Current Transformer inner diameter vs wire size

van53

Joined Nov 27, 2011
67
I may decide to use a current transformer when I build a spot welder using microwave oven transformers to measure the current. I've seen some current transformers that are 1500:5 or 3000:5 with an inner opening diameter (window) of 2.5 to 3.25 inches respectively. Reading online the insulation outside diameter of AWG 1 is around 0.435" and AWG 3 is around 0.346".

If using AWG1 or AWG3 wire through such a current transformer with an inner diameter of 2.5 - 3.25 inches, will there be any issue with regards to the open space between the wire and the transformer?

MisterBill2

Joined Jan 23, 2018
7,221
At 60 Hz line frequency there can be a small variation but if you keep the wire in the same position in the opening it should not bother your readings.

van53

Joined Nov 27, 2011
67
At 60 Hz line frequency there can be a small variation but if you keep the wire in the same position in the opening it should not bother your readings.
Thank you. By same position, do you mean the wire going through the CT should not move around when taking measurements or does the wire need to be centered when going through the CT?

van53

Joined Nov 27, 2011
67
I ended up purchasing a Crompton 8SHT-202 Current Transformer. I had sent the following question to Crompton:

Regarding the 8SHT-202 Current Transformer: For the wire that goes through the current transformers window (3.25"dia) is there a minimum wire diameter size?
For example, would a 0AWG to 4AWG wire be large enough?
Does the wire need to be centered through the window of the CT?

Below was their reply [from TE Energy / EMEA (Crompton Instruments)]:

The cable should be sized according to the nominal current expected in the circuit.
The hole size in the CT is simply designed to accept the cable that would be typical for the rated current. There is no minimum size as such. However, the 8SHT-202 is a 2000A to 5A model, so if this is the expected full load current you would need a much larger cable than ‘0’ gauge, which is good for about 250A maximum.
If your full load current is significantly less than 2000A it is advisable to choose a CT with a lower primary current rating, as the accuracy is best with current ranging from 20 to 100% of the maximum.
The cable is usually run through the centre of the window if possible, but this is not critical with a measurement CT and the cable diameter normally dictates it would not be too far out of centre anyway.
Asking specifically about if the size of the cable through the CT will have any impact on performance their answer was:

"... The actual size of cable you use through the CT will not have any bearing on the performance. If it turns out to be much smaller than the window then it is advisable to pass it through as near to centre as reasonably possible, but as mentioned before this is not critical and I doubt you would notice any difference if it were off centre. ..."

Last edited:

Joined Jan 15, 2015
5,667
Thanks for sharing their reply.

Ron

ebeowulf17

Joined Aug 12, 2014
3,274
I ended up purchasing a Crompton 8SHT-202 Current Transformer. I had sent the following question to Crompton:

Regarding the 8SHT-202 Current Transformer: For the wire that goes through the current transformers window (3.25"dia) is there a minimum wire diameter size?
For example, would a 0AWG to 4AWG wire be large enough?
Does the wire need to be centered through the window of the CT?

Below was their reply [from TE Energy / EMEA (Crompton Instruments)]:

The cable should be sized according to the nominal current expected in the circuit.
The hole size in the CT is simply designed to accept the cable that would be typical for the rated current. There is no minimum size as such. However, the 8SHT-202 is a 2000A to 5A model, so if this is the expected full load current you would need a much larger cable than ‘0’ gauge, which is good for about 250A maximum.
If your full load current is significantly less than 2000A it is advisable to choose a CT with a lower primary current rating, as the accuracy is best with current ranging from 20 to 100% of the maximum.
The cable is usually run through the centre of the window if possible, but this is not critical with a measurement CT and the cable diameter normally dictates it would not be too far out of centre anyway.
Asking specifically about if the size of the cable through the CT will have any impact on performance their answer was:

"... The actual size of cable you use through the CT will not have any bearing on the performance. If it turns out to be much smaller than the window then it is advisable to pass it through as near to centre as reasonably possible, but as mentioned before this is not critical and I doubt you would notice any difference if it were off centre. ..."
That all matches pretty well with my experience using handheld amp clamp meters.

I will say that running the wire perpendicular to the opening seems, in my experience, to make a noticeable difference. If the wire size is close to the hole size, this is taken care of automatically, but if you have a large CT hole and a small wire, you can end up passing the wire through pretty far from perpendicular, which seems to result in lower readings for any given current.