Determining the panel heat load

Tips for choosing a panel cooling solution.

By Doug Wilson

Typically mounted on a panel side wall, this air-to-air heat exchanger’s internal facing fan prevents hot spots from forming by providing a good air stir in the panel while drawing the heated air over the unit’s heat pipe core.

Whatever panel cooling solution you choose, the critical factor for a successful installation is assessing the panel’s waste heat load correctly. This loss, caused by the inefficiency of the electrical devices inside can be calculated with some accuracy or, more reliably, it can be measured by placing thermometers inside the panel to capture real-world operational readings. Both approaches have their shortcomings, however. Here are a few tips to achieve the best accuracy.

Calculating the waste heat load is critical when designing new panel installations, but it’s a somewhat inexact approach. With either measured load (in watts) or temperature readings, you can determine your cooling requirements. Predicting how devices might interact, how airflow might affect natural convection cooling and other variables means it’s best to base your estimate on the worst case.

Better to error on the high side and overcool than on the low side and discover the panel overheats. The best source is the manufacturer’s data on the device, which typically can be found online. Lacking that information, here are a few methods to estimate waste heat from common sources.

Modern variable frequency drives typically operate with 93% to 97% efficiency. Based on 1 hp being equivalent to 746 watts, you can determine the best and worst case heat load by doing the math. For example, a three-horsepower VFD would produce 3 x 746w = 2,238w x 0.07 = 157 watts of waste heat inside a panel.

Whatever panel cooling solution you choose, the critical factor for a successful installation is assessing the panel’s waste heat load correctly.

– Doug Wilson

Waste heat from transformers is harder to estimate accurately and can be a high source of heat. Power factor isn't going to have any effect on heat generation, except to cause more current to flow and increase the kVA load on the transformer. According to Cutler-Hammer, a 75-kVA, 150°F-rise, dry-type transformer has an efficiency of 97.2% at 1/4 load and 96.7% at full load. So, figure 3% loss at 75 kVA, which would represent 2,250 W. For greater precision, you need to know the amp load on the transformer and separate out the core losses (which are constant) and the winding losses (which vary as the square of the current).

The direct-sun solar load on an outdoor installation can represent 30 watts of heat penetrating the panel per sq. ft. of sun-exposed surface area.

This telcomm panel is being kept clean and cool, even in a tough outdoor installation.

Generally speaking, a PLC is a negligible source of heat, but as with the VFD calculation, base your estimate on an approximate 5% heat loss. Thus, if a PLC is rated at 1,000 watts and has an efficiency of 95%, then heat loss is 50 watts.

Measuring the waste heat load is a real-world operating measurement and is reliable, but offers its own set of challenges. Often, it’s impossible to measure a fully-loaded, operating panel with no cooling or ventilation in place (there’s a risk of failure from overheating during the measurement process).

In cool weather, it’s possible to perform this measurement because overheating generally is only a problem in hot summer months. Then the approach is simple and highly reliable. Place a few thermometers inside the panel, distributed top to bottom and not directly on or too near any heat sources. Run the panel at full load for 10 min. or more. Simultaneously, get an ambient air temperature within a few feet of the panel.

Now, you have the ΔT, the rise in panel temperature above ambient. If the highest panel reading is 91°F and the ambient is 72°F, your 19°F ΔT indicates that on hot summer days, when the facility ambient hits 85°F or more, your panel is now at 104°F and possibly heading for VFD failure. Bottom line, the ΔT is a constant and tells you exactly at what room temperature you’re going to experience problems.

Doug Wilson is applications engineer at Noren Products Inc., Menlo Park, Calif. Contact him at dougw@norenproducts.com and (650) 322-9500 x 239

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