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Field Test Best Practices: A Resource for Practical Residential Building Science

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Electrical Energy

In many field tests, the electrical energy use of the house as a whole or a specific component is the focus of the project. In retrofits, the goal is to decrease the energy use of the home. In new homes, there is typically an expected energy use of the house based on models. For any projects involving a novel system, like a new type of heating, ventilation, and air conditioning (HVAC) equipment or lighting system, measuring the energy use of that system alone are of interest. As new homes become more efficient, plug loads like TVs and computers become a larger part of the home's electrical usage and so those items may need to be monitored separately as well. Regardless of the intent of the field test, electrical energy will likely be measured for some purpose.

Safety in Electrical Panel Work

Working on a high voltage system is extremely hazardous. Work inside of a circuit breaker panel (such as hardware installation and wiring) must be done by a qualified electrician.

Current Transformer

Current transformers (CTs) allow measurement of current flow without interrupting the circuit of interest. They are commonly used in buildings field testing where inserting an ammeter in series with the circuit is impractical. A CT that measures alternating current (AC) is made of a toroidal (or similar) magnetic core with a length of wire wrapped around the torus to form a secondary winding. When a current-carrying conductor is passed through the CT ring along its cylindrical axis a magnetic field is induced in the CT core, which in turn induces a current in the secondary coil that is proportional to the current through the ring.

Calibrated Resistor / Shunt for Direct Current (DC) Measurements

Photo of a calibrated resistor shows two metallic squares with bolts on the top connected by metal.

Calibrated resistor

A calibrated resistor (also called a shunt) can be used to measure direct current (DC) with high precision. The current of interest is passed through a resistor of known resistance, R, and the voltage drop V is measured across the resistor. Per Ohm's Law, I=V/R. These types of measurements may be required when investigating the energy use characteristics of a DC component within a system. For example, you could use a shunt to measure how much power the fan is drawing inside of a furnace.

Electrical Energy Use of an Appliance on a Dedicated Circuit

Many major appliances such as water heaters, clothes dryers, and air-conditioners are powered on dedicated circuits so that the electrical energy consumption of a particular appliance may be measured at the main circuit breaker panel. In such cases, the measurement approach is nearly identical to that used to measure whole-house electrical energy consumption using (CTs) at the breaker panel.

Electrical Energy Use of Individual End Uses

Sub-metering at the appliance level may be straightforward or complex, depending on whether the appliance is powered via a dedicated circuit or if it is plugged into a wall-outlet that shares the circuit with other appliances. If the appliance is on a dedicated circuit, the electrical energy use can be measured at the breaker. In the other case, where the appliance in question is not on a dedicated circuit, there are several options.

Whole-House Electrical Energy Use

There are several approaches to collecting whole-house electrical energy use data. The most common method for acquiring research-quality data is to install current transformers and a watt transducer at the main circuit breaker panel and record the measurements with a data-acquisition system. If the home is equipped with an advanced metering infrastructure (AMI) meter, it may be possible to collect the information from the meter via a wireless home area network (HAN) connection to an automated home energy management (AHEM) system. Some manufacturers of AHEM devices allow connection to automated meter reading (AMR) type utility meters as well, using a translator box. There are pros and cons to each approach. Relying on wireless communication from the meter head avoids the hassle of having to hire an electrician to work in the breaker panel; however, the sample rate of the data will be limited by what the meter is capable of reporting. Device selection will depend on the resolution requirements of the analysis approach you plan to take as well as logistical considerations for the test site (space and access issues, for example).

Plug Pass-through Watt-Hour Meters

Some major appliances and heating, ventilation, and air conditioning (HVAC) components are powered on dedicated circuits so that their energy use may be monitored directly at the circuit breaker panel. Often, however, we are interested in monitoring individual appliances and electronics that are plugged into receptacles on shared circuits. One way to make these measurements is to create a stubby extension cord that separates the hot, neutral, and ground wires and has two additional wires to tap the voltage, so that you can easily use a current transformer (CT) to measure the supplied current and a watt transducer to calculate the power in a similar way to how these measurements are made at the circuit breaker panel.If you are looking to make a quick measurement or monitor the appliance for a short period of time, and don't require the data to be recorded using the same DAQ as other monitored systems around the house, then a much easier way to measure the electricity use of plug loads is to use portable plug watt-hour meters at the receptacle.