Plug loads estimate from electrical wiring capacity

plug-load

asked 2016-12-06 10:24:42 -0500

Amir Rezaei
841 ●1 ●7 https://www.linkedin.c...

This is mostly a plug loads estimate question. Recently, while working on a hotel project we were asked to estimate the PV capacity required for which the energy model developed for LEED was requested to be used.

The electrical engineer's estimate of total building electricity need came much higher than the one from the energy model. After investigating it turned out that the electrical engineer was basing his estimate on the total electrical wiring capacity which came to about 12 W/ft2 of electrical load versus the energy model which was based on traditional plug loads assumptions amounting to about 2 W/ft2 including lighting. This hotel has suites with major appliances and that is why the total wiring load Wattage is quite high. We weren't able to find resources to justify the actual diversity factor for the building loads and went with the assumption that very few guests use those appliances on a regular basis plus the fact that the building occupancy rate is well below 100%.

It gets harder to estimate loads for a hotel project but one can imagine a similar conversation about an office building. I am wondering if anyone has experience with estimating plug loads from the electrical wiring calculations that can share? Obviously, measured data would help with these sort of questions but I am specifically interested in best-practice examples that can be drawn from the electrical design documents with reasonable accuracy.

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answered 2016-12-06 13:03:15 -0500

mdahlhausen

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For a standard office space, the electrical designer may specify a certain number of receptacles per wall length or area, and build in capacity for space changes etc. The wiring capacity is very conservative (each receptacle at full load, summed, with a safety factor, and with no diversity factor or a very high value). Taking a diversity value on the electrical drawings is compounding several layers of somewhat arbitrary safety factors and load percentages. I think it is better practice to use measured plug load data to calculate the W/sf gain, and just say that the calculation for "expected peak electrical equipment/lighting gain in the space" from an energy modeling perspective is different than the calculation of "appropriate risk reduction to avoid tripping breakers / starting electrical fires".

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Thanks @mdahlhausen. Measurement is obviously the best way to approach this. I just keep wondering if there is a published document somewhere, perhaps from a utility company or ASHRAE, that has good estimates based on the wiring capacity calculations!

Amir Rezaei ( 2016-12-06 15:10:18 -0500 )edit

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answered 2016-12-09 17:57:55 -0500

Nick Caton
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updated 2016-12-10 11:01:15 -0500

I've done a fair amount of electrical design for commercial / residential buildings and can affirm, there likely isn't any clear line to be drawn in most cases between electrical distribution/service calculations and trended electrical load profiles. Electrical distribution design (per NEC in the USA, at least) has a lot of safety built into the standard loads & sizing factors we attribute to receptacles, motors, lights, etc. We depend on those guidelines and upsizing factors to ensure we do not undersize our systems and cause a life/safety hazard (i.e. fires).

More productively (kinda?), I suppose you could walk a path of using the associated feeder/circuit over current protection (fuses, circuit-breakers) to determine a ceiling on sustained (this is a loaded word - in electrical design terms I'm thinking minutes, not seconds) amperage draw, for the purposes of gut-checking the order of magnitude that your simulation's inputs produce for a peak demand interval... but if you polled 100 electrical designers I expect 99+ would sincerely hope that real world measurements would bear out some distance in peak sustained draw from what they sized up, else somebody is in hot water!

Long story short: No, I don't believe you can use electrical power distribution calculations to productively estimate something like a plug load density or a plug load usage profile.

Your electrical designer's calculations / specifications concerning large motor efficiencies, lighting, the likes of VFD's however are probably of much interest to the typical building simulation, so you should still buy him/her a beer when you have the opportunity!