Demand Load ???

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chicago66

Member
Hi friends,
I encountered a Breaker size issue on a sequence service switchboard.
Here is the information:
The connected load on a 500A circuit breaker is 412.5A but the demand load on the breaker is only 340A.
We have always been designing based on the fact that "Demand load" is always more accurate than "Connected load".
But the inspector argues that the load should be less than 80% of the breaker size.
I guess he was just giving us a hard time as they usually do...because that does not happen in reality...
Could anyone please tell me where in the code does it say that breakers have to be sized according to the "Connected" load?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Demand Load ???

"Connected load" means nothing. All components (e.g., conductors, overcurrent protection, and panels) is sized to handle the load that is calculated in accordance with article 220. That article takes into account the connected load, but then gives demand factors for some types of loads in some types of facilities. In addition, some loads have to be ?upsized? by 25%, as part of calculating the total load.

I do not know how you arrived at the ?demand load? value of 340 amps. But if you used article 220 methods, and that was your result, then a 500 amp breaker is acceptable. You would need to give more specific information, before I could give you more of a response.
 

chicago66

Member
Re: Demand Load ???

By "Connected load", I mean that the demand factors for the appliances are not taken into account per Article 220.
By "Demand load", I mean that demand factors are taken into account depending upon the quantities of the appliances.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: Demand Load ???

408.13 All panelboards shall have a rating not less than the minimum feeder capacity required for the load computed in accordance with Article 220.

And 220.10 states that the computed load of a feeder is determined by part I after the demand factors of parts II, II, or IV have been applied.

Thus, you do not have to size the panelboard for the "connected load", only the demand load. I'm not sure which code article extends this concept from the panelboard to the Main circuit breaker.

Steve
 

eesac

Member
Re: Demand Load ???

Most breakers are 80% rated, not 100% rated. Here is an example: 100,000 sf bldg, 50 kva of receptacle load, 5 kva of misc load. For a 80% rated breaker:
Lighting = 100,000 x 2.0 va/sf x 125% = 250 kva
Recept = 10 kva + (40 kva x 50%) = 30 kva
Misc = 5.0 kva
TOTAL = 285 kva (792 amps at 208v, 3-ph), 800/3 can be used.

For 100% rated breaker, use same calc above except do not multiply lighting by 125%, therefore Total load is 235 kva (653 amps) 700/3 100% rated breaker can be used.

Continuous loads need to add additional 25% for 80% rated breakers, not needed for 100% rated bkrs. Therefore if you add up all your loads, and multiplied by 125% of the continuous load and came up with 499 amps, you can technically use a 500 amp breaker. Loads do not have to be less than 80% of the breaker rating, even on 80% rated breakers, if they are calc'd properly. This is a very simplified calc, but I just want to try and explain 80% rated, 100% rated, and continuous loads.

Hope this didn't confuse you more.

[ March 11, 2005, 09:32 PM: Message edited by: eesac ]
 
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