Demand Load calculation

[JSelf]

I was wondering if somebody could answer this for me. If I have a demand load (lets say for lighting) of 125% I calculate my demand and then I go ahead and multiply that total by the demand of 125%. Now if I have a distribution panel that is feeding the panel with the for mentioned lighting load do I need to multiply that again by 125%? We are having a bit of an office dispute over this, I would say no because if the demand factor were 70% and you multiplied it by 70% a second time you would be underestimating that load on the main distribution panel. Any insight, We browsed through the code and didn't find anything (if its not true that could be why)

Thanks,
Jason Self

 

[peebee]

In general, the answer is no, you don't multiply demand factors against each other.

Think of it this way: calculate connected load. They all add directly such that your service entrance has a connected load equal to the total of the connected loads on all the branch circuits it eventually feeds.

Then, there will be a demand factor that applies to each level of the distribution system. Usually, that demand factor would tend to get smaller as you go on up the chain towards the service entrance. That's because the service entrance will see the average power to the whole facility, and generally will not be so affected by localized peaks in demand here or there -- in other words, when power is peaking way over on the west side, it's most likely NOT peaking way over on the east side.

Your post leaves me with a couple questions, though:

1. NEC, in Table 220.11, in no case lists any lighting demand factor over 100%. Here in Chicago, the local code does require 125% demand factor for lighting, but I think that has much more to do with unions than science -- I can't think of any rational explanation for how the measured demand would exceed the connected load, and I'd never heard of a demand factor over 100% before I moved here. Why are you using 125% -- local codes?

2. "If I have a demand load (lets say for lighting) of 125% I calculate my demand and then I go ahead and multiply that total by the demand of 125%." Have you multiplied by 125% once here, or twice? Sounds like twice already. Maybe you meant to say that you calculate your "connected" (rather than "demand") and multiply connected by 125% to calculate demand?

 

[tarzan56007]

Peepee please explain the science you used to determine that the Union in Chicago controls the areas electrical codes, I would think that the Chicago Inspection Department as well as the IBEW should take offense to that statement. I know that I personally do. P.T. Local #343, Minnesota.

 

[jbartos]

Suggestion: Reference: IEEE Std 100 "Dictionary" defines Demand Load = Demand
(1) (Electric Power Systems in Commercial Buildings) The electrical load at the receiving terminals averaged power over specified interval of time. Demand is expressed in kW, kVA, Amps, or other suitable units. The interval of time is generally 15 minutes, 30 minutes, or 60 minutes. Note: If there are two 50HP motors (which drive 45HP loads) connected to the electric power system, but only one load is operating at any time, the demand load is only 45HP but the connected load is 100HP.
(2) The average value of power or a related quantity over a specified interval of time. Note: Demand is expressed in kW, kVA, Amps, or other suitable units.
(3) The rate at which electric energy is being used.
(4) The rate of consumption, e.g., power, volume/hour.

 

[advidana]

your are geting continous loads and noncontinous loads mix up with demand factors they are not the same thing in the nec code.

 

[jbartos]

Suggestion: Reference: IEEE Std 100-2000 Dictionary:
Demand Factor Definitions:
(1) Power Operation. The ratio of the maximum coincident demand of the system, or a part of a system, to the total connected load of the system, or part of the system under consideration.
(2) Electric Power System in Commercial Building.
The ratio of the maximum demand of a system to the total connected load of the system. Notes: 1. Since demand load cannot be greater than the connected load, the demand factor cannot be greater than unity. 2. Those demand factors permitted by the NEC (e.g. Services and Feeders) must be considered in sizing electric system (with few exceptions, this is 100%); otherwise the circuit may be sized to support anticipated load.
(3) Ratio of the maximum coincident demand of a system, or part of a system, to the total connected load to the system, or part of the system, under consideration. The resultant is always 1 or less and can range from 0.8 to 1.0 to as low as 0.15 to 0.25 for some plants with very low diversity.
(4) The ratio of operating load demand of a system or part a system to the total connected load of the system or part of the system under consideration.

 

[peebee]

Re: "The resultant is always 1 or less and can range from 0.8 to 1.0 to as low as 0.15 to 0.25 for some plants with very low diversity." (as posted by jbartos),

Exception: here in Chicago, 125% demand factor is used all the time, it's written into the Chicago Electrical Code. Somehow, here in Chicago, it's apparently possible that the maximum coincident demand can exceed the total connected load.

tarzan: There's lots of requirements here in Chicago that seem to serve little purpose other than ensuring plenty of work for the electricians. The above discussion seems to be sufficient justification for this. I don't know why you personally would take offense to this as you're in Minnesota, not Chicago.