CT Burden Calculator

It means 400 A in the primary produces 5 A in the secondary, a turns ratio of 80:1. Divide the secondary current by 5 and multiply by 400 to get the primary current: a meter reading 1.5 A on the secondary means 120 A in the primary. The same arithmetic applies to 200/5 (40:1), 1200/5 (240:1) and any other ratio in the chart below.

Work out the full-load current first. For a three-phase transformer it is kVA x 1000 divided by (1.732 x line-to-line volts). A 500 kVA transformer at 480 V draws about 601 A. Pick a standard ratio whose primary rating is at or above that current, here 800:5, or check the rating factor on the nameplate of a 600:5 CT before using it. The utility or the meter specification may require a particular ratio.

It is an accuracy class, but 0.5 is not one of the metering classes in IEEE C57.13-2016 Table 8, which lists classes such as 0.3, 0.6 and 1.2. A 0.5 class appears on CTs built to other standards. This tool publishes no limits for it, so read the limits from the CT data sheet.

Add the resistance of the lead wires, the meter or relay burden converted to ohms, and the connection resistance. Lead resistance is twice the one-way length times the wire resistance per foot. A device burden in VA converts to ohms as VA divided by the rated secondary current squared. Compare the total with the burden on the CT nameplate.

It is the metering accuracy class followed by the highest standard burden at which that class holds. IEEE C57.13 says a rating of 0.3 B-1.8 implies 0.3 at B-0.1, B-0.2, B-0.5, B-0.9 and B-1.8. A rating written for one burden only, such as 0.3 @ B-0.5, is not guaranteed at the others.

C200 is a relaying class. The CT delivers 200 V to a standard burden at 20 times rated current without more than 10% ratio error. For a 5 A secondary the standard burden is 2 ohm, so 20 x 5 A x 2 ohm = 200 V. The C means leakage flux does not noticeably affect the ratio, so performance can be calculated from the excitation curve.

A metering CT is rated for accuracy at 100% of rated current and at 10% (or 5% for some classes) of it. A relaying CT is rated for how far it can carry fault current before it saturates. IEEE C57.13 limits ratio error for C and T classes to 3% at rated current and 10% at 20 times rated current.

For CTs with non-gapped cores, IEEE C57.13 defines the knee of the excitation curve as the point where the tangent is at 45 degrees to the abscissa. IEC 61869-2 defines the knee differently, so check which standard a published knee-point value follows before comparing it with another.

No. IEEE C57.13 states that current transformers should never be operated with the secondary circuit open because hazardous peak voltages may result. Short the secondary before disconnecting the burden.

Those are the NEC Chapter 9 Table 8 values for stranded, uncoated copper at 75 C. Real leads run cooler or hotter, and solid wire is slightly lower. For long runs or tight margins, use the measured loop resistance.

No. It adds resistances, which is a conservative arithmetic sum. Real burdens have different power factors, so a result within a few percent of the rating needs a closer check with the device data sheet.