load calcs w/kilns

[nhfire77]

My wife is ceramics teacher and we are making our garage into a studio for her. We already have local approvals etc.

There is no electric service there, yet. We have the POCO onboard with the kilns, we are getting a dedicated pole mount transformer, for free on a new pole for free, when did they get so nice?

We are putting a new service (main not yet determined) in we will only need a maximum of 20 circuits including the kilns on two pole breakers with ties.

My question is what do you use to figure the load calc. They are 1ph 120/240 kilns, one at 60A one at 40A.

Does 210.19(A), 215.3, and 230.42 apply to kilns like a dryer? Or does 424.3(B) for electric heat?



Also, does anyone have an opinion on whether you would direct wire or use plug disconnects? My wife has seen both types fail at other places with kilns. Now I don't know if they were installed properly, so I'm not sure why they failed.

Obviously the plugs are more convenient, but that seems to be a weak link that would wear out, any opinions?

 

[Smart $]

60A@240V is 14,400VA
40A@240V is 9,600VA
————————————
Total VA is 24,000VA

While a kiln is more broadly viewed as an oven, NEC demand factors apply to ovens only for [food] cooking purposes. Similar if you try to classify them under a dryer, as NEC demand factors only apply to electric clothes dryers. I think classifying them as electric heat is a stretch, too

I believe you will have to classify both at their full rating as non-continuous loads. While it may be questionable whether you will have to classify them as a continuous load, I think not. I assume they will be operating for periods of three hours or longer, but thermostatically temperature controlled where the current draw is periodic rather than continuous.

 

[augie47]

I agree with Smart$. I don't think there is an option to derate here and doubt you would want to. If you can control the operation of the kilns you could adjust accordingly..... the kilns would be 100 amp and 200 amp service which is pretty standard should be adequate unless you have a heck of an additional load.

 

[ceb58]

I agree with $ on the rating. I see no way you could derate since the kilns really do not fall under any thing you mentioned. Depending on your other loads you should be fine with a 200amp service. As far as hard wire verses plug unless there would be a need to move the kilns often I would opt for hard wire just because over time the heating and cooling of the plugs could cause the jaws in the plug to weaken and lose some of there contact.

 

[nhfire77]

.... I would opt for hard wire just because over time the heating and cooling of the plugs could cause the jaws in the plug to weaken and lose some of there contact.

I agree, this seems to be a cause of failures, the jaws let the plug loosen, and just enough arcing would ensue to fry it. Most of the installs I have seen were plugs, however, the kiln is awkward and heavy and I do not want to move it... even if she does.

 

[K8MHZ]

60A@240V is 14,400VA
40A@240V is 9,600VA
————————————
Total VA is 24,000VA

While a kiln is more broadly viewed as an oven, NEC demand factors apply to ovens only for [food] cooking purposes. Similar if you try to classify them under a dryer, as NEC demand factors only apply to electric clothes dryers. I think classifying them as electric heat is a stretch, too

I believe you will have to classify both at their full rating as non-continuous loads. While it may be questionable whether you will have to classify them as a continuous load, I think not. I assume they will be operating for periods of three hours or longer, but thermostatically temperature controlled where the current draw is periodic rather than continuous.

I used to build and repair kilns. The control of choice was and still seems to be a 'kiln sitter' which turns the kiln off at a certain temperature by using cones that melt. This is just a max temp control. The 'rheostat' controls, many times, are simple duty cycle controllers (and not really rheostats) independent of the kiln sitter with a range from on all the time to 10 percent on and 90 percent off. The same controls were used on electric ranges for many years. Since the capability of a three hour cycle time is present, I would wire for a continuous load even though in reality one would not exist as most kilns need much less than a 100 percent duty cycle to reach their set point at the desired time.