Definite Purpose Contactor for Powder Coat Oven
I recently finished building a powder coat oven. It's 3' H X 2' W X 6' Deep inside dimensions. It has four oven elements inside which add up to around 12,000 watts. They are wired in parallel and measure approximately 4.7 ohms. I've tested it and it does 400 degrees F. with no problem. Using a clamp meter separarately on the wires inside it's apparently drawing 51.4 amps. The wiring all the way from the subpanel where I'll be using the oven is four-wire 6 gauge 240 VAC. That's L1, L2, Neutral and ground.
The design that I'm using here is on a website that shows construction and gave a materials list. I changed the framework design but basically I followed the instructions. Here is the link and I recomment it to anyone who is interested in doing their own powdercoating.
As I mentioned, I'm seeing slightly more than the rated resistive load. I've checked Amapcity charts on the internet and they show generally that enclosed 6 ga wiring is OK for 60 amps. My concern is the Cutler-Hammer C25 BNF240A Definite Purpose Contactor (DPC) that I'm using, It's single phase, two pole and rated at 50 amps resistive load. The control coil is powered by 120 VAC.
I've been trying to locate a 60 amp DPC so that I am not over-extended. So far, the only ones I've seen on the internet are-three pole for 60 amps or greater resistive load.
My question then is this: Would it be appropriate to use only two of the three poles that are offered on a 60 amp or larger three-pole DPC? I'm thinking it would but I'm not sure as I've not got a great deal of experience with this sort of equipment.
I plan on updating the oven at some point to a P.I.D. (proportional, integrated, differential?) unit with a solid state relay to obtain more control over temperature but that's down the road.
Any suggestions regarding three pole DPC usage on two pole power would be greatly appreciated. There are literally hundreds of three-pole DPC's on eBay at excellent prices so availability is no problem. The one I have is the largest I can find for two-pole.
Thanks in advance.
Inside View showing elements.
Electrical Control Box (Pre-installation. The DPC is the black box in the center).
Last edited by gnm109; 12-01-2009 at 08:06 PM.
Better to use two of the available three than need three and only have two. I don't know if it is legal, but I've seen it a lot.
PS I have even done it......shhhhh
Perfectly ok to use 2 of the 3 poles.
No problem using only 2 poles of the contactor.
Really nice job on the oven there.
If the contactor is going to whacking on and off a lot, consider a mercury relay. They are often used in electric heating ovens. Use a contactor as a safety backup actuated by a high-limit thermostat to shut things down if the primary contactor welds itself shut. You might thank yourself if you leave the shop and the oven goes haywire!
Thanks for the responses! I was hoping that two out of three poles would be OK. I notice that resistive loads come on gradually and apparently that's the reason that the DPC is rated higher for resistive loads than for Full Load Amps as with a motor. Once I install a DPC rated for 60 amps resistive load, I'll be good, although the one I have works OK.
I'm going to be using my Mill and lathe to build special sidecar mounts out of steel. I've done some already and powdercoated them in a dedicated toaster oven with good results. The oven I have is large enough to take a motorcycle frame or four car wheels at the same time. It has an upper and lower trolley to roll the parts in once they're coated.
I used metal studs for the base and 2X2X2" U channel for the top, sides and front and back. It's insulated with rock wool, sometimes called mineral wool.
Last edited by gnm109; 12-02-2009 at 02:29 AM.
Cool. I have always wanted to try powder painting but it seemed very expensive and equipment intensive. Where do you spray the powder? Is there a booth required or does it just go all over? I think this place has good stuff (elements, themocouples, etc.,) for your oven and I believe they will sell to individuals via credit card.
Lemme run you over the hurdles:
According to the NEC all conductors above ground potential serving an electrical load have to be opened simultaneously by a single device. Meaning your 230 single phase needs a pair of contacts; one pair on each leg. OK you got that but you're concerned with your preent DPC ampacity which from your description seems to be marginal to me. If you wish to divide your load your will need a second DPC. There's room for one in the enclosure.
My suggetion is to control your heater bank with two DPC's wired to break both legs of the line current to the divided loads respectively. The DPC coils can be energized simultaniously by the control device (Thermostat? Temperature controller?) in series with an over-ride switch. You will, of coure, need a means of circuit protection and a disconnect serving the whole oven.
The terminal at the lower left: are they ceramic binding posts? Are the small multple leads from the switched side of the DPC leading to the element? Are they supported and passed through openings with components rated for the oven's max temperature?
Is the temp control via thermocouple? I see some termocouple sized wire in the lower photo. Is there a separate over-temp switch?
I can't see it in the photos but is the wiring in the oven rated for the oven's skin temperature? Looks like 90degree C WEO cord and 75 Degree THHN conductor but no ratings are visible.
Are the devices and wiring in the electrcal control enclosure attached to the oven exterior rated for the oven's skin temperature.
Is there a bonded ground connected from the frame of the oven to the electrical ground of your electrical service? Oops! I see one with a fat gree wire hooked to it.
My NEC dates from 1993 so I can't quote chapter and verse. It wouldn't hurt to find an up to date copy and work through the stuff on industrial ovens.
Last edited by Forrest Addy; 12-01-2009 at 11:53 PM.
Try Square D 8910DPA62V02. This is a 60A two pole definite purpose contactor with a 120V operating coil (other operating voltages available).
I use these quite a bit in my real job (Sports Lighting Contractor) and have found them to be very reliable.
If you have already purchased a 3 pole contactor, there is nothing wrong with using any 2 of the 3 poles for your purposes.
Last edited by tmc_31; 12-02-2009 at 02:05 AM.
Interesting questions you raise Mr. Addy. Responses are in red above to the best of my knowledge. Remember, this is not my design but is based on a very succesful oven from the website link.
Originally Posted by Forrest Addy
I just want to et a bit more capacity to keep things wihin limits.
Thanks for the response.
Last edited by gnm109; 12-02-2009 at 03:06 AM.