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.

  • 2
    Stepdown transformers are NOT isolation transformers (unless they are). Commented Oct 14, 2018 at 7:29
  • 1
    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) Commented Oct 14, 2018 at 14:18
  • 1
    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
    Commented Nov 22, 2018 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
    Commented Nov 22, 2018 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
    Commented Nov 22, 2018 at 3:14

1 Answer 1


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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.