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Power limited

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  • Power limited

    After many hours of research I finally decided on an oven design and size for my application. Shortly after, I realized I am severely power limited in my current shop and updating the power panel is not an option for me at this time. I will only have 30A available to run my new oven, so I have some questions.

    I originally wanted to duplicate the oven build in the link below which is 6x5x4 (interior dimensions) but have decided to knock that down to 5x4x3 in an attempt to keep the required BTUs down.

    So my primary concern is building an oven that I *think* will run on a 30A circuit and then finding out that it won’t. The BTU calculators I’ve seen vary wildly which is not helping my cause. I’ve also noticed that some of them account for “normal” and “good” insulation and the delta between the BTUs required for those is significant. According to one calculator, for a 5x4x3 oven that will need to raise the temp 350 degrees (worst case in the PNW), I’m looking at 11,500 BTUs (3400 W) with “normal” insulation or 6900 BTUs (2000 W) with “good” insulation. This swing seems huge to me. What would be the difference between “normal” and “good” insulation?

    Also, what is the max wattage I can get away with on a 30A 220v circuit? Is it really as simple as V x A = W? If so, it looks like 3400W only requires 16A and I'm well within what I have available...does that sound right?
    My powder coat oven build Garages, Workshops & Tools
    Last edited by N2950H; 11-10-2015, 02:14 PM.

  • #2
    I'd use 240 for my calculations, but generally speaking... you may apply a load that is up to 80% of the circuit rating. That would give you as much as 5760 watts, minus any other items... So Make it 5000 and give yourself room for a fan or some lighting, and the electronics needed to control it. (Electronics will be in the double digit area, so not gonna need much there)


    • #3
      I would add that 3400 watts for that size oven isn't going to work and even 6k will be on the light side. IMO the calculator by the heater shop isn't much good. It was never intended to calculate temperature rise in an oven. The excel calculator posted in the first sticky does a much better job. If you're not able to run that then just do a simple wattage to cubic foot calculation.


      • #4
        To be clear I wasn't going to build it with just 3400W, I wanted to use the max I could with my 30A service. I was using that number as a reference from the calculator.

        All of the links but one on the first sticky appear to be broken (and that's the one I've been using), but using the method Ed suggests I get a requirement of 9000W (5x4x3x150). I simply do not have that much power available. P=IE gives me 7200W for a 30A 240v circuit. How hard and fast is that 80% rule (giving me 5760W as Scott pointed out)? Obviously I don't want to be unsafe...but heating elements won't spike like air compressors and welders, right?

        Let's say I called 6000W my hard limit given my current power situation...would you say it's not worth the gamble on my 5x4x3 oven not reaching temp? If I wanted to stick to the cubic feet x 150 rule, then I'd have to do something like 4.5x3x3 which is significantly smaller than I want to do.

        I really appreciate your help, guys. I'm itching to start my oven build but I need to get these details sorted out first.


        • #5
          If it's any consolation to know... My oven is 40" x 44" x 72" and it came with (And I still run) two 2850 elements in it and can actually exceed 500 degrees. The time to reach 400 degrees from cold is around 20 minutes... so it can be done. With 3 x 3 x 5 you could at least reach temp and cure parts (As long as you keep it powder, and don't try going up to ceramics)... It may take some patience at times when you have large mass items in the oven. But for best results... Ed's completely correct on design.


          • #6
            The 150 watts per c.f. is what I consider optimal but that doesn't mean you can't get by with less. Scott is in Texas so I'm sure his ambient start temp is probably higher than mine here in Ohio, especially in winter. If you're even further North then you need to take that into consideration. I personally wouldn't go below 100 watts per cf, but as Scott mentioned a lot depends on what you intend to coat, as heavier parts will need to soak much longer to reach temp than small, less bulkier parts.

            At 5x4x3 you would be right at 100 watts per cu.ft. with 6000 watts. The 80% rule has nothing to so with spikes, it's a continuous load consideration.

            I would maybe consider a 6X3X3 size, but you don't list where you are located so that might be a little on the light side.
            Last edited by ed_denu; 11-11-2015, 09:10 PM.


            • #7
              I think I mentioned it in my original post, but I'm in the PNW (Seattle). I will consider the info you guys have provided before finalizing my design this weekend. I very much appreciate both of your input in this thread and the many others I have read here. It's guys like you who are giving me the confidence to move forward with my own build. Cheers!


              • #8
                Haven't quite settled on a design yet as I still have a few nagging concerns I'm sorting out.

                Something that caught my eye today was the TC Coatings oven that I've seen several other people mimick when building their own. I started looking at the specs on their site, and they claim it runs on only 27A which sounds like it would work with my current 30A limitation. This surprised me because 40x44x72 (75 cu ft) seems like it would need more than that. Most of what I read says 150 W per cubic foot is ideal which, for this size oven, would be 11,250 W. This looks like a two burner oven, so I'm guessing it doesn't use that much power.


                If this oven would work, I want to fabricate one just like it as the size would work well for me. So my questions are:

                1) Does it seem reasonable to you that this oven (75 cu ft) could run off only 27A?

                2) Would you feel comfortable running that, by itself, off a 30 A circuit? I realize that is running at 90% capacity rather than the above recommended 80%, but I still don't have a good handle on the importance of adhering to that.

                Appreciate the input.


                • #9
                  This oven runs as it is stated... I have one of their older ones that has even lower wattage elements... and only 2. 2850 watts each. Mine runs at max draw of 23.75 watts (OK, a little more due to the rest of the electronics and fans and stuff, but heat-wise... That's it) Takes about 20-25 minutes to get to 400 from dead cold... Average temp here is fairly high... Texas weather. The ovens are actually "Ted's Fabrication" ovens. Good quality build. I changed the electronics out on mine after a year or two... You can use those dimensions and run it with lower wattage elements and get closer to your limits without going so close to 100%


                  • #10
                    I've just about got my oven framed up now and I'm almost ready to start installing the fan and lighting. Can I just use a standard porcelain lampholder and appliance lightbulb, or do I need to look for equipment that is meant to handle higher temps?


                    • #11
                      Originally posted by N2950H View Post
                      I've just about got my oven framed up now and I'm almost ready to start installing the fan and lighting. Can I just use a standard porcelain lampholder and appliance lightbulb, Yes or do I need to look for equipment that is meant to handle higher temps?
                      These will work fine


                      • #12
                        Build has been in progress for a few weeks and so far is going well. What's left is fabbing up the door, wire everything up, insulate it, and put the outer skin on. I'm about as ignorant as they come when it comes to electrical theory and safety but have pored over photos online and tried to emulate other successful builds. I'll post up some photos shortly in the hopes that if I'm doing something dumb, you guys will let me know. I also have a couple questions.

                        Here's what I'm building:
                        - Interior dimensions are 44 x 40 x 72
                        - Two 3000W elements wired with two more installed but not hooked up due to power limitations
                        - Ted's control box for four elements
                        - All electrical components recessed

                        So, my questions:

                        1) Should the oven structure itself be grounded somehow?

                        2) I'm not quite sure how to hook my elements to the contactors provided with the box from Ted. I did find a wire diagram on this site (below) which seems to be representative of what I'm trying to do, and that seems to indicate I can wire two elements in series to each contactor, am I reading this correctly? If so, that works out great because I am going to start by running only two elements and see if I can get to temp. If not, I will upgrade my panel (or add a generator into the equation somehow) and hook up the other two. So unless you guys tell me this is an awful plan, I will hook two heaters up in series to a single contactor, and I will coil and stow inside the box the wires coming from the other two elements and if needed I will connect them to the other contactor.

                        Thanks for looking.
                        Last edited by N2950H; 06-11-2016, 05:24 PM.


                        • #13
                          Here are some of the build pics:

                          IMG_20160427_182556_hdr by Chris Johnson, on Flickr

                          IMG_20160429_170324 by Chris Johnson, on Flickr


                          • #14

                            IMG_20160429_170333 by Chris Johnson, on Flickr

                            IMG_20160429_171313 by Chris Johnson, on Flickr


                            • #15

                              IMG_20160429_171320 by Chris Johnson, on Flickr

                              IMG_20160429_190634 by Chris Johnson, on Flickr