Category: Current Transformers

Easy Steps to Choosing the Right Current Transformer

There’s nothing more frustrating than arriving at the project site to complete a meter installation only to realize you do not have the right current transformer in your toolkit. Not having the right tools for any job is a waste of both time and money. To avoid this, a little pre-planning goes a long way. 

Current transformers are available in a variety of styles, sizes, and amperage ranges. They vary in output and accuracy as well. With so many variables, it is sometimes difficult to choose exactly the right CT for a project. Or is it?

Whether you need assistance with choosing the right CT, or have a related inquiry, contact us at DENT Instruments.

Answer the Following Questions to Find the Right Current Transformer

Selecting the right current transformer can be as easy as answering a few questions about your project, the site, and the goals. You may be able to answer some of these questions before even setting foot on your project site. Others, such as knowing whether there is a space constraint in your electrical panel, are best answered after a site visit. Answer a few questions up front to save major headaches down the road.

QUESTION 1: WHAT TYPE OF POWER METER ARE YOU USING?

One thing to keep in mind is that just because a current transformer is compatible with a meter does not mean that it’s the best choice. For example, did you know that all DENT CTs are compatible with both ELITEpro and PowerScout series instruments? Even though they work together, a clamp-on CT isn’t the best choice to use with a PowerScout. Why? Because part of the appeal of a clamp-on CT in the first place is that it’s easy and convenient to move between panels. In fact, you’re paying more for that extra convenience. The PowerScout, along with other submeters in the industry, are designed to be permanently installed so why pay for the convenience of a clamp-on when you’re not moving it anyway?

Some ways meter choice impacts CT choice:

  1. CT inputs- is your meter designed for mV output CTs or amp output? Common industry standards are 333mV, 1A, or 5A. DENT meters are compatible with 333mV.
  2. Will the meter be installed permanently (such as with a PowerScout or other submeter) or will you be moving the meter from location to location (such as with energy audits)?
  3. Does the meter have the ability to work with flexible Rogowski coils either by themselves or with an amplifier/integrator?

QUESTION 2: HOW MANY AMPS DO YOU PLAN ON MEASURING?

Perhaps one of the most important questions to answer is how many amps will be measured. You will typically know this ahead of a site visit because it’s typically dictated by your project goals. If your goal is to measuring a lighting load in a small office, the CT required will be much smaller than if you plan on measuring a full building load for a large complex.

Keep in mind that best CT performance occurs when the current flow is between 10% and 100% of the CT full-scale value. For example, let’s say you wanted to measure four lighting circuits with #12 wires and 20A breakers. When the lights are on, the amperage measures 45 amps. The ideal CT for this example is a 50A split core current transformer.

But what about a Rogowski coil? They are easy to install and work over a broad range. Keep in mind that the best CT accuracy occurs when the load operates as close to the full rating of the CT as possible. If the load is under 20A, generally speaking a Rogowski coil is not the right choice because it is simply too big for that load. In addition, current values below 5A may cause the meter to read 0 amps.

What happens if you move your meter between many different loads? Sometimes the best solution in this case is to keep two different sets of CTs in your toolkit – one set for small loads (for example, a set of 50A split cores) and one set for larger loads, like Rogowski coils. This way, you’re covered for many different environments.

QUESTION 3: DO YOU REQUIRE A REVENUE-GRADE CT?

Thinking about your project type and goals, it’s important to keep in mind what the end-data will be used for. If you’re doing a measurement and verification (M&V) project, a standard accuracy (1% accuracy) may be accurate enough to achieve your project goals. If you are using a revenue grade meter for tenant submetering or billing purposes, every bit of accuracy counts – and a revenue grade CT would be ideal.

Examples for when to use a Standard Accuracy CT:

  1. Load studies
  2. Measurement & Verification applications

Examples for when to use a Revenue-Grade CT:

  1. Demand Metering
  2. Tenant Submetering
  3. Tenant Billing
  4. Your meter is also revenue grade

QUESTION 4: HOW LONG WILL YOUR PROJECT LAST?

Some CTs are easier to install and move around than others. Available CT styles generally include:

  • Split Core – removable leg or hinge design
  • Clamp-On – clothes-pin design, one handed operation
  • Rogowski Coil – flexible “rope-style” CT
  • Solid Core – rigid; conductor must be inserted through window

Split core, clamp-on, and Rogowski coil CTs are designed to be installed without disconnecting any wires. With the solid core, you must disconnect the conductor to feed it through the window opening of the CT. This can be an inconvenience under certain circumstances and probably not handy if you plan on moving the meter around often.

No matter which type of CT you choose, if possible, always de-energize the circuit to be monitored and follow full safety precautions outlined in your equipment manuals.

QUESTIONS 5 & 6: HOW MUCH “FREE” SPACE DO YOU HAVE IN THE PANEL? HOW BIG IS THE CONDUCTOR TO BE MEASURED?

Space constraints can be a real problem in most electrical panels. It’s possible that your meter is not the only piece of monitoring equipment installed. When multiple meters and CTs are already crammed in, extra small or flexible CTs become even more attractive. (Note: NEC does not allow equipment to exceed 75% of the electrical panel space.)

Also important to consider: What is the size of the conductor that you’ll be measuring? Is it 20 gauge wire or are you measuring around a buss bar? A split core may be ideal for a small wire, but there’s no chance that will work around a buss bar. Generally speaking, CTs with larger window openings also are designed to measure higher amps.

NEED HELP SELECTING A CT FOR YOUR PROJECT?

If you’ve read through these questions and are still unsure of which CT is best, remember that we are here to help! Contact DENT Instruments to discuss your project requirements. We will help you select equipment tailored to your project needs.

Extending the Leads on Your DENT Current Transformer

One of the most frequently asked questions in our tech support department is this: “Can I get CTs with longer leads or extend the leads myself? What’s the maximum length for CT leads?”

Standard lead length for a DENTcurrent transformer is 3 meters (with a 7 meter option available for some models). Occasionally, because of electrical room layout or other factors, there is a requirement for longer leads on a current transformer. Therefore, extending the CT leads is necessary. The following is a brief overview of options for extending CT leads based on CT type. Note that the length recommendations listed below apply to PowerScout Series and ELITEpro XC instruments ONLY. To extend RoCoil leads for use with the TCA-5, please contact Tech Support to get advice specific to your application.

For questions outside the scope of this post, please contact DENT Technical Support.

Split-Core / Hinged Split-Core Current Transformers (Including Revenue Grade Hinged Split Cores)

MAXIMUM LENGTH

Split-Core Current Transformers leads can be extended up to 500 feet using extensions of the same wire type/AWG.

WIRE TYPE

600V Rated VW-1, 105°C, 20AWG, black and white twisted pair, cURus

PURCHASING OPTIONS
  1. Purchase wire by-the-foot for DIY splicing from DENT @ $1/foot using part number WIRE 20 AWG TWPR
  2. Wire may be purchased in 1000 foot spools from Interconnect Sales with part number UL1015-20-0-9-Twist

Sure-Connect sealed butt splices from McMaster-Carr (part 9895K13) which require a crimping tool and hot air gun. Always follow manufacturer’s instructions.

RoCoil Current Transformers

MAXIMUM LENGTH

RoCoil CT leads can be extended up to 100 feet using extensions of the same wire type/AWG.

WIRE TYPE

“Bare” RoCoil: Shielded cable with thermoplastic insulation, 1000VAC VW-1, 2x26AWG, 80°C, cURus

PURCHASING OPTIONS
  1. Purchase wire by-the-foot for DIY splicing from DENT @ 1.50/foot using part number CBL ROCOIL
  2. RoCoil mV ONLY: DIY splicing is not advised for RoCoil mV CTs. Contact us for details.

Sure-Connect sealed butt splices from McMaster-Carr (part 9895K11) which require a crimping tool and hot air gun. Always follow manufacturer’s instructions.

Solid Core (Toroid) Current Transformers

MAXIMUM LENGTH

Solid Core (Toroid) CT leads can be extended up to 500 feet using extensions of the same wire type/AWG.

WIRE TYPE

UL1015 24 AWG 7 Strand Black & White Twisted Pair (1 twist per inch), 600V rated

PURCHASING OPTION

DENT does not currently stock this wire. It can be purchased by the spool (10,000 ft) through Interconnect Sales.

Sure-Connect sealed butt splices from McMaster-Carr (p/n 9895K11) which require a crimping tool and hot air gun. Always follow manufacturer’s instructions.

FOR MORE INFORMATION

To download a PDF copy of this information, click here.

If you would like to order lead wire or need more information on CT lead extensions, please do not hesitate to contact DENT Sales or Technical Support.

Measuring DC Loads with an ELITEpro XC

The ELITEpro XC current input channels can accept +/-1VDC. The ELITEpro XC Portable Power Data Loggers analog channels can accept any sensor that has a 0-10 V or 0-20 mA or 4-20 mA signal. What does this mean for DC measurements? Non-DENT sensors may be used on the current and/or analog inputs to measure DC loads. Note that when the DC transducer is connected to the ELITEpro XC CT input, DC power will be reported as long as the DC voltage is connected to the voltage inputs. When the DC transducer is connected to the analog inputs, DC power will not be reported.

Below are two options for measuring DC loads with an ELITEpro XC instrument.

USING THE AEMC MR521 AC/DC CURRENT PROBE

The AEMC MR521 can be used either on the CT inputs or the analog inputs.

USING THE MR521 ON THE CT INPUTS

When using the AEMC MR521 on the ELITEpro XC CT inputs, you may select from two amperage ranges: 100A or 1000A.

  1. 150A:10 mV/A range = 100 Amps DC max
  2. 1500:1 mV/A range = 1000 Amps DC Max

USING THE MR521 ON THE ANALOG INPUTS

If the CT is used on the analog inputs, the full 150A or 1500A range can be used.

OTHER INFORMATION

Note that the AEMC MR521 has a 50 hour battery life and a 2-5% accuracy. List price is $389 (cannot be ordered from DENT). Also keep in mind that a minimum of 80VDC is needed to power the ELITEpro XC.

USING THE NK TECHNOLOGIES DT SERIES DC CURRENT TRANSDUCERS

The NK DT Series current transducers can be used on DC currents from 0-50A or 0-100A depending on the NK DT model chosen. The NK DT Series have 3 available output ranges that must be chosen at the time of purchase.

USING THE NK DT ON THE CT INPUTS

If you wish to use the NK DT on the ELITEpro XC CT inputs, select an NK DT model with 333mV out.

USING THE NK DTON THE ANALOG INPUTS

If you wish to use the NK DT on the ELITEpro XC analog inputs, select an NK DT model with 10VDC output.

POWER SUPPLY OPTIONS

The NK DT Series current transducer has two power supply options. If you plan on using the CT with the ELITEpro XC’s CT input channels and power the NK DT transducer via the ELITEpro XC’s 6VDC power out, choose the 5VDC power supply option. Cannot be ordered from DENT.

FOR MORE INFORMATION

For questions outside the scope of this document, please contact DENT Technical Support at: support@dentinstruments.com / 541-388-4774.

Optimizing Performance from Rogowski Coil Current Transformers

WHAT IS A ROGOWSKI COIL?

A Rogowski coil, named after Walter Rogowski, is an electrical device for measuring alternating current (AC) or high-speed current pulses. It consists of a helical coil of wire with the lead from one end returning through the center of the coil to the other end so that both terminals are at the same end of the coil. The whole assembly is then wrapped around the straight conductor whose current is to be measured.

Since the voltage that is induced in the coil is proportional to the rate of change of current in the straight conductor the output of the Rogowski coil is usually connected to an electrical (or electronic) integrator circuit in order to provide an output signal that is proportional to the current.

The relationship between voltage and rate of change of current is explained in the following equation where V is voltage and M is a constant:

WHY CHOOSE A ROGOWSKI COIL CURRENT TRANSFORMER?

Flexible Rogowski Coils, such as the DENT RoCoil, are designed for easy placement around cable bundles, large busbars, or within tight breaker panels. Their design has several advantages over traditional split core current transformers including:

  1. Open-ended, flexible design allows it to be wrapped around a live conductor without disturbing it.
  2. Rogowksi coils feature an air core rather than an iron core, which results in low inductance and faster response time to changing currents.
  3. Highly linear, even when subjected to large currents such as those used in electric power transmission, welding, or pulsed power applications.
  4. Largely immune to electromagnetic interference.

HOW TO MAXIMIZE PERFORMANCE FROM A ROGOWSKI COIL CURRENT TRANSFORMER

Rogowski coil accuracy is usually calibrated with the conductor centered in the CT window. In practice, however, the CT typically hangs on the conductor which can introduce measurement errors. Note that the error is greatest when the CT connector hangs on the conductor. The following diagram is an example of the type of error that could be introduced simply by moving the conductor to different positions within the opening of the Rogowski coil.

Best practice for Rogowski coil installation is to center the conductor as best as conditions allow. In addition, it’s a good idea to keep any other conductors (those you do not wish to measure) as far away from the Rogowski coil as is practical.

Also, keep in mind that although a Rogowski coil is ideal for large amperage loads, accuracy of the coil may be reduced on smaller loads (<20A). Additionally, the meter you are using may “snap to zero” if the loads is under a certain threshold. This may result in recording “0” during times when a small load is actually present (such as during after hours or weekends in a facility when most loads are shut off). Check with DENT Instruments for current transformer recommendations on your specific project – we would be happy to provide advice on choosing the right sensor.

For more information on DENT RoCoil current transformers, see the RoCoil page here:

Need a quote for current transformers or power meters? Contact us today!