How do you know a new circuit breaker is protecting the circuit properly?

http://groups.google.com/group/alt.home.repair/browse_frm/thread/fdd208ac66d94c74/a542c9b51e25efaf?lnk=arm#a542c9b51e25efaf
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It was posted at alt.home.repair
The guy has a new oven that is 30A. He wants to change the existing 50A breaker to 30A. Is this required?
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What do you want to protect? The wire or the equipment?

http://groups.google.com/group/alt.home.repair/browse_frm/thread/fdd208ac66d94c74/a542c9b51e25efaf?lnk=arm#a542c9b51e25efaf
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The usual USA situation is that the panel circuit breaker is intended to protect the wiring and the wire size determines the circuit breaker ampacity. You need to determine the gauge of the wire and the termperature rating of the wire. Then consult Table 310-16 of the National Electrical Code. Temperature derating may apply. Because most circuit breakers have a 75C terminal rating, you are usually limited to the 75C rating column of the table. #8 AWG wire is required for 50A at 75C copper wire.
Ovens should load the circuit to only 80% of its capacity. A minimum fuse would be 30 x 1.25 = 37.5A, rounded up to the next standard of 40A.
Best to consult a local electrician. Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz
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On Tue, 05 Dec 2006 03:39:11 GMT Bill Kaszeta / Photovoltaic Resources
| Ovens should load the circuit to only 80% of its capacity. A minimum fuse | would be 30 x 1.25 = 37.5A, rounded up to the next standard of 40A.
Can you cite the specific NEC rule that applies to circuits with only one outlet (as opposed to the rules, like 210.23(B), that only apply to multiple outlets) that says no more than 80% for non-continuous loads?
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Bill Kaszeta / Photovoltaic Resources wrote:

Residential ovens are not likely to be considered a continuous load. For minimum circuit rating US-NEC 220.55 can be used, which accounts for the non-continuous nature and "diversity" if multiple units are connected. NEC 210.19-A-3 also has minimum rating rules (and has a rule on tapping cooktops to heavier circuits that include an oven).
The breaker also protects the oven. Maximum breaker ratings for an oven are in NEC 422 and in general can not exceed a maximum, if marked on the oven, or 150% of the rated oven current (or next larger standard size). If the oven (or associated literature) is marked "maximum 30A" the breaker size must be reduced to 30A. Otherwise it depends on the oven rating.
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If you read the original message you will see that the guy wanted to know how to test a circuit breaker. The simple answer is you can't. You should buy a circuit breaker with an approval mark (not a CE mark, as this is not an approval). Typical approval marks are as was stated for North America UL, CSA or possibly TUV. For European TUV, VDE, BSI, NEMKO, DEMKO, SEMKO, NF, etc. Regarding the CE mark this is only a manufacturer claiming that his product meets the requirement, but unfortunately this is not always the case.
Hope the above helps.
BillB
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On Dec 5, 2:41 pm, snipped-for-privacy@abc.net wrote:

The first sentence in the OP says that he wants to change a 50A breaker to a 30A breaker because he bought an oven that is 30A
MY question was...is he required to do this? Everyone agrees that more protection is better than less, but is it necessary?
I would say if the 30A plug on the oven will plug into the 50A outlet then it "should" be OK. For this to be true, however, the oven should have it's own overcurrent protection. I know toaster ovens and microwaves do. My computer has a fuse in it, too.
20A plugs will not work in 15A outlets, but 15A plugs will work in 20A outlets. I think receptacles are keyed so that you can not use them in the wrong situation.
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