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Without an isolation (or step down) transformer. Your 20 ampere circuit breaker can happily trip when oven hot wires touches ground by using proper hot, neutral and grounding wire sizes. No problem about it.

But when you add an isolation (or 220/110v step down) transformer. There is now the core size that can limit the current. So when you use small size transformer, and the wire touches ground in the oven (given proper wire sizes for all), the 20 ampere circuit breaker can no longer trip because the core limits the current, right?

What must then be the proper transformer size given 20 ampere circuit breaker. Does it mean I need to buy a 20 ampere 220v-110v transformer even if the oven is only 1 ampere just to make it trip?

Note I live in a country with only 220v output and let's say I need to use an important equipment that is rated only at 110v. I don't know the answer above so even if the answer is obvious. Please confirm it.

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    Stepdown transformers are NOT isolation transformers (unless they are). – Harper Oct 14 '18 at 7:29
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    Can you give us photos of the transformer nameplate? This strongly depends on whether we're talking a small distribution transformer, or a plug-in type "transformer"/"travel power converter" (which come in a few different types) – ThreePhaseEel Oct 14 '18 at 14:18
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    This guy is a waste of time , no upvotes and challenges answers with no knowledge and using pirate panels in his country , not even a ground on his pannel , he has a non contact detector by 1 post or a photo of 1. – Ed Beal Nov 22 '18 at 3:01
  • Because when I first joined.. I couldn't up vote... beginners at stack exchange can't up vote.. it's only now that I'm aware I can up vote already... and I just upvoted the answers by Ed and others (3 hours ago). And I don't know comments should be upvoted too (I upvote comments now). – Samzun Nov 22 '18 at 3:01
  • I will upvote everthing now and if you see i didn't, then i will leave voluntarily. I'm really sorry. – Samzun Nov 22 '18 at 3:14
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You can't rely on the primary-side circuit breaker to detect all faults that can occur on the secondary side. You need to install separate protection for the secondary, appropriate to its ratings.

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