oversized by a certain percentage, then I'd go less than 100% by approximately an equivalent amount to the oversizing.

The only two motel jobs I've been involved with were engineered, so no calculations were involved on my part. A PTAC with resistance heat is the typical motel heat and A/C source. They don't come in too many different sizes, so yours may be oversized slightly because of that. Hard to say.

frank

JD.

The arithmetic rule you are querying is called 'diversity'. A safe rule of thumb is to apply 25% diversity to your maximum demand rule. So if you calculate a max demand of 100kVa then supply distribution protection etc to fail at 75kVa. This difference is based upon the chance rule of occupation and opperation of themostats etc switching on and off the load etc. It is a good safe rule and used by engineers everywhere. Just think about your household electric cooker. If you cabled etc to allow for all elements.ovens.grills.hot plates to be on all day you would really have to overwire and protect. Local/National regulations may apply to your calculations so go with these but you would be safe to apply 'diversity' and so save some installation costs.

Frank Doncaster England