# Isolation transfomer outputs 250v and ends up tripping breaker

**URL:** <https://community.victronenergy.com/t/isolation-transfomer-outputs-250v-and-ends-up-tripping-breaker/30618>\
**Category:** Q&A and troubleshooting\
**Tags:** isolation\_tranformer, marine, diy\
**Created:** [30 April 2025 11:40 UTC](https://community.victronenergy.com/t/isolation-transfomer-outputs-250v-and-ends-up-tripping-breaker/30618 "2025-04-30T11:40:45Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![brann](https://dub1.discourse-cdn.com/victron/user_avatar/community.victronenergy.com/brann/32/18419_2.png) [@brann](https://community.victronenergy.com/u/brann)\
**Post date:** [30 April 2025 11:40 UTC](https://community.victronenergy.com/t/isolation-transfomer-outputs-250v-and-ends-up-tripping-breaker/30618/1 "2025-04-30T11:40:45Z")

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My AC shore power is 235V 50hz. the unit’s jumper are configured to output 230V.  
When I check with a voltmeter I get 250V 50hz. After a few minutes (sometimes 5 sometimes 20), the 16A breaker on the shore power trips (not the gfci, I’m talking about the individual breaker protecting the circuit from overcurrent). The transformer is 3kw so 16A is plenty (and I wasn’t drawing any current from the boat anyways).

If that helps, on the boat end of the transformer there’s a multiplus and a BMS NG, but that part should be.. er.. isolated 🙂

what could be causing this?  
Thank you!  
Brann

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**Author:** ![MikeD](https://dub1.discourse-cdn.com/victron/user_avatar/community.victronenergy.com/miked/32/1522_2.png) [@MikeD](https://community.victronenergy.com/u/MikeD)\
**Post date:** [1 May 2025 04:23 UTC](https://community.victronenergy.com/t/isolation-transfomer-outputs-250v-and-ends-up-tripping-breaker/30618/3 "2025-05-01T04:23:54Z")

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The Auto transformers are usually wound to increase the voltage by 6%, so a slight increase in the voltage is normal. As per Ludo, try using an insulated screwdriver to tighten the breaker terminals. Cheap breakers also can ‘wear out’ tripping at lower currents than rated after a few years…

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**Author:** ![brann](https://dub1.discourse-cdn.com/victron/user_avatar/community.victronenergy.com/brann/32/18419_2.png) [@brann](https://community.victronenergy.com/u/brann)\
**Post date:** [1 May 2025 10:08 UTC](https://community.victronenergy.com/t/isolation-transfomer-outputs-250v-and-ends-up-tripping-breaker/30618/4 "2025-05-01T10:08:39Z")

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I switched to another outlet and I dont have the issue so this was indeed the breaker’s fault.

with regards to the 6% increase, I had no idea it did that, 250V sounds like a lot!  
Thank you for the information anyway 🙂