{"id":532,"date":"2026-10-08T21:36:13","date_gmt":"2026-10-08T13:36:13","guid":{"rendered":"http:\/\/www.aranciamiss.com\/blog\/?p=532"},"modified":"2026-10-08T21:36:13","modified_gmt":"2026-10-08T13:36:13","slug":"what-is-the-secondary-current-of-a-current-transformer-4a56-d937f4","status":"publish","type":"post","link":"http:\/\/www.aranciamiss.com\/blog\/2026\/10\/08\/what-is-the-secondary-current-of-a-current-transformer-4a56-d937f4\/","title":{"rendered":"What is the secondary current of a Current Transformer?"},"content":{"rendered":"

Hey there! If you\u2019ve ever dug into power distribution gear, you\u2019ve probably heard the term \u201ccurrent transformer\u201d (CT) thrown around a ton \u2013 but let\u2019s get real, most of the explanations online make it sound like rocket science. I\u2019m gonna break down one of the most confusing parts for new folks (and even some vets who skip the fine print): the secondary current of a CT. And yeah, since I\u2019m the guy selling these things, I\u2019ll tie in what that means for you as a buyer too. Let\u2019s cut the jargon, okay? Current Transformer<\/a><\/p>\n

<\/p>\n

First, let\u2019s start with the basics so we\u2019re all on the same page. A current transformer is basically a power tool that steps down high current (think hundreds or thousands of amps running through a main power line) into something safe for humans and regular gadgets to handle. Like, you can\u2019t exactly plug a meter or a relay into a line carrying 10,000 amps and expect it not to blow up, right? That\u2019s where CTs come in. They wrap around that main power line (the primary conductor, if you wanna be fancy) and spit out a smaller current on the other side \u2013 that\u2019s the secondary current. But it\u2019s not just a random number; there\u2019s a reason every CT has a sticker with \u201c5A\u201d or \u201c1A\u201d printed on it. That\u2019s your secondary current.<\/p>\n

Wait, hold on, why those two numbers? Ninety-nine percent of the time, secondary current is either 5 amps or 1 amp when the primary is at its rated current. Rated primary current is the max current the CT is designed to handle regularly. So if your CT is rated for 100A primary, its secondary is 5A \u2013 that means when 100A is flowing through the main line, the secondary side puts out 5A. Makes sense, right? The ratio is super simple here: secondary current = primary current divided by the turns ratio. If the CT has 20 turns on the secondary, primary is 1 turn, that\u2019s a 20:1 ratio \u2013 100A \/ 20 = 5A. Easy peasy.<\/p>\n

But here\u2019s the part most people miss (and what makes or breaks a CT working for your setup): the secondary current isn\u2019t fixed. It changes proportionally with the primary current. If your primary current jumps to 200A, secondary goes to 10A. If it drops to 50A, secondary drops to 2.5A. That\u2019s the whole point \u2013 it\u2019s a mirror, just scaled down, so you can hook it up to meters, protectors, whatever without frying anything.<\/p>\n

Now, why two standard ratings (5A and 1A)? Back in the day, 5A was the go-to because old meters and relays were built for that. Everyone had 5A gear, so CTs were made to match. But lately, 1A is getting more popular, especially in high-voltage setups. Why? Because 1A is a smaller current, so you can use thinner wire on the secondary side, less loss, and it\u2019s a bit safer for long runs. If you\u2019re running secondary wires 100+ feet, 1A is way better than 5A \u2013 less voltage drop, no weird errors messing up your readings.<\/p>\n

But here\u2019s where I see so many buyers mess up: they don\u2019t pay attention to the \u201cburden\u201d of the secondary circuit. Wait, burden? That\u2019s the amount of resistance (or load) on the secondary side, right? Like, if you hook up a meter and a relay, together they add up to some resistance \u2013 that\u2019s your burden, measured in volt-amperes (VA) or ohms. The CT\u2019s rated accuracy only applies if the burden doesn\u2019t exceed what it\u2019s supposed to handle. If your burden is too high, the secondary current gets all wonky \u2013 it\u2019s not perfectly proportional anymore, your readings are off, maybe the CT overheats. That\u2019s a big no-no. For example, if you have a CT rated for 5A, 5VA burden, and your secondary circuit adds up to 7VA, the CT might start saturating during high currents, leading to bad data or even tripping relays when they shouldn\u2019t. I\u2019ve had customers come to me saying their CTs are giving weird readings, and 9 times out of 10 it\u2019s a burden problem they didn\u2019t account for.<\/p>\n

Another key point: when the primary current is way higher than rated \u2013 like during a short circuit \u2013 the secondary current doesn\u2019t keep going up proportionally. That\u2019s CT saturation, and it\u2019s actually a good thing for protection CTs, but bad for metering ones. Metering CTs need linear secondary current all the way up to their rated current, so your energy readings stay accurate. Protection CTs, though, need to handle huge fault currents (like 20x or 50x rated primary) and still output a secondary current that\u2019s predictable enough for relays to trip correctly. So secondary current behavior changes based on whether the CT is metering or protection.<\/p>\n

Let\u2019s use a real example to make this concrete. Say you\u2019re a small factory with a 200A main power line. You need to hook up an energy meter to track usage, so you pick a 200:5 CT \u2013 that means when primary is 200A, secondary is 5A. The CT\u2019s burden rating is 2VA. You hook up a meter that has 0.5VA burden, so total is 0.5VA, which is way under 2VA. Perfect \u2013 your secondary current will be exactly proportional, so your meter readings are spot-on. But if you add a power analyzer that pulls 2VA more, now total burden is 2.5VA, which is over. Your secondary current might be a tiny bit less than it should be, so your meter undercounts your usage. That\u2019s money you\u2019re leaving on the table, and it\u2019s avoidable if you just check that burden.<\/p>\n

Now, as a CT supplier, I can\u2019t tell you how many times people ask me, \u201cCan I get a CT with any secondary current I want?\u201d The short answer is: not really, because the standard 5A and 1A are there for compatibility. Custom secondary currents exist, but they\u2019re rare, and usually only for specific applications. The whole power industry is built around 5A and 1A gear, so mixing and matching would mean you have to build all your own secondary stuff, which is expensive. So stick to the standards unless you have a really good reason not to.<\/p>\n

Another thing I wish more people knew: the secondary side of a CT should never be open-circuited when the primary is carrying current. That\u2019s a huge safety hazard. If you leave the secondary wires disconnected while power\u2019s flowing through the primary, the secondary side will build up a super high voltage \u2013 like thousands of volts \u2013 because there\u2019s no load to dampen the current. That can arc, burn out gear, or even hurt someone. I\u2019ve had a customer who thought it was fine to disconnect a CT to test a meter, and almost got a shock that landed him in the ER. Never open a CT\u2019s secondary when the primary is live. Always short the secondary terminals first if you need to work on it. That\u2019s non-negotiable.<\/p>\n

Wait, let\u2019s circle back to why secondary current matters so much for your operations. If you\u2019re a facility manager, your energy bills are based on those secondary current readings \u2013 if they\u2019re wrong, you\u2019re overpaying. If you\u2019re an electrician installing a new substation, the protection relays rely on secondary current to trip during faults \u2013 if that\u2019s off, you could have a fire or a major outage. So getting the secondary current right isn\u2019t just a technical detail, it\u2019s a big deal for your wallet and your safety.<\/p>\n

Now, when you\u2019re shopping for a CT, here\u2019s my quick checklist for secondary current: first, pick standard 5A or 1A based on your setup (secondary wire length, existing gear). Second, confirm the rated ratio matches your primary current (don\u2019t get a 200:5 CT for a 300A line \u2013 that\u2019ll under-read). Third, calculate your total secondary burden and make sure it\u2019s at or below the CT\u2019s rated burden. Fourth, make sure it\u2019s the right type: metering or protection, depending on what you\u2019re using it for.<\/p>\n

I\u2019ve been in this CT game for years, and I see the same mistakes over and over. Folks look at the primary current and buy a CT without checking the secondary, or ignore the burden, or forget about the open-circuit risk. The secondary current is the heart of what the CT does \u2013 it\u2019s the signal that lets you measure power, keep your equipment safe, keep the lights on. Mess that up, and the whole system falls apart.<\/p>\n

So if you\u2019re working on a project right now, or you\u2019re tired of your CTs giving wonky readings or causing headaches, don\u2019t guess. Hit me up to chat through your needs, I\u2019ll help you pick the right CT with the perfect secondary current for your setup. No sales pitches, just straight talk \u2013 I\u2019ve got 10+ years of experience helping folks get this right the first time, so you don\u2019t have to waste time or money on the wrong gear.<\/p>\n

<\/p>\n

Thanks for sticking with me through all this \u2013 I hope I made secondary current make sense, instead of just another confusing power term. Now go check that CT burden, and if you need help, I\u2019m here.<\/p>\n

LCD<\/a> References:<\/p>\n

    \n
  1. Electrical Safety Authority. (2021). Current Transformer Safety and Application Guide. ESA Publications.<\/li>\n
  2. Westinghouse Electric Corporation. (1995). Industrial Power Distribution Handbook. McGraw-Hill.<\/li>\n
  3. International Electrotechnical Commission. (2018). IEC 60044-1: Instrument Transformers \u2013 Part 1: Current Transformers. IEC Standards.<\/li>\n
  4. IEEE Power & Energy Society. (2020). Recommended Practice for Instrument Transformers Applications in Power Systems. IEEE Std C57.13.<\/li>\n<\/ol>\n
    \n

    Jian Xin Technical Limited<\/a>
    Jian Xin Technical Limited is well-known as one of the leading current transformer manufacturers and suppliers in China. If you’re going to buy high quality current transformer with low price, welcome to get pricelist from our factory. Also, customized service is available.
    Address: Jianxin Industry Park, Longtan Load, Yuhang District, Hangzhou, China. 311121
    E-mail: marketing@jianxintechnical.com
    WebSite:     https:\/\/www.jianxintechnical.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

    Hey there! If you\u2019ve ever dug into power distribution gear, you\u2019ve probably heard the term \u201ccurrent … What is the secondary current of a Current Transformer?<\/span>Read more<\/a><\/p>\n","protected":false},"author":8,"featured_media":532,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[495],"class_list":["post-532","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-current-transformer-4ee1-d97f6e"],"_links":{"self":[{"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/posts\/532","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/users\/8"}],"replies":[{"embeddable":true,"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/comments?post=532"}],"version-history":[{"count":0,"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/posts\/532\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/posts\/532"}],"wp:attachment":[{"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/media?parent=532"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/categories?post=532"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.aranciamiss.com\/blog\/wp-json\/wp\/v2\/tags?post=532"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}