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What is a QMB333TW?
QMB panelboard, switchboard with QMB distribution.
Products sold in the United States.
The QMB333TW was a device designed for use with Class T fuses on a 480Y/277V system (see next paragraph for more history on this device). This device is now OBSOLETE. The substitution would be a QMJ363T, which uses Class J fuses, and is a 3-pole device. (Note: If replacing an existing QMB333TW in a panel, in order to fill up the space, a Blank Filler catalog number QMB1BLW may also be needed.)
HISTORY: The QMB333TW was designed for use with Class T fuses on a 480Y/277 Vac system. The 1990 National Electrical Code (NEC) 240-60(a) exception permitted this application. The wording in the 1993 NEC 240-60(a) exception was clarified to support the proper application of a 300 Volt type fuse ``in single-phase line-to-neutral circuits supplied from 3-phase, 4-wire, solidly grounded neutral systems where the line-to-neutral voltage does not exceed 300V.`` 300 Volt type fuses are permitted only when applied in accordance with NEC 240.60(a). As the device is now obsolete, consider using a 3-pole QMJ363T (uses Class J fuses) for this application.
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