{"id":4131,"date":"2023-01-05T14:16:14","date_gmt":"2023-01-05T13:16:14","guid":{"rendered":"https:\/\/www.gt-engineering.it\/normative-tecniche\/%tipologia_normativa%\/circuiti-di-controllo\/"},"modified":"2025-08-18T15:42:45","modified_gmt":"2025-08-18T14:42:45","slug":"control-circuits","status":"publish","type":"normativa_tecnica","link":"https:\/\/www.gt-engineering.it\/en\/technical-standards\/ul-and-nfpa-standards\/ul-508a\/control-circuits\/","title":{"rendered":"UL 508A &#8211; Control Circuits"},"content":{"rendered":"\n\n    \n        <section\n            data-name=\"Contenuto testuale con immagine\"\n            class=\"bow-gte-testo-media u-spacer-base\"\n            id=\"\"\n        >\n            \n<div class=\"o-text-media px-side-spacer sm:px-side-spacer-sm lg:px-side-spacer-tablet 2xl:px-side-spacer-desktop flex flex-wrap items-start md:block\">\n\n    \n        \n                    <div\n                class=\"h-auto mb-6 md:mb-4 lg:mb-8 xl:mb-9 float-left mr-6 md:mr-4 lg:mr-8 xl:mr-9  overflow-hidden shadow-content-image w-full md:w-1\/2\"\n            >\n                \r\n\r\n    \r\n        \r\n            \r\n                \r\n                    <picture\r\n                        class=\"\"\r\n                                            >\r\n                                                                                                        <source\r\n                                media=\"(max-width: 150px)\"\r\n                                width=\"150\"\r\n                                height=\"150\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/branch_feeder1-150x150.png.webp\"\r\n                                type=\"image\/webp\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 300px)\"\r\n                                width=\"300\"\r\n                                height=\"260\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/branch_feeder1-300x260.png.webp\"\r\n                                type=\"image\/webp\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 768px)\"\r\n                                width=\"768\"\r\n                                height=\"665\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/branch_feeder1-768x665.png.webp\"\r\n                                type=\"image\/webp\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 1024px)\"\r\n                                width=\"1024\"\r\n                                height=\"886\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/branch_feeder1-1024x886.png.webp\"\r\n                                type=\"image\/webp\"\r\n                            \/>\r\n                                                                            <img\r\n                            class=\"a-image transparent border-image h-auto  w-full\"\r\n                                                        srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/branch_feeder1.png.webp\"\r\n                                                        title=\"branch_feeder1\"\r\n                                                        type=\"image\/webp\"\r\n                        \/>\r\n                    <\/picture>\r\n\r\n                \r\n\r\n                    \r\n    \r\n            <\/div>\n        \n                    <div class=\"relative w-full\">\n                \n<div class=\"a-text font-text text-base normal mt-8 first:mt-0 space-y-4\">\n    <p>According to NEC Art.100 any circuit that controls another circuit through a relay or equivalent device can be considered a control circuit.&nbsp;This circuit can be&nbsp;<strong>upstream<\/strong> or <strong>downstream <\/strong>of the <strong>Branch Circuit Protection Device (BCPD)<\/strong>.<\/p>\n<p>The control circuit is usually supplied by a transformer that lowers&nbsp;the voltage below <strong>600 V AC<\/strong>&nbsp;and has no current limits (UL508A). On the other hand, according to NFPA 79&nbsp;the maximum voltage must not exceed <strong>120 V AC<\/strong>&nbsp;or <strong>250 V DC <\/strong>(9.1.2.2). Moreover, autotransformers cannot be used. The control transformer is not necessary if:<\/p>\n<ul>\n<li>the supply voltage is already 120 V AC;<\/li>\n<li>the maximum short-circuit current is less than 1000 A.<\/li>\n<\/ul>\n<p>With regard to the protection devices that can be used in a&nbsp;control circuit:&nbsp;<\/p>\n<ul>\n<li>if the control circuit is downstream of a BCPD, <strong>Supplementary protections<\/strong> (UL1077) can be used, since, in case of&nbsp;short-circuit,&nbsp;the upstream BCPD is supposed to be able to interrupt the fault currents;<\/li>\n<li>if the control circuit is upstream of a BPCD, a UL489 cicuit breaker or a suitable fuse UL248&nbsp;is required.<\/li>\n<\/ul>\n<p>The control transformer can be protected <strong>only<\/strong> <strong>on the<\/strong> <strong>primary side<\/strong> (referring to <strong>Table 42.1<\/strong>) or, to avoid unwanted tripping, on<strong> both the primary and secondary side<\/strong> (<strong>Table 42.2<\/strong>).<\/p>\n<\/div>            <\/div>\n        \n        \n                <div class=\"clear-both\"><\/div>\n        \n\n    <\/div>\n        <\/section>\n\n    \n","protected":false},"excerpt":{"rendered":"<p>According to NEC Art.100 any circuit that controls another circuit through a relay or equivalent device can be considered a control circuit.\u00a0This circuit can be\u00a0upstream or downstream of the Branch Circuit Protection Device (BCPD).<\/p>\n","protected":false},"author":1,"featured_media":4132,"parent":4095,"menu_order":9,"template":"","meta":{"_acf_changed":false,"footnotes":""},"normative-tecniche":[59],"class_list":["post-4131","normativa_tecnica","type-normativa_tecnica","status-publish","has-post-thumbnail","hentry","tipologia_normativa-ul-and-nfpa-standards"],"acf":{"sottotitolo":"Voltage supply, short-circuit current e control devices","allegati":null},"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.2 - 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