{"id":1802,"date":"2026-07-27T10:32:55","date_gmt":"2026-07-27T02:32:55","guid":{"rendered":"https:\/\/jutrion.com\/?page_id=1802"},"modified":"2026-07-27T10:38:58","modified_gmt":"2026-07-27T02:38:58","slug":"%e5%a4%89%e5%9c%a7%e5%99%a8%e9%9b%bb%e6%b5%81%e8%a8%88%e7%ae%97%e6%a9%9f","status":"publish","type":"page","link":"https:\/\/jutrion.com\/ja\/tools\/transformer-current-calculator\/","title":{"rendered":"\u5909\u5727\u5668\u5168\u8ca0\u8377\u96fb\u6d41\u8a08\u7b97\u6a5f"},"content":{"rendered":"\n<!-- ===== JUTRION Transformer Current Calculator \u2014 paste into a WordPress \"Custom HTML\" block ===== -->\n<div class=\"jt-calc\">\n  <p class=\"eyebrow\">Power distribution<\/p>\n  <h1>Transformer Full-Load Current Calculator<\/h1>\n  <p class=\"tool-lede\">Calculate the rated line current on both the primary and secondary of a transformer from its kVA rating, the two voltages, and the phase arrangement \u2014 the starting point for sizing the incoming and outgoing protection, cables, and CTs.<\/p>\n\n  <div class=\"jt-card\"><div class=\"jt-grid\">\n    <div class=\"jt-inputs\">\n      <div class=\"jt-field\"><label>Phase<\/label>\n        <div class=\"jt-seg\" id=\"jt-phase\"><button type=\"button\" data-ph=\"3\" aria-pressed=\"true\">Three-phase<\/button><button type=\"button\" data-ph=\"1\" aria-pressed=\"false\">Single-phase<\/button><\/div>\n      <\/div>\n      <div class=\"jt-field\"><label for=\"jt-kva\">Rating (kVA)<\/label><input type=\"number\" id=\"jt-kva\" value=\"1000\" min=\"0.1\" step=\"1\" inputmode=\"decimal\"><\/div>\n      <div class=\"jt-two\">\n        <div class=\"jt-field\"><label for=\"jt-vp\">Primary voltage (V)<\/label><input type=\"number\" id=\"jt-vp\" value=\"11000\" min=\"1\" step=\"1\" inputmode=\"numeric\"><\/div>\n        <div class=\"jt-field\"><label for=\"jt-vs\">Secondary voltage (V)<\/label><input type=\"number\" id=\"jt-vs\" value=\"400\" min=\"1\" step=\"1\" inputmode=\"numeric\"><\/div>\n      <\/div>\n      <div class=\"jt-note\"><span>\u24d8<\/span><span>Use the line-to-line voltage for a three-phase transformer. Current is the rated full-load value; inrush on energisation is far higher for a brief moment.<\/span><\/div>\n    <\/div>\n    <div class=\"jt-readout\">\n      <div><div class=\"jt-rl\">Secondary full-load current<\/div><div class=\"jt-big\"><span id=\"jt-is\">\u2014<\/span> <span>A<\/span><\/div><\/div>\n      <div class=\"jt-rec row\"><div class=\"k\">Primary full-load current<\/div><div class=\"v\" id=\"jt-ip\">\u2014<\/div><\/div>\n      <div class=\"jt-rec row\"><div class=\"k\">Voltage ratio<\/div><div class=\"v\" id=\"jt-ratio\">\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>Formulas used<\/h2>\n    <div class=\"jt-eq\">Three-phase: I = S \/ (\u221a3 \u00d7 V)\nSingle-phase: I = S \/ V\nRatio: V_primary \/ V_secondary<\/div>\n    <p>where <b>S<\/b> is the apparent power (VA), <b>V<\/b> the line voltage on the side being calculated, and <b>I<\/b> the line current on that side. The apparent power is the same on both windings (losses aside), so the low-voltage side always carries the higher current. A 1000 kVA, 11 kV \/ 400 V transformer draws about 52 A on the primary but delivers about 1443 A on the secondary.<\/p>\n\n    <h2>What the currents are used for<\/h2>\n    <ul>\n      <li><b>Secondary current<\/b> sizes the main LV breaker or ACB, the busbar, and the outgoing distribution.<\/li>\n      <li><b>Primary current<\/b> sizes the HV protection and the incoming cable or ring-main unit.<\/li>\n      <li>Both feed the CT ratios for metering and protection on each side.<\/li>\n    <\/ul>\n    <div class=\"jt-note\"><span>\u26a0<\/span><span><b>Full-load current is not the whole story.<\/b> Energising inrush can reach 8\u201312 \u00d7 the rated current for a few cycles, and the secondary fault level is set by the impedance voltage u<sub>k<\/sub>. Size protection to carry the full-load current, ride through inrush, and break the prospective fault current.<\/span><\/div>\n\n    <h2>Frequently asked questions<\/h2>\n    <div class=\"jt-faq\">\n      <details><summary>Why is the secondary current so much higher?<\/summary><p>Power is conserved across the windings, so as the voltage steps down the current steps up by the same ratio. The low-voltage side therefore carries the larger current and needs the heavier busbar and protection.<\/p><\/details>\n      <details><summary>Which voltage do I enter for three-phase?<\/summary><p>The line-to-line voltage on each side (for example 11000 V and 400 V), because the \u221a3 relationship uses the line voltage.<\/p><\/details>\n      <details><summary>Does this give the fault current too?<\/summary><p>No. This is the rated full-load current. The prospective short-circuit current at the secondary depends on the impedance voltage u<sub>k<\/sub> \u2014 use a short-circuit calculation for that.<\/p><\/details>\n    <\/div>\n\n    <div class=\"jt-cta\">\n      <h2>Size the protection next<\/h2>\n      <p>With both currents known, select the primary and secondary breakers, ATS, and CTs \u2014 or ask JUTRION for a suitable set for your market.<\/p>\n      <div class=\"jt-cta-btns\">\n        <a class=\"jt-btn\" href=\"https:\/\/jutrion.com\/products\/\">Browse JUTRION products<\/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\/acb\/\">ACB<\/a> \u00b7 <a href=\"https:\/\/jutrion.com\/mccb\/\">MCCB<\/a> \u00b7 <a href=\"https:\/\/jutrion.com\/ats\/\">ATS<\/a><\/p>\n    <\/div>\n\n    <div class=\"jt-std\"><strong>Standards referenced<\/strong>IEC 60076 (power transformers) \u00b7 IEC 60038 (standard voltages) \u00b7 basic three-phase power relationships.<\/div>\n    <p class=\"jt-fine\">Engineering estimate for guidance only. Final protection, cable, and CT selection must account for inrush, the impedance voltage, the fault level, and the applicable standards, verified by a qualified person.<\/p>\n  <\/div>\n<\/div>\n\n<script>\n(function(){\n  var $=function(id){return document.getElementById(id);};\n  var kva=$('jt-kva'),vp=$('jt-vp'),vs=$('jt-vs'),isEl=$('jt-is'),ipEl=$('jt-ip'),ratioEl=$('jt-ratio');\n  var phase=3;\n  document.querySelectorAll('#jt-phase button').forEach(function(b){b.addEventListener('click',function(){phase=parseInt(b.dataset.ph,10);document.querySelectorAll('#jt-phase button').forEach(function(x){x.setAttribute('aria-pressed',x===b?'true':'false');});calc();});});\n  function amp(S,V){return (phase===3)?(S\/(Math.sqrt(3)*V)):(S\/V);}\n  function fmt(x){return x>=100?Math.round(x):(Math.round(x*10)\/10);}\n  function calc(){\n    var S=parseFloat(kva.value)*1000,Vp=parseFloat(vp.value),Vs=parseFloat(vs.value);\n    if(!(S>0&&Vp>0&&Vs>0)){isEl.textContent='\u2014';ipEl.textContent=ratioEl.textContent='\u2014';return;}\n    isEl.textContent=fmt(amp(S,Vs));\n    ipEl.textContent=fmt(amp(S,Vp))+' A';\n    ratioEl.textContent=(Vp\/Vs).toFixed(2)+' : 1';\n  }\n  [kva,vp,vs].forEach(function(el){el.addEventListener('input',calc);el.addEventListener('change',calc);});\n  $('jt-copy').addEventListener('click',function(){var b=$('jt-copy');var txt='Transformer: '+kva.value+' kVA, '+vp.value+' V \/ '+vs.value+' V, '+(phase===3?'3-phase':'1-phase')+'\\nPrimary FLC: '+ipEl.textContent+'\\nSecondary FLC: '+isEl.textContent+' A\\nRatio: '+ratioEl.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 distribution Transformer Full-Load Current Calculator Calculate the rated line current on both the primary and secondary of a transformer from its kVA rating, the two voltages, and the phase arrangement \u2014 the starting point for sizing the incoming and outgoing protection, cables, and CTs. Phase Three-phaseSingle-phase Rating (kVA) Primary voltage (V) Secondary voltage (V) \u24d8Use the line-to-line voltage for a three-phase transformer. Current is the rated full-load value; inrush on energisation is far higher for a brief moment. Secondary full-load current \u2014 A Primary full-load current \u2014 Voltage ratio \u2014 Copy result Formulas used Three-phase: I = S \/ (\u221a3 \u00d7 V) Single-phase: I = S \/ V Ratio: [&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-1802","page","type-page","status-publish","hentry"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/pages\/1802","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/comments?post=1802"}],"version-history":[{"count":1,"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/pages\/1802\/revisions"}],"predecessor-version":[{"id":1803,"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/pages\/1802\/revisions\/1803"}],"up":[{"embeddable":true,"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/pages\/1778"}],"wp:attachment":[{"href":"https:\/\/jutrion.com\/ja\/wp-json\/wp\/v2\/media?parent=1802"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}