Current Transformers are devices which convert high power signals to lower, more manageable signals. The most common type of transformer is the "donut style" transformer. This transformer is a ring which encircles the wire carrying the signal and via induction transmits a proportional signal through two leads attached to the donut.
Current Transformer Glossary
Donut
Donut is the slang term used to describe the class of current transformers which are shaped like a toroid.
Primary Line
The primary line is the line which carries the current to be measured. For normal operation the primary line will pass through the center of the transformer once.
Primary Wrap
It is possible to manipulate the ratio of a transformer to make it fit into a specific application. One way of doing this is to add primary or secondary wraps. A primary wrap is added whenever the primary line is passed through the center of the current transformer. The process of wrapping a transformer is explained in detail here.
Ratio
The ratio of a current transformer indicates the multiple between the current in the secondary lines and the current in the primary lines. For example: a 50:5 transformer will transmit 5 Amperes through the secondary line when the primary line is carrying 50 Amperes.
Secondary Line
The secondary lines are the two smaller lines which carry signals from the transformer to the measuring device. These lines usually carry much lower current than the primary.
Secondary Wrap
It is possible to manipulate the ratio of a transformer to make it fit into a specific application. One way of doing this is to add primary or secondary wraps. A secondary wrap is added whenever one of the the secondary lines is passed through the center of the current transformer. The process of wrapping a transformer is explained in detail here.
Toroid
Toroid is the proper name for the shape of most transformers. A toroid is a solid ring with a hollow center (much like a donut)
Wrap
It is possible to manipulate the ratio of a transformer to make it fit into a specific application. One way of doing this is to add primary or secondary wraps. The process of wrapping a transformer is explained in detail here.
Related Topics:
Chart of Donut Style Current Transformer Ratios
Current Transformer Wire Length Table
As the distance between the transformer and meter increases, the signal intensity falls. For all of the current transformers the maximum distance is determined by its VA burden and also the VA burden of the meter being used. Here is a table of the maximum recommended wire length for all of our current transformers using the recommended 16 guage copper wire.
Catalog Number |
CT Ratio |
Burden VA |
Analog (.2 VA) |
Digital (1 VA) |
01293 |
50:5 |
2 VA |
9 Ft. |
5 Ft. |
01306 |
75:5 |
2 VA |
9 Ft. |
5 Ft. |
01297 |
100:5 |
2 VA |
9 Ft. |
5 Ft. |
01298 |
150:5 |
2 VA |
9 Ft. |
5 Ft. |
01299 |
200:5 |
2 VA |
9 Ft. |
5 Ft. |
01313 |
250:5 |
2 VA |
9 Ft. |
5 Ft. |
01300 |
300:5 |
2 VA |
9 Ft. |
5 Ft. |
01305 |
400:5 |
2 VA |
9 Ft. |
5 Ft. |
01301 |
500:5 |
2 VA |
9 Ft. |
5 Ft. |
02303 |
600:5 |
2 VA |
9 Ft. |
5 Ft. |
02459 |
750:5 |
2 VA |
9 Ft. |
5 Ft. |
02304 |
1000:5 |
2 VA |
9 Ft. |
5 Ft. |
37020 |
100:5 |
2 VA |
9 Ft. |
5 Ft. |
37021 |
150:5 |
5 VA |
24 Ft. |
20 Ft. |
37022 |
200:5 |
5 VA |
24 Ft. |
20 Ft. |
37023 |
300:5 |
12.5 VA |
61 Ft. |
57 Ft. |
37024 |
500:5 |
20 VA |
98 Ft. |
95 Ft. |
37025 |
1000:5 |
25 VA |
123 Ft. |
119 Ft. |
Wrapping Donut Style Current Transformers
It is often the case that a specific application will require a current transformer with an odd ratio which is not available commercially. In these instances it is possible to use additional wraps of either the primary line or the secondary lines to change the ratio of a standard transformer to have the desired ratio. The formula for computing the ratio of a CT is as follows:
Xnew / Ynew =
(Xstandard + (Ystandard * # of secondary wraps)) / Ystandard* # of primary wraps
Xnew = The desired numerator
Xstandard = The numerator of the ratio of the standard, stock transformer
Ynew = The new denominator; for 99% of applications this will be constrained to equal 5
Ystandard = The denominator of the ratio of the standard, stock transformer
For example, if two primary wraps and negative three secondary wraps were added to a 75:5 transformer, the end ratio would be:
(75 + (5 * -3)) / (5 * 2) or 30:5
Primary wrap refers to wrapping the line with the current being measured around the body of the CT. Each primary wrap beyond the first decreases the ratio of the transformer; in fact, two wraps divides the ratio by two, three wraps divides the ratio by three, etc.
Secondary wrap refers to wrapping the lines running from the CT to the meter through the body of the CT. A secondary wrap in the same direction as the primary loop is a positive wrap, while a secondary wrap in the opposite direction of the primary loop is a negative wrap. Each secondary wrap changes the ratio of the CT by a value equal to the denominator of the CT (usually 5) and are either negative or positive, depending on direction.
All of the transformers retailed by Simpson have a secondary value of 5 Amps, and ratio tables for these tables are available here in PDF format. The top row of the table shows the numerator of the stock transformer ratio and the number of primary and secondary wraps are shown along the right. To use the chart, simply find the number in the table which is closest to the desired value, then order the transformer with the same numerator and add the requisite number of wraps.