Panelboard Load Calcs per NEC
Panelboard Load Calcs per NEC
(OP)
I know that to estimate the load for a building the receptacle demand is calculated by taking 100% of the first 10kW plus 50% of all recept load over 10kW.
I have been told that this applies to panelboards also, however this does not seem logical,i.e. suppose I have 5 panels and they each have 20kW of receptacle load. If I follow the rule and take 100% of the first 10kW on each panelboard I already have 50kW of load. How does this square with the rule for the entire building that I only need to allow for 100% of the first 10kW for the entire building receptacle load?? Where is this found in the NEC??






RE: Panelboard Load Calcs per NEC
NEC just says non-dwelling units, so Table 220.13 applies to the entire facility, AFAIK. Since NEC uses 180 VA per receptacle and takes 50% of that value for demand, that's generally conservative in most facilities when you average this over all the receptacles.
Obviously this is a **minimum** requirement. If you know that there will be specific loads connected, you need to account for these.
RE: Panelboard Load Calcs per NEC
The more load a given board feeds the more diversity it gets to take.
RE: Panelboard Load Calcs per NEC
RE: Panelboard Load Calcs per NEC
RE: Panelboard Load Calcs per NEC
The rule you indicated is for 'service' sizing, not panelboards.
For panelboards as davidbeach said..see article 220.
As you come closer to the loads, the demand factor goes up and you have to rely on more accurate information on loads.
You just can not generalize one rule meant for certain applications and apply to all.
RE: Panelboard Load Calcs per NEC
RE: Panelboard Load Calcs per NEC
I'll usually take lighting at 100%, HVAC at 75%.
Receptacles somewhere between 50 and 100% depending on type of installation (eg, in an office, I'll take every PC receptacle at 240va and 100% or so, you'd be nuts to say only 50% of the computers will be running at a time).
My calcs might be slightly conservative, but I'd rather have a bit of spare capacity than not enough. . . . Clients get much more upset about main breakers tripping than a bit of extra capacity. . . .
RE: Panelboard Load Calcs per NEC
I believe everyone that has ever designed an electrical system would agree that it is very unlikely that all of the connected load of an electrical system is in simultaneous use, and hence has a demand load of less than 100%.
Your very example indicates that. If you assume that the nameplate of a PC is wrong, it is still very unlikely that a PC will draw more than its power supply rating, which can range from 200 to 500 watts. And yet if you read the National Electrical Code very carefully, especially article 220-3(B)(1): "Specific Appliances or loads. An outlet for a specific appliance or other load not covered in (2) through (11)shall be computed on the ampere rating of the appliance or looad served".
I don't believe that the use of demand factors is conservative. Conservatism would be strict adherence to article 220-3(B)(1)in the face of reality.
RE: Panelboard Load Calcs per NEC
I have to second peebee..
The receptacles for computers or equipment that you know will be connected do not count toward "general purpose" receptacles. General purpose would be those they are there for convenicence for once in while use such as cleaning or temporary heating or portable lighting etc..
You have to take all loads that you know will run at all the time or even most of the time, need to be taken at 100%.