{"id":1804,"date":"2026-07-27T10:33:47","date_gmt":"2026-07-27T02:33:47","guid":{"rendered":"https:\/\/jutrion.com\/?page_id=1804"},"modified":"2026-07-27T10:36:09","modified_gmt":"2026-07-27T02:36:09","slug":"cable-derating-iz-calculator","status":"publish","type":"page","link":"https:\/\/jutrion.com\/tl\/tools\/cable-derating-iz-calculator\/","title":{"rendered":"Cable Derating &amp; Iz\u00a0Calculator"},"content":{"rendered":"\n<!-- ===== JUTRION Cable Derating & Iz Calculator \u2014 paste into a WordPress \"Custom HTML\" block ===== -->\n<div class=\"jt-calc\">\n  <p class=\"eyebrow\">Power distribution<\/p>\n  <h1>Cable Derating &amp; I<sub>z<\/sub> Calculator<\/h1>\n  <p class=\"tool-lede\">Combine the ambient, grouping, and thermal-insulation correction factors with a cable&#8217;s tabulated current-carrying capacity to find the real installed capacity I<sub>z<\/sub>, and check it against the breaker rating on the I<sub>n<\/sub> \u2264 I<sub>z<\/sub> rule. Read the base capacity and factors from the wiring tables adopted for the project.<\/p>\n\n  <div class=\"jt-card\"><div class=\"jt-grid\">\n    <div class=\"jt-inputs\">\n      <div class=\"jt-field\"><label for=\"jt-it\">Tabulated capacity I<sub>t<\/sub> (A)<\/label><input type=\"number\" id=\"jt-it\" value=\"76\" min=\"1\" step=\"1\" inputmode=\"numeric\"><\/div>\n      <div class=\"jt-two\">\n        <div class=\"jt-field\"><label for=\"jt-ca\">Ambient factor C<sub>a<\/sub><\/label><input type=\"number\" id=\"jt-ca\" value=\"0.87\" min=\"0.1\" max=\"1.2\" step=\"0.01\" inputmode=\"decimal\"><\/div>\n        <div class=\"jt-field\"><label for=\"jt-cg\">Grouping factor C<sub>g<\/sub><\/label><input type=\"number\" id=\"jt-cg\" value=\"0.8\" min=\"0.1\" max=\"1\" step=\"0.01\" inputmode=\"decimal\"><\/div>\n      <\/div>\n      <div class=\"jt-two\">\n        <div class=\"jt-field\"><label for=\"jt-ci\">Insulation factor C<sub>i<\/sub><\/label><input type=\"number\" id=\"jt-ci\" value=\"1.0\" min=\"0.1\" max=\"1\" step=\"0.01\" inputmode=\"decimal\"><\/div>\n        <div class=\"jt-field\"><label for=\"jt-in\">Breaker rating I<sub>n<\/sub> (A)<\/label><input type=\"number\" id=\"jt-in\" value=\"50\" min=\"1\" step=\"1\" inputmode=\"numeric\"><\/div>\n      <\/div>\n      <div class=\"jt-note\"><span>\u24d8<\/span><span>C<sub>a<\/sub> corrects for ambient temperature, C<sub>g<\/sub> for grouping with other loaded circuits, C<sub>i<\/sub> for contact with thermal insulation. Use 1.0 where a factor does not apply.<\/span><\/div>\n    <\/div>\n    <div class=\"jt-readout\">\n      <div><div class=\"jt-rl\">Installed capacity I<sub>z<\/sub><\/div><div class=\"jt-big\"><span id=\"jt-iz\">\u2014<\/span> <span>A<\/span><\/div><\/div>\n      <div class=\"jt-rec row\"><div class=\"k\">Combined derating<\/div><div class=\"v\" id=\"jt-fac\">\u2014<\/div><\/div>\n      <div class=\"jt-rec\"><div class=\"k\">Coordination \u2014 I<sub>n<\/sub> \u2264 I<sub>z<\/sub><\/div><div style=\"margin-top:6px\"><span class=\"jt-pill na\" id=\"jt-chk\">\u2014<\/span><\/div><\/div>\n      <div class=\"jt-rec row\"><div class=\"k\">Min. base I<sub>t<\/sub> for this I<sub>n<\/sub><\/div><div class=\"v\" id=\"jt-need\">\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>Why tabulated capacity is not the installed capacity<\/h2>\n    <p>Cable tables give a base current-carrying capacity I<sub>t<\/sub> for a reference condition \u2014 a stated ambient, one circuit, a defined installation method. Real installations are hotter, grouped, or wrapped in insulation, all of which reduce how much current the conductor can carry before it overheats. The installed capacity is the base value scaled by every applicable factor:<\/p>\n    <div class=\"jt-eq\">I<sub>z<\/sub> = I<sub>t<\/sub> \u00d7 C<sub>a<\/sub> \u00d7 C<sub>g<\/sub> \u00d7 C<sub>i<\/sub><\/div>\n    <p>Protection is only valid when the breaker rating sits below the <i>corrected<\/i> capacity, not the table value:<\/p>\n    <div class=\"jt-eq\">I<sub>b<\/sub> \u2264 I<sub>n<\/sub> \u2264 I<sub>z<\/sub><\/div>\n    <p>To work backwards, the minimum base capacity a cable must have for a chosen breaker is I<sub>t<\/sub> \u2265 I<sub>n<\/sub> \/ (C<sub>a<\/sub> \u00d7 C<sub>g<\/sub> \u00d7 C<sub>i<\/sub>) \u2014 the figure shown on the right.<\/p>\n\n    <h2>Where each factor comes from<\/h2>\n    <ul>\n      <li><b>C<sub>a<\/sub> \u2014 ambient:<\/b> below 1.0 when the ambient exceeds the table reference (usually 30 \u00b0C in air, 20 \u00b0C in ground); above 1.0 for cooler surroundings.<\/li>\n      <li><b>C<sub>g<\/sub> \u2014 grouping:<\/b> below 1.0 when several loaded circuits share a route, tray, or conduit and heat each other.<\/li>\n      <li><b>C<sub>i<\/sub> \u2014 thermal insulation:<\/b> below 1.0 where a cable is in contact with or surrounded by thermal insulation that traps heat.<\/li>\n    <\/ul>\n    <div class=\"jt-note\"><span>\u26a0<\/span><span><b>Use the correct table set.<\/b> Base capacities and correction factors depend on the insulation type, installation method, and the standard adopted (IEC 60364-5-52 or the local wiring rules). Mixing tables gives a wrong I<sub>z<\/sub>.<\/span><\/div>\n\n    <h2>Frequently asked questions<\/h2>\n    <div class=\"jt-faq\">\n      <details><summary>Which current do I compare against I<sub>z<\/sub>?<\/summary><p>The breaker rating I<sub>n<\/sub>. The design current I<sub>b<\/sub> must be \u2264 I<sub>n<\/sub>, and I<sub>n<\/sub> must be \u2264 the corrected I<sub>z<\/sub>. If I<sub>n<\/sub> exceeds I<sub>z<\/sub>, upsize the cable \u2014 never upsize the breaker to stop tripping.<\/p><\/details>\n      <details><summary>Can a factor be greater than 1.0?<\/summary><p>Yes \u2014 the ambient factor exceeds 1.0 when the surroundings are cooler than the table reference, which slightly raises the capacity. Grouping and insulation factors are 1.0 or below.<\/p><\/details>\n      <details><summary>Do the factors multiply or add?<\/summary><p>They multiply. Three factors of 0.9 give 0.9 \u00d7 0.9 \u00d7 0.9 \u2248 0.73, so combined effects reduce capacity faster than any single factor suggests.<\/p><\/details>\n    <\/div>\n\n    <div class=\"jt-cta\">\n      <h2>Match the protective device<\/h2>\n      <p>With the installed capacity known, confirm the breaker rating and curve stay within I<sub>n<\/sub> \u2264 I<sub>z<\/sub>.<\/p>\n      <div class=\"jt-cta-btns\">\n        <a class=\"jt-btn\" href=\"https:\/\/jutrion.com\/mccb\/\">Explore JUTRION MCCBs<\/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\/mccb\/\">MCCB<\/a> \u00b7 <a href=\"https:\/\/jutrion.com\/mcb\/\">MCB<\/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 60364-5-52 (current-carrying capacity, correction factors) \u00b7 IEC 60364-4-43 (I<sub>b<\/sub> \u2264 I<sub>n<\/sub> \u2264 I<sub>z<\/sub>).<\/div>\n    <p class=\"jt-fine\">Estimate for guidance only. Base capacities, correction factors, and the final cable and breaker selection must be taken from the adopted wiring tables and verified by a qualified person for the actual installation.<\/p>\n  <\/div>\n<\/div>\n\n<script>\n(function(){\n  var $=function(id){return document.getElementById(id);};\n  var it=$('jt-it'),ca=$('jt-ca'),cg=$('jt-cg'),ci=$('jt-ci'),inn=$('jt-in'),\n      izEl=$('jt-iz'),facEl=$('jt-fac'),chk=$('jt-chk'),needEl=$('jt-need');\n  function calc(){\n    var It=parseFloat(it.value),a=parseFloat(ca.value),g=parseFloat(cg.value),c=parseFloat(ci.value),In=parseFloat(inn.value);\n    if(!(It>0&&a>0&&g>0&&c>0)){izEl.textContent='\u2014';facEl.textContent='\u2014';chk.className='jt-pill na';chk.textContent='\u2014';needEl.textContent='\u2014';return;}\n    var f=a*g*c,Iz=It*f;\n    izEl.textContent=Iz.toFixed(1);\n    facEl.textContent='\u00d7 '+f.toFixed(3);\n    if(In>0){var pass=In<=Iz;chk.className='jt-pill '+(pass?'ok':'no');chk.textContent=pass?('PASS \u2014 In '+In+' A \u2264 Iz '+Iz.toFixed(0)+' A'):('FAIL \u2014 In '+In+' A > Iz '+Iz.toFixed(0)+' A');needEl.textContent='\u2265 '+(In\/f).toFixed(1)+' A';}\n    else{chk.className='jt-pill na';chk.textContent='enter In';needEl.textContent='\u2014';}\n  }\n  [it,ca,cg,ci,inn].forEach(function(el){el.addEventListener('input',calc);el.addEventListener('change',calc);});\n  $('jt-copy').addEventListener('click',function(){var b=$('jt-copy');var txt='Tabulated It: '+it.value+' A\\nFactors: Ca '+ca.value+' \u00d7 Cg '+cg.value+' \u00d7 Ci '+ci.value+'\\nInstalled Iz: '+izEl.textContent+' A ('+facEl.textContent+')\\nCoordination: '+chk.textContent+'\\nMin base It: '+needEl.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 Cable Derating &amp; Iz Calculator Combine the ambient, grouping, and thermal-insulation correction factors with a cable&#8217;s tabulated current-carrying capacity to find the real installed capacity Iz, and check it against the breaker rating on the In \u2264 Iz rule. Read the base capacity and factors from the wiring tables adopted for the project. [&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-1804","page","type-page","status-publish","hentry"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/pages\/1804","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/comments?post=1804"}],"version-history":[{"count":1,"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/pages\/1804\/revisions"}],"predecessor-version":[{"id":1805,"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/pages\/1804\/revisions\/1805"}],"up":[{"embeddable":true,"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/pages\/1778"}],"wp:attachment":[{"href":"https:\/\/jutrion.com\/tl\/wp-json\/wp\/v2\/media?parent=1804"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}