kWh calculation

Hey guys,

I'm working on plans for a cabinet shop that was built without a permit. They got caught (after being in operation for years), and now they have to submit plans and get a building permit.

I know I can use the billing history from the Electric Company to show that the existing service, feeders and panels are sized properly based on the Average Monthly Usage or Total Usage for the past 12 months.

I would much rather do this than try to figure out how much power the CNC, Edge Bander, Table Saws, Vacuum system, etc. use and doing a load calc from that. My problem is that I am unsure how to show this data on the plans. Should I just note that the Total Usage for the past 12 months was 64156 kWh with a 208 3 phase service? Should I try to break this down more?

Thanks,
Dan

P.S.- This is in the Central Ohio Area.

I never put load data on the plans. I put panel ratings and conductor sizes, and I base my selections on loads. But if the authority wants to see the load data, I submit a separate calculation. For new projects, the load calculation is generally a set of panel schedules, with each panel?s loads including the loads from any sub-panel it feeds, and with the main panel?s loads including all downstream panels. In your place, I would put together a simple calculation that shows max demand per month over a 12 month period, that shows the largest demand over that period, and that shows there is at least a 25% spare capacity to account for the possibility of a larger demand in a future month. This is similar to the calculation I would do, if an existing facility is planning to add load. I show that the measured peak demand, plus 25%, still leaves room for the new load.

By the way, neither average nor total load is of interest here. What is important is the maximum load seen during the 12 month period.

Thanks Charlie! I'll have to call the PoCo later to get the peak power during the past 12 months. I just tried and their automated answering message tells me they are more concerned with the 17,000 people that still don't have power and to please call back at a later date.

Just out of wild curiosity, can I convert kWh to W by dividing the length of time in hours and multiplying by 1000? It seems like I'm missing a step.

DanZ said:

Just out of wild curiosity, can I convert kWh to W by dividing the length of time in hours and multiplying by 1000? It seems like I'm missing a step.

Click to expand...

You can. But it would be meaningless, for your intended purposes.

Tell me, if a particular highway has a speed limit of 60 mph, and if a person travels along that road for a distance of 180 miles, and if the trip lasts 3 hours, was that person ever at risk of getting a speeding ticket? Do you know enough to say for certain? Perhaps he stopped at a fast food restaurant, and perhaps he was gliding along at 45 mph for a while, and perhaps there was a 10 mile stretch that he took at 95 mph.

Just knowing the total distance traveled and the average speed does not tell you the peak speed at which the car traveled. Just knowing the total KWH used and the average watts does not tell you the peak demand.

I thought it would be worthless, but I also thought about how to convert it, and it seemed too easy. :wink:

I just wanted to make sure my grasp of the logic was right.

Thanks!

It may make sense for a place that had a fairly constant load during X hours of the day (e.g. lights and motors/machines for 10 hours a day). But most places are rather variable in load. Off hours things could screw this up too (computers and refrigeration that run 24/7).