# Shunt sizing

**URL:** <https://community.victronenergy.com/t/shunt-sizing/34812>\
**Category:** DIY\
**Tags:** marine, smartshunt, diy\
**Created:** [5 June 2025 16:40 UTC](https://community.victronenergy.com/t/shunt-sizing/34812 "2025-06-05T16:40:46Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Bail](https://avatars.discourse-cdn.com/v4/letter/b/e0b2c6/32.png) [@Bail](https://community.victronenergy.com/u/Bail)\
**Post date:** [5 June 2025 16:40 UTC](https://community.victronenergy.com/t/shunt-sizing/34812/1 "2025-06-05T16:40:46Z")

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Hi  
Looking to use a smart shunt on marine battery.

Simple…but the house battery also starts one of the engines a 175hp outboard. Other engine has its own dedicated battery.

How do I size the shunt as there will be a high if short current draw for motor starting.  
If a rate for CCA I would need rather large shunt.

I welcome and advice

Thanks  
Paul

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**Author:** ![pwfarnell](https://dub1.discourse-cdn.com/victron/user_avatar/community.victronenergy.com/pwfarnell/32/2341_2.png) [@pwfarnell](https://community.victronenergy.com/u/pwfarnell)\
**Post date:** [5 June 2025 17:31 UTC](https://community.victronenergy.com/t/shunt-sizing/34812/2 "2025-06-05T17:31:13Z")

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Size your shunt for the house loads, probably a 500A shunt would be best choice.

There are many posts where people have used the SmartShunt on a starter battery without any issue, but it may not register the short surge current if it is over the shunts rating.

See answers on old community, starter current sjould not be an issue.

> **[What is the maximum SmartShunt \*peak\* current? - VictronEnergy](https://communityarchive.victronenergy.com/questions/271077/what-is-the-maximum-smartshunt-peak-current.html)**

> **[Using the 500amp Smart Shunt with a starting motor battery - VictronEnergy](https://communityarchive.victronenergy.com/questions/120026/using-the-500amp-smart-shunt-with-a-starting-motor.html)**

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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:** [5 June 2025 20:56 UTC](https://community.victronenergy.com/t/shunt-sizing/34812/3 "2025-06-05T20:56:11Z")

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The outboard manual should give the cranking amps of the starter motor, for a 175Hp, I would expect this to be about 250A. Size you shunt for at least this, plus what your house load might be at cranking time. As suggested, 500A would most likely cope.