We^re looking at power supplies and we found a power supply that is not Class 2 rated because it is 100.4VA and the 1999 NEC code book section 725, tables 11(a) and 11(b) chapter 9 states that the Power source maximum nameplate rating is 100VA. I^ve seen several vendors advertise their "DeviceNet" power supplies over 100VA - Deltron, Allen-Bradley. Does this violate NEC code if I were to use these supplies with DeviceNet? If I supply overcurrent protection on a supply that is greater the 100VA, but fuse it at 4A, would this be considered Class 2? Allen-Bradley even advertises its 5.5A DeviceNet power supply as Class 2. The cut sheet states its fuse is Class 2 type accepted and the "nameplate rating of 100VA to be consistent with NEW Class 2 peak outputs current 5.25A". Would this mean if I used a Class 2 type accepted fuse on any power supply 100VA or greater it would meet NEC Class 2 code? Answered by Brian Taylor, Physical Layer SIG Member, e-mail: briant@ra.rockwell.com A23) Editor^s note: A response was offered by Brian Taylor, Rockwell Automation Product Manager. Mr. Taylor is not a regular Dr. DeviceNet volunteer (although he is an active member of the Physical Layer SIG). Despite the commercial nature of the reply, DeviceNet News felt that this question and answer was of interest to many readers of Dr. DeviceNet. When DeviceNet is installed as building wiring, installers must adhere to National Electric Codes (NEC) codes for materials and installation practices. As a class 2 system, DeviceNet must be powered from a Class 2 listed power supply as defined by the NEC. The 1787-DNPS power supply has the required NEC Class 2 listing needed for standard installations of DeviceNet. This power supply was developed with two main characteristics: * Meets all of the DeviceNet specifications for a power supply * Meets National Electric Codes (NEC) for Class 2 Power limited power sources. To meet the DeviceNet Specification the power supply must have the following characteristics: * Rise time of 250ms at full load and with a 7000 uF. capacitive load across output. This requires a significant amount of additional reserve current capability and extra current regulation circuitry * Ripple of 250mv at full rated power. Requires large bulk storage capacitors. * Voltage setting to within 1% that requires precision voltage regulation circuitry. * Voltage regulation to within 1V over full range of rated currents. * Superior Isolation from line, no loop currents and low power buss noise. This requires Linear design. Linear designs are not as efficient and require larger transformers. * Linear designs are not as likely to latch up during short power interruptions or cold starts as switchers tend to do. The 1787-DNPS was specifically designed not latch up as other power supplies have been known to do. This requires special sensing circuits. * Virtually short circuit proof/safe requires sophisticated regulation and current fold back circuits to eliminate the need for customer replaceable fuses. To meet NEC and UL requirements for a Class 2 Listed power source: * No hot surfaces for customers to touch, this requires a larger than normal enclosure design. * Low heat rise in enclosure requires vented enclosure. * IEC type power connection, No exposed hazardous voltages inherent in terminal screw designs. * To obtain the maximum 5.25 Amps from the supply sophisticated current regulation circuits were required (3 layers deep). * To obtain Class 2 listing with no fuses sophisticated safety circuits are required. * To obtain the maximum NEC current while maintaining DeviceNet rise time requirements, an over sized transformer is required. The oversized transformer adds additional weight to the product over a switcher. In short if you want a power supply that will not be tagged by your local inspector as not being Class 2 listed and you want a power supply that was designed specifically for DeviceNet netw