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Once upon a time, the only reason building codes addressed electrical metering was to enable local utility billing efforts. Today, though, metering is seen as a critical tool in efforts to boost our buildings’ energy efficiency. With the understanding that we can’t manage what we don’t measure, the latest energy standards are calling for greater use of submetering of individual building systems – and that means even greater use of current transformers (CTs).

Current Transformers – and How to Protect Them

Chuck Ross
Once upon a time, the only reason building codes addressed electrical metering was to enable local utility billing efforts. Today, though, metering is seen as a critical tool in efforts to boost our buildings’ energy efficiency. With the understanding that we can’t manage what we don’t measure, the latest energy standards are calling for greater use of submetering of individual building systems – and that means even greater use of current transformers (CTs).

In smaller systems, CTs might be installed directly on the panel. In these applications, the devices can measure the usage of individual circuits. This capability can help facility owners comply with the latest standards calling for submetering of lighting, HVAC systems, interior lighting, exterior lighting and receptacles.

CTs have an additional role to play in local distribution systems above 400 amps. In these installations – which could include high-end residential, commercial and industrial settings –  they literally take the load off utility metering equipment by transforming a high primary current into a low secondary current. The primary current continues into the customer’s service entrance, while the secondary current is used by the utility’s revenue meter to measure consumption. This protects that meter from the damaging heat such high amperages can create.

In these high-amperage projects, CTs are housed in dedicated cabinets. They’re most commonly installed outside a facility, so the unit’s NEMA rating is an important consideration – NEMA 3R is standard. With this rating, enclosures have been proven to prevent water ingress due to snow, sleet and light rain. Because these cabinets require ventilation, they do not offer Type 3 protection against windblown dust.
Photo courtesy of Milbank Manufacturing
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