With Dimension tool on I can dimension the intersection of straight lines no problem.
I have watched tutorials and when the cursor hovers over the circular edge the line is highlighted in blue and the dimension is added.
But when I try to select a circle or arc by hovering over the edge nothing happens?
By “Label tool” do you mean the Text tool and manually type the dimension?
You should be able to add dimensions to circles and arcs with the Dimension tool. Are you clicking on the arc or circle? Is the arc or circle inside a component or group? If so, you need to enter the group or component to access the arc or circle itself.
Ignore my previous post.
Bingo…Thx DaveR,
I was selecting the group but only with one click…so when I selected the Dim Tool the group would deselect.
If I doodle click the group all is fine.
Amazing, I have been using SU for years and never realised this.
Thx again.
That’s the secret. 
Please replace my 'Doodle" with “double”…LOL!!!
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I’m glad I didn’t have a mouthful of beer or something. 