{"id":1800,"date":"2026-07-27T10:33:03","date_gmt":"2026-07-27T02:33:03","guid":{"rendered":"https:\/\/jutrion.com\/?page_id=1800"},"modified":"2026-07-27T10:36:19","modified_gmt":"2026-07-27T02:36:19","slug":"may-tinh-ty-so-ct-va-ganh-nang","status":"publish","type":"page","link":"https:\/\/jutrion.com\/vi\/tools\/ct-ratio-burden-calculator\/","title":{"rendered":"C\u00f4ng c\u1ee5 t\u00ednh t\u1ef7 s\u1ed1 CT &amp; g\u00e1nh n\u1eb7ng"},"content":{"rendered":"\n<!-- ===== JUTRION CT Ratio & Burden Calculator \u2014 paste into a WordPress \"Custom HTML\" block ===== -->\n<div class=\"jt-calc\">\n  <p class=\"eyebrow\">Power metering<\/p>\n  <h1>CT Ratio &amp; Burden Calculator (Metering &amp; Protection)<\/h1>\n  <p class=\"tool-lede\">Select a reference current-transformer ratio from the primary current, then add up the secondary burden from the connected meters and relays and the pilot leads, so the CT&#8217;s rated burden (VA) and class can be chosen. The exact accuracy class and saturation still come from the CT datasheet.<\/p>\n\n  <div class=\"jt-card\"><div class=\"jt-grid\">\n    <div class=\"jt-inputs\">\n      <div class=\"jt-two\">\n        <div class=\"jt-field\"><label for=\"jt-ip\">Primary current (A)<\/label><input type=\"number\" id=\"jt-ip\" value=\"630\" min=\"1\" step=\"1\" inputmode=\"numeric\"><\/div>\n        <div class=\"jt-field\"><label for=\"jt-sec\">Secondary<\/label>\n          <select id=\"jt-sec\"><option value=\"5\" selected>5 A<\/option><option value=\"1\">1 A<\/option><\/select>\n        <\/div>\n      <\/div>\n      <div class=\"jt-field\"><label for=\"jt-dev\">Connected device burden (VA)<\/label><input type=\"number\" id=\"jt-dev\" value=\"3\" min=\"0\" step=\"0.1\" inputmode=\"decimal\"><\/div>\n      <div class=\"jt-two\">\n        <div class=\"jt-field\"><label for=\"jt-len\">Lead length one-way (m)<\/label><input type=\"number\" id=\"jt-len\" value=\"10\" min=\"0\" step=\"0.5\" inputmode=\"decimal\"><\/div>\n        <div class=\"jt-field\"><label for=\"jt-cs\">Lead cross-section (mm\u00b2)<\/label><input type=\"number\" id=\"jt-cs\" value=\"2.5\" min=\"0.5\" step=\"0.5\" inputmode=\"decimal\"><\/div>\n      <\/div>\n      <div class=\"jt-note\"><span>\u24d8<\/span><span>Lead burden rises with the square of the secondary current, so a 1 A CT loses about 25\u00d7 less in the leads than a 5 A CT \u2014 the usual choice for long runs.<\/span><\/div>\n    <\/div>\n    <div class=\"jt-readout\">\n      <div><div class=\"jt-rl\">Reference CT ratio<\/div><div class=\"jt-type\" id=\"jt-ratio\">\u2014<\/div><\/div>\n      <div class=\"jt-rec row\"><div class=\"k\">Lead burden<\/div><div class=\"v\" id=\"jt-vlead\">\u2014<\/div><\/div>\n      <div class=\"jt-rec row\"><div class=\"k\">Total burden<\/div><div class=\"v\" id=\"jt-vtot\">\u2014<\/div><\/div>\n      <div class=\"jt-rec row\"><div class=\"k\">Select CT rated burden<\/div><div class=\"v\" id=\"jt-vsel\">\u2014<\/div><\/div>\n      <button type=\"button\" class=\"jt-copy\" id=\"jt-copy\">Copy result<\/button>\n    <\/div>\n  <\/div><\/div>\n\n  <div class=\"jt-content\">\n    <h2>Ratio and burden together<\/h2>\n    <p>A CT is specified by two things: its <b>ratio<\/b> (primary current to a 5 A or 1 A standard secondary) and its <b>rated burden<\/b> (the VA it can drive at rated secondary while holding its accuracy class). Load the CT beyond its rated burden and the accuracy collapses \u2014 and a protection CT may saturate and fail to operate the relay.<\/p>\n    <div class=\"jt-eq\">Lead burden VA = I_sec\u00b2 \u00d7 \u03c1 \u00d7 (2 \u00d7 L) \/ S\nTotal burden = device VA + lead burden VA\nSelect CT: rated burden \u2265 total burden<\/div>\n    <p>The leads count twice (go and return), and copper resistivity \u03c1 \u2248 0.0175 \u03a9\u00b7mm\u00b2\/m. The device VA is the sum of the connected meters, transducers, and relays at rated secondary. The tool rounds up to the next standard CT burden (2.5, 5, 10, 15, 30 VA).<\/p>\n\n    <h2>Why 1 A for long runs<\/h2>\n    <p>Because lead burden scales with the square of the secondary current, dropping from 5 A to 1 A cuts the lead loss to one twenty-fifth. For switchrooms with long pilot cables, a 1 A CT keeps the burden within a modest rating; for short local metering, 5 A is common and convenient.<\/p>\n    <div class=\"jt-note\"><span>\u26a0<\/span><span><b>Metering and protection CTs are not interchangeable.<\/b> A metering CT (class 0.2 \/ 0.5) is accurate around rated current and saturates early to protect instruments. A protection CT (class 5P \/ 10P with an ALF) must stay linear up to the fault current. Match the class to the job.<\/span><\/div>\n\n    <h2>Frequently asked questions<\/h2>\n    <div class=\"jt-faq\">\n      <details><summary>What if my burden is well below the CT rating?<\/summary><p>A lightly loaded metering CT is fine and stays accurate. A lightly loaded protection CT is also fine \u2014 but never exceed the rated burden, or the accuracy and saturation behaviour are no longer guaranteed.<\/p><\/details>\n      <details><summary>Does the accuracy class come from this calculation?<\/summary><p>No. The class (0.2, 0.5, 5P10, 10P20, and the accuracy limit factor) is a CT property from the datasheet. This tool sizes the burden the CT must be rated to drive; the class is chosen to suit metering or protection.<\/p><\/details>\n      <details><summary>How do I pick the primary rating?<\/summary><p>Choose a standard primary at or just above the maximum load current so the CT runs near its accurate range without saturating in normal service. This tool rounds up to the nearest standard CT primary.<\/p><\/details>\n    <\/div>\n\n    <div class=\"jt-cta\">\n      <h2>Complete the metering scheme<\/h2>\n      <p>With the ratio, burden, and class set, match the CT and the associated protection relay to the switchboard.<\/p>\n      <div class=\"jt-cta-btns\">\n        <a class=\"jt-btn\" href=\"https:\/\/jutrion.com\/relay\/\">Explore JUTRION relays<\/a>\n        <a class=\"jt-btn ghost\" href=\"https:\/\/jutrion.com\/contact\/\">Get a free quote<\/a>\n      <\/div>\n      <p class=\"jt-related\">Related products: <a href=\"https:\/\/jutrion.com\/relay\/\">Relay<\/a> \u00b7 <a href=\"https:\/\/jutrion.com\/acb\/\">ACB<\/a> \u00b7 <a href=\"https:\/\/jutrion.com\/products\/\">All products<\/a><\/p>\n    <\/div>\n\n    <div class=\"jt-std\"><strong>Standards referenced<\/strong>IEC 61869-2 (current transformers) \u00b7 standard secondary currents 5 A \/ 1 A \u00b7 standard burdens 2.5\u201330 VA.<\/div>\n    <p class=\"jt-fine\">Estimate for guidance only. The final CT ratio, rated burden, accuracy class, and saturation performance must be confirmed by a qualified person against the CT datasheet and the metering or protection scheme.<\/p>\n  <\/div>\n<\/div>\n\n<script>\n(function(){\n  var $=function(id){return document.getElementById(id);};\n  var ip=$('jt-ip'),sec=$('jt-sec'),dev=$('jt-dev'),len=$('jt-len'),cs=$('jt-cs'),\n      ratioEl=$('jt-ratio'),vleadEl=$('jt-vlead'),vtotEl=$('jt-vtot'),vselEl=$('jt-vsel');\n  var PRIM=[50,75,100,150,200,250,300,400,500,600,630,750,800,1000,1200,1250,1500,1600,2000,2500,3000,4000,5000],\n      BURD=[2.5,5,10,15,30],RHO=0.0175;\n  function nextP(x){for(var i=0;i<PRIM.length;i++){if(PRIM[i]>=x)return PRIM[i];}return null;}\n  function nextB(x){for(var i=0;i<BURD.length;i++){if(BURD[i]>=x)return BURD[i];}return null;}\n  function calc(){\n    var Ip=parseFloat(ip.value),Is=parseFloat(sec.value),dv=parseFloat(dev.value),L=parseFloat(len.value),S=parseFloat(cs.value);\n    if(!(Ip>0)){ratioEl.textContent='\u2014';vleadEl.textContent=vtotEl.textContent=vselEl.textContent='\u2014';return;}\n    var p=nextP(Ip);ratioEl.textContent=(p?p:Math.round(Ip))+' \/ '+Is+' A';\n    var vlead=(L>0&&S>0)?(Is*Is*RHO*(2*L)\/S):0;\n    vleadEl.textContent=vlead.toFixed(2)+' VA';\n    var vtot=(dv>0?dv:0)+vlead;\n    vtotEl.textContent=vtot.toFixed(2)+' VA';\n    var sel=nextB(vtot);\n    vselEl.textContent=sel?('\u2265 '+sel+' VA'):('> 30 VA \u2014 special');\n  }\n  [ip,sec,dev,len,cs].forEach(function(el){el.addEventListener('input',calc);el.addEventListener('change',calc);});\n  $('jt-copy').addEventListener('click',function(){var b=$('jt-copy');var txt='CT ratio: '+ratioEl.textContent+'\\nDevice burden: '+dev.value+' VA\\nLead burden: '+vleadEl.textContent+'\\nTotal burden: '+vtotEl.textContent+'\\nSelect CT rated burden: '+vselEl.textContent;navigator.clipboard.writeText(txt).then(function(){var o=b.textContent;b.textContent='Copied \u2713';setTimeout(function(){b.textContent=o;},1500);}).catch(function(){b.textContent='Press Ctrl+C';});});\n  calc();\n})();\n<\/script>\n<!-- ===== end block ===== -->\n\n","protected":false},"excerpt":{"rendered":"<p>Power metering CT Ratio &amp; Burden Calculator (Metering &amp; Protection) Select a reference current-transformer ratio from the primary current, then add up the secondary burden from the connected meters and relays and the pilot leads, so the CT&#8217;s rated burden (VA) and class can be chosen. The exact accuracy class and saturation still come from [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1778,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-1800","page","type-page","status-publish","hentry"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/pages\/1800","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/comments?post=1800"}],"version-history":[{"count":1,"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/pages\/1800\/revisions"}],"predecessor-version":[{"id":1801,"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/pages\/1800\/revisions\/1801"}],"up":[{"embeddable":true,"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/pages\/1778"}],"wp:attachment":[{"href":"https:\/\/jutrion.com\/vi\/wp-json\/wp\/v2\/media?parent=1800"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}