Panelboard Load Calcs per NEC 1

[Chuk1t]

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??

 

[dpc]

Article 220.13 (2002 version) This is only one option for load calcs in the NEC - you really have to read the whole article. This rule applies only to non-dwelling units, so apartment buildings, etc are not covered by this - they have their own rules. In the average building there are a lot of receptaclees that end up behind bookcases, etc and are never or rarely used.

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.

 

[davidbeach]

As dpc says, that rule only applies to nonresidential and it applies to each location in the system. So each panel board has to be sized for 100% of the first 10kW connected plus 50% of the remaining connected to that panelboard. Moving upstream, where multiple panels are fed from a single board, that board is sized for 100% of the first 10kw plus 50% of the remaining that it feeds.

The more load a given board feeds the more diversity it gets to take.

 

[mad2000]

how do i determine recept/lighting load calcs.

 

[davidbeach]

NEC Article 220.

 

[rbulsara]

chukit:

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.

 

[edunnpe]

Panelboard sizing is related to feeder sizing. The calculations for feeders applies to the panelboard bus. If the individual circuit breakers or fuses are protecting branch circuits (as opposed to feeders), then branch circuit sizing of the overcurrent device and branch circuit conductor applies. In the case here where the first 10KW of receptacle loads is taken at a demand factor of 100% while the balance is computed at 50%, this is a feeder calculation. And yes, the formula applies at each next feeder above all the way to the service conductors. NEC Articles clearly indicated whether the calculations are for services, feeders, and branch circuits as defined.

 

[peebee]

For what it's worth -- 100% x 10kW + 50% of load >10kW seems like it's cutting things just a little too close for many many installations.

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. . . .

 

[meihm]

Peebee, with respect, article 220-13 establishes permissible demand loads for general purpose receptacles rated at not more than 180 VA each.

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.

 

[rbulsara]

mehim:

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%.