{"id":3473,"date":"2026-09-23T12:50:36","date_gmt":"2026-09-23T04:50:36","guid":{"rendered":"http:\/\/www.growhob.com\/blog\/?p=3473"},"modified":"2026-09-23T12:50:36","modified_gmt":"2026-09-23T04:50:36","slug":"what-is-the-ratio-error-of-a-current-transformer-4edb-c4b157","status":"publish","type":"post","link":"http:\/\/www.growhob.com\/blog\/2026\/09\/23\/what-is-the-ratio-error-of-a-current-transformer-4edb-c4b157\/","title":{"rendered":"What is the ratio error of a Current Transformer?"},"content":{"rendered":"

Hey everyone,
\nIf you\u2019re working with current transformers (CTs) in power systems\u2014whether you\u2019re an electrical engineer, a maintenance tech, or someone just getting into the game\u2014you\u2019ve probably heard the term \u201cratio error\u201d thrown around. But let\u2019s be real, it\u2019s not just another jargon word to memorize. For us CT suppliers, ratio error is the backbone of why our products work (or why they don\u2019t<\/em> work) when your facility is measuring current, protecting equipment, or keeping the grid stable. Let\u2019s break this down like we\u2019re chatting over coffee\u2014no boring textbook fluff, just the stuff that actually matters. Current Transformer<\/a><\/p>\n

<\/p>\n

First, let\u2019s start with the basics: what\u2019s a CT, anyway? In case you\u2019re new, a current transformer is a device that steps down high AC currents (think: thousands of amps running through power lines) to a safe, low level (usually 5A or 1A) that your meters, relays, or sensors can handle. It\u2019s like a translator between the big, scary power world and the small, precise devices you use to monitor it.<\/p>\n

Now, the \u201cratio\u201d part. Every CT has a nameplate ratio<\/em>\u2014that\u2019s the number like 200:5, which means if the primary current (the big one going through the line) is 200A, the secondary current should be exactly 5A. That\u2019s the ideal scenario. But in the real world, nothing\u2019s perfect. That\u2019s where ratio error comes in.<\/p>\n

So, what exactly is ratio error<\/em>? Let\u2019s keep it simple: it\u2019s the difference between the actual secondary current your CT produces and the secondary current it should<\/em> produce, divided by that ideal secondary current, multiplied by 100 to get a percentage. The formula looks like this (I\u2019ll keep it super casual):
\nRatio Error (%) = [( (Ideal Secondary Current) \u2013 (Actual Secondary Current) ) \/ Ideal Secondary Current] \u00d7 100<\/p>\n

Wait, let\u2019s test that with an example. Suppose you have a 200:5 CT. If the primary current is 200A, the ideal secondary is 5A. But say your actual secondary current is 4.95A. Plugging in the numbers: [(5 \u2013 4.95)\/5] \u00d7100 = 1% ratio error. Easy, right?<\/p>\n

But hold up\u2014why does this matter so much? Let\u2019s get to the \u201cso what?\u201d part. If ratio error is off, your measurements are wrong. If your meter reads 1% less current than what\u2019s actually flowing, your energy bills could be off (either you overpay or underpay, and trust me, utilities hate that). Worse, if you\u2019re using a CT for protection\u2014like a relay that trips when current gets too high\u2014wrong ratio error could make the relay trip too early, leading to downtime, or not trip at all when there\u2019s a fault, which can fry expensive equipment. For our team here, ratio error isn\u2019t just a spec on a sheet\u2014it\u2019s about making sure your facility stays online, your bills are accurate, and no one\u2019s dealing with a avoidable outage.<\/p>\n

Now, what causes ratio error? Let\u2019s talk about the real-world factors that throw this off, because that\u2019s what we troubleshoot every day. First up: load on the secondary side<\/em> (we call this \u201cburden\u201d in the trade, but let\u2019s just say the stuff connected to the CT\u2019s output). Every CT has a rated burden\u2014like meters, wires, relays, all have resistance. If you connect more devices than the CT is designed for (or the wrong ones), that changes the secondary current. For example, if a CT is rated for a 5-amp burden but you plug in a 10-amp device, the actual secondary current won\u2019t line up with the nameplate. That\u2019s a common mistake we see, especially with folks who mix and match CTs and devices without checking specs.<\/p>\n

Next, magnetization in the CT core<\/em>. CTs work by inducing current in the secondary coil via the primary coil\u2019s magnetic field. The core (usually silicon steel) has to carry that magnetic field, but it\u2019s not perfect. Some of the energy gets lost as the core magnetizes and demagnetizes every cycle. That \u201clost\u201d energy means the secondary current is a little lower than it should be\u2014boom, ratio error. The quality of the core is huge here; that\u2019s why we don\u2019t cut corners on core material. A cheap, thin core will have way higher ratio error than a high-grade, laminated one.<\/p>\n

Then there\u2019s primary current level<\/em>. Ratio error isn\u2019t the same at all current levels. At very low primary currents (like 10% of the CT\u2019s rated primary), the core doesn\u2019t get fully magnetized, so the ratio error can spike. At way<\/em> too high primary currents (over the CT\u2019s rating), the core saturates\u2014magnetism maxes out, so it can\u2019t induce more current, leading to huge ratio error. That\u2019s another thing we warn customers about: never use a CT above its rated current, it\u2019ll mess up measurements and could even damage the device.<\/p>\n

Wait, what about phase angle error<\/em>? No, that\u2019s a different thing, but people mix it up. Ratio error is about magnitude (how big the current is), phase angle error is about timing (how much the secondary current lags the primary). We focus on both, but ratio error is the one that trips people up for basic measurements.<\/p>\n

Now, let\u2019s talk about why our CTs keep ratio error super low<\/em>\u2014since that\u2019s what you actually care about, right? We\u2019ve been making CTs for years, so we\u2019ve fixed a lot of these issues. First, we use high-quality, grain-oriented silicon steel cores that reduce magnetization loss. We test every single CT before it leaves our warehouse\u2014no exceptions. Each unit goes through a ratio test at different current levels (low, rated, high) to make sure ratio error is well within industry standards. For our standard CTs, ratio error is usually under 0.2% at rated burden, which is way better than the minimum required. That\u2019s the kind of precision you need if you\u2019re monitoring energy or protecting equipment.<\/p>\n

But wait, not all ratio error is bad? No, wait\u2014actually, in most cases, you want it to be as close to zero as possible. The only time a little ratio error might be \u201callowed\u201d is if it\u2019s within the accuracy class. Oh right, accuracy classes! You\u2019ve probably seen terms like 0.5 class or 1.0 class. That\u2019s exactly about ratio error. A 0.5 class CT means the ratio error is \u22640.5% at rated burden and rated current. A 1.0 class is \u22641%. For metering CTs, you want 0.5 or better because accuracy matters for billing and monitoring. For protection CTs, maybe 5P class, which is for fault conditions, so ratio error can be a little higher, but still within limits to make sure the relay trips when it needs to.<\/p>\n

Let\u2019s get real about a common mistake we run into: people buying the cheapest CT they can find, not checking the accuracy class, and then wondering why their meter is off by a few percent. We\u2019ve had customers come to us after dealing with that\u2014they saved $50 on a cheap CT, but now their energy bills are off by hundreds every month, plus they had to fix a relay that didn\u2019t trip. Don\u2019t do that. Ratio error is worth investing in.<\/p>\n

Another thing: burden matching<\/em>. If you\u2019re adding new devices to your CT\u2019s secondary, you can\u2019t just plug them in and hope for the best. You have to calculate the total burden of all connected devices and make sure it\u2019s within the CT\u2019s rated burden. For example, if your CT is rated for 15 VA (volt-amps, which is how burden is measured) and you add two meters that use 8 VA each, total is 16 VA\u2014you\u2019re over, so ratio error will go up. That\u2019s why our tech support team is always here to help you calculate burden, no question.<\/p>\n

Wait, let\u2019s answer a question we get all the time: \u201cCan ratio error be corrected?\u201d Yes, but only up to a point. Some CTs have taps\u2014adjustable primary turns\u2014so you can tweak the ratio slightly, but that\u2019s not fixing big ratio error from a bad core or over-burdening. The best fix is getting the right CT for your application, installing it correctly (making sure secondary connections are tight, no loose wires), and not overloading the secondary side.<\/p>\n

Let\u2019s wrap this up with a quick example of what happens when ratio error is ignored. A manufacturing plant we worked with a while back used a cheap 1.0 class CT for their main motor current monitoring. The ratio error was 1.2%\u2014so their meter said the motor was drawing 988A, but it was actually 1000A. That might not sound like much, but over a year, that added up to thousands of dollars in overpaid energy bills. Worse, the relay on the motor was set based on the meter reading, so it almost tripped unnecessarily a few times, costing them production downtime. They switched to our 0.2 class metering CT, and now their readings are spot-on, no more wasted cash, and the relay works perfectly. That\u2019s the difference ratio error makes.<\/p>\n

So, to recap: Ratio error is the gap between a CT\u2019s ideal and actual secondary current, shown as a percentage. It\u2019s caused by core magnetization, secondary burden, and current level. Too much ratio error messes up measurements and protection. For reliable power, you need a CT with low ratio error, matched to your burden and application.<\/p>\n

If you\u2019re working on a project and need to pick the right CT\u2014whether it\u2019s for metering, protection, or something else\u2014hit us up. We can walk you through choosing the right accuracy class, calculating burden, and making sure ratio error is as low as it needs to be. No confusing jargon, no pushy sales stuff, just real advice from people who work with CTs every single day.<\/p>\n

<\/p>\n

Contact us today to chat through your needs and find the perfect CT for your setup.<\/p>\n

Toroidal Transformer<\/a> References:<\/p>\n

    \n
  1. IEEE Std C57.13-2021, Standard Requirements for Instrument Transformers<\/li>\n
  2. Electrical Measurement and Measuring Instruments, S. A. T. Al-Azzawi, 2018<\/li>\n
  3. Current Transformer Applications, Cooper Power Systems, 2020<\/li>\n<\/ol>\n
    \n

    Dongguan Hensiron Electric Co., Ltd.<\/a>
    As one of the most professional current transformer suppliers in China, we have world-leading production equipment and strong manufacturing capabilities. Please feel free to buy high quality current transformer made in China here from our factory. Customized orders are welcome.
    Address: Building 4, Xinxing Industrial Zone, Wangao Road, Wanjiang Street, Dongguan City, China
    E-mail: jessica@dghensiron.com
    WebSite:     https:\/\/www.dghensiron.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

    Hey everyone, If you\u2019re working with current transformers (CTs) in power systems\u2014whether you\u2019re an electrical engineer, … What is the ratio error of a Current Transformer?<\/span>Read more<\/a><\/p>\n","protected":false},"author":672,"featured_media":3473,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3436],"class_list":["post-3473","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-current-transformer-4212-c4e3e8"],"_links":{"self":[{"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/posts\/3473","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/users\/672"}],"replies":[{"embeddable":true,"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/comments?post=3473"}],"version-history":[{"count":0,"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/posts\/3473\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/posts\/3473"}],"wp:attachment":[{"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/media?parent=3473"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/categories?post=3473"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.growhob.com\/blog\/wp-json\/wp\/v2\/tags?post=3473"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}