# When using two CTs to monitor the two main phases, do I just sum their RMS current? **URL:** \ **Category:** Hardware\ **Created:** [28 December 2023 19:56 UTC](https://community.openenergymonitor.org/t/when-using-two-cts-to-monitor-the-two-main-phases-do-i-just-sum-their-rms-current/25229 "2023-12-28T19:56:05Z")\ **Posts on this page:** 1\ **Showing post:** 5
**Author:** ![Bill.Thomson](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/bill.thomson/32/1834_2.png) [@Bill.Thomson](https://community.openenergymonitor.org/u/Bill.Thomson)\ **Post date:** [30 December 2023 00:44 UTC](https://community.openenergymonitor.org/t/when-using-two-cts-to-monitor-the-two-main-phases-do-i-just-sum-their-rms-current/25229/5 "2023-12-30T00:44:10Z")
@moeburn If you’ve got 100 Amp _service,_ then one question that needs to be answered is how big is the transformer that feeds your house. Even with _200 Amp service,_ the max continuously available current is _typically_ 104 Amps. And _that’s_ only if the transformer (25 kVA in this example) is feeding _one_ house, which is usually _not_ the case. See [US 200 Amp Service explained or Do I need a 200 Amp CT?](https://community.openenergymonitor.org/t/us-200-amp-service-explained-or-do-i-need-a-200-amp-ct/4219) for an explanation of _why_ you don’t actually have 100 Amps of continuously available current, and how to determine the rating of the transformer that feeds your house. The term 100 Amp service confuses most folks because it means the weakest link in the _Service Entrance Wire, Meter Base, Meter, Main Breaker, Load Center (circuit breaker box)_ chain, is rated at 100 Amps. It’s _not_ the _continuously available_ current rating. --- _[View the full topic](https://community.openenergymonitor.org/t/when-using-two-cts-to-monitor-the-two-main-phases-do-i-just-sum-their-rms-current/25229)._