Mike Dolt Enterprises
Mike Dolt Enterprises
User: William Teller, Location: Texas
Asked on: Wednesday, June 2, 2021 at 8:45 pm EDT
Answered on: Thursday, June 20, 2024 at 11:41 am EDT
Question: I occasionally design renovations to existing facilities. Sometimes I have accurate as-built drawings that show the existing load of a panel and sometimes not. If you count 80 percent of the capacity of each branch breaker that is connected, you often come up with an existing calculated load that is far greater than the main breaker of the panel. Now if you have a main panel and disconnect that is rated at 1000A and have 4 sub distribution panels that are 600A and the load is more or less evenly distributed, then I would think that you could divide the peak demand load (in this case 425A at the service.) by 4 to estimate how much load is on each sub distribution panel. But, I was told you can not do this by a senior electrical engineer. I was told that if you add load to a panel, even if it is say 13 amps on an unused 20A 120A/1P breaker you would have to assume: 1) all of the peak demand load on the service is on the distribution panel that feeds that panel and that most of that load is on the branch panel you are updating. Or, 2) you have do a load analysis of that upstream distribution panel and every panel that is fed from it. Does that seem reasonable to you.FYI, doing a 30 day load measurement is often not practical and the same senior engineer says that you have to do a load measurement of the panel and upstream distribution panel for a whole year to have meaningful results.I appreciate any wisdom on this mater.
As-Built Drawings Not Available
When accurate as-built drawings are not available, estimating the existing load of a panel can be challenging. While it may be tempting to use a percentage-based approach (e.g., 80% of branch breaker capacity), this method can often overestimate the actual load.
Peak Demand Load Distribution
If the peak demand load is known, distributing it evenly among sub-distribution panels can be a reasonable estimation, but it has certain limitations: Load Variation
The load on each sub-distribution panel may not be evenly distributed, and some panels may have higher loads than others.
Voltage Drop Considerations
Excessive load on a single sub-distribution panel can result in voltage drop issues, which may affect the performance of electrical equipment.
Senior Electrical Engineer's Perspective
The senior electrical engineer's concerns are valid. Adding load to a panel, even a small amount, can potentially increase the load on the upstream distribution panel and main breaker.
Load Analysis
A thorough load analysis of the upstream distribution panel and all panels that are fed from it is the most accurate way to determine the existing load. This analysis involves calculating the total connected load of all equipment and appliances connected to each panel.
Practicality of Load Measurements
While a 30-day load measurement can provide valuable insights, it may not always be practical in all situations. However, a year-long load measurement is not necessary for meaningful results. Short-term measurements (e.g., 7-14 days) can provide a snapshot of the typical load profile and identify potential issues.
Recommendations Obtain As-Built Drawings
If possible, always request as-built drawings to have an accurate representation of the existing electrical system.
Use a Conservative Approach
When as-built drawings are not available, use a conservative approach in estimating the existing load.
Consider Load Analysis
If the calculated existing load exceeds the panel or main breaker capacity, perform a load analysis to verify the actual load.
Monitor Load Measurements: Short-term load measurements can be helpful in identifying potential issues and verifying the estimated load. Flag for review