## Overview

• How big of a CT window opening is needed to fit around a particular conductor diameter (gauge)?
• How big of a CT window opening is needed to handle a circuit rated for a particular current (amps)?

This page covers recommended current transformer (CT) sizes for various conductor wire gauge sizes. These only apply to standard round conductors. If you are measuring busbars or multiple parallel conductors making up each phase, you may need to use our CTB family of busbar CTs available with openings from 1.5 x 3.5 in. up to 8 x 24 in.

## Definitions

• AWG—American wire gauge
• kcmil—In the North American electrical industry, conductors larger than 4/0 AWG are generally identified by the area in thousands of circular mils (kcmil), where 1 kcmil = 0.5067 mm². A circular mil is the area of a wire one mil in diameter. One million circular mils is the area of a rod with 1000 mil = 1 inch diameter.
• MCM—one thousand circular mils. This is an older name for kcmil and is an identical unit. It is still in widespread use in the United States.
• mm2—Square millimeters. This is the area of the copper conductor and is the International standard wire gauge measurement.

## United States Wire Gauges

The AWG wire gauge standard specifies diameters for bare conductors. However, all current carrying conductors except busbars are insulated, which adds additional thickness. When sizing a CT, you must select an opening size that can accommodate the insulated wire. Insulation thickness can vary, so the following table provides guidance, but cannot always guarantee any given conductor will fit a particular CT opening. If possible, use the CT with the smallest opening that will fit the conductor you are monitoring, because this will give you the best accuracy.

Wire Gauge Typical Amps THHN Insulated Wire Diameter Recommended CT Opening Sizes (I.D.)
14 AWG 15A 0.109″ 0.30″ – 0.75″
12 AWG 20A 0.128″ 0.30″ – 0.75″
10 AWG 30A 0.161″ 0.30″ – 0.75″
8 AWG 40-55A 0.213″ 0.30″ – 0.75″
6 AWG 55-75A 0.249″ 0.30″ – 0.75″
4 AWG 70-95 0.318″ 0.35″ – 0.75″
3 AWG 85-110A 0.346″ 0.50″ – 0.75″
2 AWG 95-130A 0.378″ 0.50″ – 1.00″
1 AWG 110-150A 0.435″ 0.75″ – 1.00″
1/0 AWG 125-170A 0.474″ 0.75″ – 1.00″
2/0 AWG 145-195 0.518″ 0.75″ – 1.00″
3/0 AWG 165-225A 0.568″ 0.75″ – 1.25″
4/0 AWG 195-260A 0.624″ 0.75″ – 1.25″
250 kcmil 215-290A 0.678″ 0.75″ – 1.25″
300 kcmil 240-320A 0.730″ 1.00″ – 1.25″
350 kcmil 260-350A 0.777″ 1.25″
400 kcmil 280-380A 0.821″ 1.25″
500 kcmil 320-430A 0.902″ 1.25″
600 kcmil 355-475A 0.993″ 1.25″
750 kcmil 400-535A 1.126″ 1.25″ – 2.00″
1000 kcmil 455-615 1.275″ 2.00″

#### Notes

• We generally recommend that the CT opening shouldn’t be more than twice the wire diameter unless there are no other options.
• Information to compile this table came from the 2017 edition of the National Electric Code and from Southwire Corporation’s 2010 datasheet for SIMpull THHN insulated copper building wire.
• Typical Amps range based on 60°C to 90°C temperature ratings fo conductor insulation.

## International Wire Gauges

These are defined by IEC 60228.

Gauge AWG Equivalent
0.5 mm2 ~20 AWG
0.75 mm2 ~18.5 AWG
1 mm2 ~17 AWG
1.5 mm2 ~15.5 AWG
2.5 mm2 ~13 AWG
4 mm2 ~11 AWG
6 mm2 ~9.5 AWG
10 mm2 ~7 AWG
16 mm2 ~5 AWG
25 mm2 ~3 AWG
35 mm2 ~2 AWG
50 mm2 ~1/0 AWG
70 mm2 ~2/0 AWG
95 mm2 ~3/0 AWG
120 mm2 ~4/0 AWG
150 mm2 ~300 kcmil
185 mm2 ~350 kcmil
240 mm2 ~450 kcmil
300 mm2 ~600 kcmil
400 mm2 ~800 kcmil
500 mm2 ~1000 kcmil