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The Midwest CT Series


  • The VA output capacity is from 2VA to 15VA.
  • Standard leads are 24" in length.
  • Screw terminals are available at no extra cost when specified on order.
  • 600 Volt insulation is standard. A High-Voltage option is available at an additional cost.
  • Optional taps and optional mounting brackets are also available.

Options for the CT Series:

  • Mounting Bracket
  • Special Lead Lengths
  • Extra Taps
  • Fungus Treatment to meet Military Specifications

The donut transformer can be ordered with the optional mounting bracket for quick installation. CARE MUST BE TAKEN TO ENSURE THE SECONDARY LEADS ARE CONNECTED AT ALL TIMES when current is passing through the primary conductor.

The Midwest current transformer core is made with grain oriented steel. The core is insulated and then wound with the secondary windings. The 24" leads are permanently fastened in place and then a PVC coating is applied over the entire current transformer. Optional mounting brackets are available.

Ratio Modifications

The current ratio between the primary and secondary winding is defined by the following formula:

Ns x Is = Np x Ip


Ip = Primary Current
Is = Secondary Current
Np = No. of turns on the Primary Winding
Ns = No. of turns on the Secondary Winding

On a 300:5 current transformer,

Is = 5 Amps when Ip = 300 Amps,
      the number of primary turns is 1.
Ns x 5 = 300 x 1
Ns = 60

The ratio of the current transformer can be modified by altering the number of secondary turns. Forward or backwinding the secondary leads through the window of the current transformer will add or subtract secondary turns respectively.

By adding secondary turns the same primary current will result in a decrease in secondary current output. By subtracting secondary turns the same primary current will result in greater secondary output.


On a 300:5 current transformer, if a 325:5 current ratio is desired it is necessary to add five turns to the secondary winding.
Ns x 5 = 325 x 1
Ns = 65

Deducting 5 secondary turns will create a transformer with a current ratio of 275:5.
Ns x 5 = 275 x 1
Ns = 55

Ratio Modifications are achieved as shown:

Ratio Modification drawing 1
To add secondary turns, the white leads should be wound through the CT from the side opposite the polarity mark.
Ratio Modification Drawing 2
To subtract secondary turns, the lead from the left terminal or the white lead should be wound through the CT from the same side as the polarity mark.
Current Transformer Polarity
Ratio Modification Drawing 3
When wired as shown, the voltage on the white lead will be in phase with the voltage on the power lead connecting the line to the load.