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CT Sizing Guide

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For PV system monitoring, existing public resources provide a great method for determining proper CT sizing.  PHOTON International magazine provides a free, up to date database of nearly all commercially available inverters which includes full electrical specifications.  Using this database, installers, integrators and others can determine the necessary CT size as long as they have the inverter model name.

The database is hosted at http://www.photon.info/photon_site_db_wechselrichter_en.photon?ActiveID=1241 .  The process begins by selecting the inverter manufacturer from the drop down on the top left.  The full list of models will be populated below.  There will be a separate entry listed in cases where a given inverter is available for multiple voltage configurations; choosing the proper entry is essential for proper CT sizing.  Once the correct model is found, locate the field “Imax AC” (maximum AC current) under the AC-Part heading.  The value listed will be the per phase current of the inverter.  Please note that the database is hosted in Europe, and so commas may be used in place of decimal points.  For example, the value “90,3 A”  is equivalent to 90.3A.

If only one inverter is to be monitored by the DECK system, select the CT size which is closest to but greater than the Inom value.  The CT size selected should never be smaller than the Imax value.  If the meter will be monitoring several inverters, add the Imax values for each inverter to determine the total maximum current and choose the CT based on that value.  Note that an additional ‘safety factor’ is not necessary for sizing CTs.

Physical sizing of CTs
CTs are specified according to physical dimensions as well as current rating.  The physical size of the CT is commonly referred to as ‘window size’.  The total diameter of the wire must be smaller than the short dimension of the window.  If multiple parallel conductors are to be run through a single CT, be sure that sufficient space is available for bundling.

The following table lists typical wire diameter figures for insulated conductors.  These are based on information available for THHN, THWN and MTW wire and may not directly match your intended product.

Wire

Gauge

Typical

Amps

Typical Insulated

Wire Diameter

16AWG

8A

0.12"

14AWG

15A

0.13"

12AWG

20A

0.15"

10AWG

30A

0.17"

8AWG

40-50A

0.24"

6AWG

50-70A

0.27"

4AWG

80-100A

0.32"

3AWG

90-110A

0.35"

2AWG

100-130A

0.38"

1AWG

110-150A

0.44"

1/0AWG

125-170A

0.48"

2/0AWG

145-200A

0.53"

3/0AWG

165-225A

0.58"

4/0AWG

200-260A

0.64"

250 kcmil

215-290A

0.71"

300 kcmil

240-320A

0.77"

350 kcmil

260-350A

0.81"

400 kcmil

280-380A

0.86"

500 kcmil

320-430A

0.95"

600 kcmil

355-475A

1.04"

750 kcmil

400-535A

1.14"

1000 kcmil

500-750A

1.30"

1250 kcmil

600-800A

1.52"

1500 kcmil

700-900A

1.64"

1750 kcmil

750-1000A

1.75"

2000 kcmil

800-1200A

1.85"

1

[1] Table created by Continental Control Systems (2010), used with permission. 


References-

http://www.photon.info/photon_site_db_wechselrichter_en.photon?ActiveID=1241
http://www.ccontrolsys.com/w/CT_Opening_Size_vs._Conductor_Size





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