DB Loading Schedule & Load Calculations
In this article, we will discuss how to prepare DB loading schedule, and the branch circuit load calculations related to it including, total connected loads. loads unbalance %, total demand loads, demand factors, and max. demand loads.
The distribution board is part of the distribution system, which distributes electrical power to branch circuits, feeding all the loads (lights, sockets, mechanical loads, etc). Create a DB loading schedule shows how the loads are distributed to the three phases (red, blue, yellow), making the maintenance easier, and helps to achieve load balance across the three phases.
In terms of installation, there are two types of DB that commonly used the Surface mount, and flush mount. Surface mount means the DB install on the wall while the flush design to fit inside hole inside the wall.
Firstly, as seen in the below gif, just gather all the data regarding your DB like panel No., IP rating, main breaker size, location .. etc.
Let’s say we want to prepare a load schedule for the following loads list. it’s required a list for all loads beside it rating and information.
When distributing the loads on the three phase feeder (red, blue, yellow), we should take into account achieve the load balanced system where the sum of the current passing through three lines will flow through the neutral. In balance condition, there will be no current flow through the neutral. It is recommended that the load unbalance percentage does not exceed 10%.
As shown, there is two side in the load schedule we will start to distribute side by side in each line as shown below:
Note that, any edit in the load schedule will compulsory an edit to the electrical layouts as well.
After that, calculate the total connected load (T. C. L.) that equal to the summation of the loads connected to each phase. T. C. L. is the total power consumed by all the loads in case all will run at the same time.
Load unbalance percentage shall not exceed 10%, and it can be determined by:
But this result will be true only if all loads are running at the same time (Demand factor=100% or 1). Since the case of the total connected loads is can be impossible to occur or some loads may be off for a certain time. This will introduce us to the concept of the total demand load and the max. demand load. Total demand load equal to T. C. L. with applying demand factor.
NEC defines Demand factor as the ratio of the maximum demand of a system, or part of a system, to the total connected load of a system or the part of the system under consideration. National electric code, 2017
Demand factors values vary depending on the load type applying demand factor for lighting loads referring to Table 220.42 in the NEC, Heating and Air-Conditioning Load demand factors are illustrated in 220.82 (B) Dwelling Unit. Table 220.56 illustrated demand Factors for Kitchen Equipment. Read NEC article 220.
Max. demand load is used to size the components of the system like C.B, feeder sizing, is same as the demand factor but with considering Future extension/growthing factor in the calculation and it can be calculated as follow:
Back to our example, consider branch-circuits with connected loads of 900 VA, 1489 VA, 2300 VA and 2405 VA and demand factors of 100%, 90%, 75% and 65% respectively.
- Total connected load = 2233+2465+2396= 7094 VA
- Unbalanced % = (2465–2233)/2465=(232/2465)*100=9.41%
- Total demand load = (900*1)+(1489*0.9)+(2300*0.75)+(2405*0.65)= 5528.35 VA.
- Max. demand load = 5528.35*125%= 6910.44 VA
With this, we come to the end of this article. I would like to thank Eng. Ahmed elsobky for his amazing course on udemy. Also, thanks to everyone who read my posts. If anyone has comments or suggestions on how to improve the post, please add them below or you want to share more information about the topic discussed above, please don’t hesitate to discuss through Gmail or add me on LinkedIn