A customer writes:

I was thinking about purchasing one of your JH series kilns. The one I am most interested in is the JD230-JH. However, when looking at the electrical specs it says it requires 3 gauge wire. Right now my studio has 90amp service running to it via 4 gauge wire. I was assuming I could add an 80 amp breaker at the subpanel to connect the kiln, but I am only able to run 4 gauge wire to the studio since the conduit will not fit 3
gauge wire.

I was wondering why 3 gauge wire was recommended for this kiln when 4 gauge 75C THNN wire is rated to 85 amps and the kiln only draws 57 amps.

L&L Answer:

Here is how we do our calculations and lets see how it applies to your situation.

We start with this table:

We take the full load amps of the kiln, which for the JD230-JH is 57 and multiply by 1.25. This gives us the 25% safety factor the National Electrical code wants. So, for this kiln, it is 71.25 amps. Then, for the fuse we do to the first standard size above that which is 80. For wire size, wee use the worse case situation for our public information (60 Deg C rated wire). You can see how the 71.25 is just above the rating for #4 wire if it is 60 Deg C. If you are using THHN wire, however, you can see that the cart tells you that you can safely run 71.25 amps through it with #4 wire.

You will be fine to use the #4 wire!

By the way - this website has a great resource for calculating wire size based on all the factor like length of run: http://wiresizecalculator.net/