I had a customer call and say that he's got a 11.5KW oven (240V 1Phase)that has been operating for several months on a 50A circuit. Recently it has tripped the breaker several times to the point that it will only hold for a short period of time. Is there something(like a high limit???) that could have gone out in the unit that would cause this to exceed the 11.5 kw rating?I realize that the 50A is close to the nameplate, but it has worked fine since June. This was a new unit when it was installed. Thoughts? BTW, he was unable to check anything since he had no equipment to do it with.
Commercial oven
- Thread starter nizak
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
1. Warranty. If the breaker holds with the range disconnected/unplugged, it's someone else's problem.
2. See #1.
Added: Okay, seriously (hey, it could happen!), it sounds like a short. I'd start in the receptacle, then in the terminal compartment.
growler
Senior Member
- Location
- Atlanta,GA
Allways check out the simple stuff first. The most simple thing that will cause a breaker to trip like this is a poor connection. Check breaker connections to the bus ( snap or bolt in) for arcing and check to make sure the conductors are tight.
I assume that you have already put the amp. meter on it to check for above normal current draw.
growler
Senior Member
- Location
- Atlanta,GA
The only thing that you can do to a heating element to cause it to draw more current is to reduce the resistance of the element. You could possibly short it out in some way but a heating element is more likely to burn out than short. You could have a short to ground ( not a dead short but some sort of carbonization that would offer resisitance ).
Check the current draw of the equipment before trying to say you have a problem and then perform a good visual inspection. Check for a short to ground ( just low resistance ).
Heating elements are designed to run at full power until either you reach the desired temperature or get close and then power is reduced or shut off.
There are lots of things that can cause a breaker to trip, even the ambiant temerature in the panel. Maybe they installed something else with a high current draw and the breaker is close to this one ( happens all the time).
e57
Senior Member
- Location
- San Francisco, CA
Seen that once or twice - not so common IMO... A breaker below it would have to get pretty hot - to, or past the point it should be tripping at, and for quite some time to effect another breaker.
I have also seen a thermostat in an Pannini press not quite open causing just an arcing on the contacts before do something like this - took a while to figure out. I didn't notice it the first time and changed out the breaker because it was tripping before it should (at ~20 for a 30A CB) - then it took several weeks for it to start happening again - changed the breaker then watched it for a while. Two bad breakers doesn't just happen... The bad thermostat would sort of chatter when it should have been off and seemed that it was too - meanwhile the amp-clamp was all over the place, but did not trip the breaker. My therory was that the very quick spikes in amperage were not enough to trip, but damaged the breaker over time - maybe deforming the bi-metal strip slowly heating it instead of quickly - over time until it tripped at a much lower amperage. One of the few times I have delved into equipment repair - but replacing the thermostat cured it.
Either way an amp-clamp or logger might be in order IMO if it is a simular situation.
charlie
Senior Member
- Location
- Indianapolis
Since this is a commercial oven, the 47.9 ampere draw may be too much for a 50 ampere circuit. Look at the usage of the oven. If the door is opened a lot, the element will be on for a large portion of the time and the 125% should have been applied. If that is the case, a 60 ampere circuit should have been installed instead of the 50 ampere circuit.
What size circuit does the manufacturer recommend and is this designed to be a commercial oven?
What size circuit does the manufacturer recommend and is this designed to be a commercial oven?
Charlie, I went to the restraunt and found the oven to be 11kw 1 phase 240V. When I went to start it the fan came on first, then in a couple of seconds it made a loud bang and tripped the B/C breaker and the main feeding the sub panel. I took the back panel off and found the wires connected to the heating elements melted together. Upon talking to the owner he told me that a service tech was there in July and replaced melted wires once already, appears to be the same problem again. Service tech told the owner that the circuit wires feeding the unit were too small(#6 CU THHN), I disagree and feel there is an internal problem with the elements. I checked line voltage(no load) and it read 246. Any ideas on what may cause the wiring internally to fail twice? Thanks.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Seems odd that wires feeding the unit would cause internal wiring to melt but the #6 THHN wouldn't, not to mention that the OCPD is 50 amps which would protect the #6's indefinitely. The service tech is missing something. :roll:
charlie
Senior Member
- Location
- Indianapolis
I now suspect that the service tech doesn't know what he is doing.
e57
Senior Member
- Location
- San Francisco, CA
I'l second (or third) that - since he says the wire "melted" - one can assume he is not using the correct wire.... What type of insulation were on the "melted" wires? Since the correct stuff would be difficult to "melt".... Especially 'together'....I now suspect that the service tech doesn't know what he is doing.
growler
Senior Member
- Location
- Atlanta,GA
What about the idea of useing high temp wiring for this application ( it's an oven).
It would be interesting to know what type of insulation is used on the cable that melted. The high temp insulation doesn't melt.
Darn you are fast e57, I didn't see your comment.
e57
Senior Member
- Location
- San Francisco, CA
Take a look at the temp ratings of the original wiring, and get a match to it, and make sure the terminals also rate for the temp.
You can find this type of wire here... BTW - This ia a great company name
You can find this type of wire here... BTW - This ia a great company name
Not to be labor the subject, if the thermostat on the unit has a max setting of 500 degrees wouldn't that be the time when the unit would be drawing the full 11KW rating? If the thermo was set at 350 for instance wouldn't the draw be less? . The unit is under warranty, I suggested to the owner that he demand a new oven since this is the second time this has happened and they have both occured at peak holiday periods when the use is critical.Thank you all.
charlie
Senior Member
- Location
- Indianapolis
Most are either all on or all off. The thermostat will determine whether the element has power applied or not. Once it reaches its set temperature, the thermostat will cycle the element on and off to keep the oven within its set limits.
e57
Senior Member
- Location
- San Francisco, CA
No the unit should be cycling on and off. Longer periods of 'on' for higher temps and like wise shorter for lower temps. The draw will be essentially the same at 200F, as it would be for 500. The only factor would be if it has seperate "broil" elements - but those too would be below the units total rating to include the convection fan.
F, or C either way it sounds low IMO
See here - much of this stuff is 200C/392F for the purpose. The wiring area should also be sealed +/or shielded from the oven temps as even a small hole will pump hot air into the wiring area and turn it into (for the lack of a better analogy) the space shuttle... Also - the terminations need to be CLEAN and rated for the tempatures as well - any ol' reminal will eventually fail just as well, or even contribute to conductor failure.
I say new oven or new appliance guy... But don't let him blame you....
growler
Senior Member
- Location
- Atlanta,GA
I'm not sure if this is the right way to handle things but it is what I do in situations like this. Once I see a defect in a product I call the manufacturer and ask to speak with tech rep. or engineer and explain what I have found and ask how they wish to handle things. They may even agree to pay for the service call and they may have the correct specs. ( wire type ) for you to correct the problem. It may have left the factory as a defective product and they may even be aware of the problem. Quality control isn't quite what you would think at some factories.
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