The ones I have from Brushy Mt. and Miller Bee supply are 13 3/4". The ones advertised by Mann Lake and Betterbee say they are 14". Western Bee Supply will make them whatever you like. They are usually between 13 3/4" and 14". 13 3/4" leaves a lot more extra space with eight standard frames than a ten frame Langstroth does.
Having been around this industry since 1979, the standard 8-frame width varies depending on geographical region of the country more than anything. I've commented on that out here before. The closest I've seen to an actual "standard" is 13 3/4". More on that at link below:
I made all my 8-frame mediums in my shop. I made them 13 and 3/4" as that was the most common dimension when I inquired with the major suppliers. I thought if I ever bought other boxes/tops etc., I wanted them all to be interchangeable with my homemade boxes.
So far, I've made everything but the frames. So either size fits. If you're making your own equipment, pick a size and go for it.
If you are going to buy your 8-frame stuff, why worry about the dimensions? Just buy from the same company and (hopefully) they'll all be the same size (but this will depend on their suppies which change from year to year). Or buy a set-up and then make your own to those sizes.
I know Kelley's has a uniquedimension to their 8-frame equipment due to their saws, but I can't remember what the difference was.
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