The rule in 695 will not only require larger service/feeder conductors, but may very well require a larger transformer. Look at what the starting current is on a large motor. I have seen electric fire pumps up to 250 hp and know that some are even larger. Don
[This message has been edited by resqcapt19 (edited 07-28-2002).]
Re: Voltage Drop Requirements in 2002 NEC#81229 07/28/0201:26 PM07/28/0201:26 PM
647 appears to address a very specific application via SDS, and as the article would deal with 'sensitive' electronics the nature of VD being mandatory supplements/modifies ( 90.3) chap's 1-4. note how the evil nuetral* is done away with...
647.3 General. Use of a separately derived 120-volt single-phase 3-wire system with 60 volts on each of two ungrounded conductors to a grounded neutral conductor shall be permitted for the purpose of reducing objectionable noise in senstive electronic equipment locations provided that the following conditions apply: (1) The system is installed only in commercial or industrial occupancies. (2) The system’s use is restricted to areas under close supervision by qualified personnel. (3) All of the requirements in 647.4 through 647.8 are met.
my question here is, does this system supply normal 120V equipment , or is said 'equipment' rated/listed to run off the 60v/60v SDS ???
Re: Voltage Drop Requirements in 2002 NEC#81232 07/28/0204:10 PM07/28/0204:10 PM
yes Joe, i can imagine that, in addition to VD, any form of a 'ground loop' would also be a concern,the issue of overall power quality at hand.... as such we are directed in 647 to 250.30 .....
(B) Ungrounded Systems. The equipment of an ungrounded separately derived system shall be grounded as specified in 250.30(B)(1) and (2). (1) Grounding Electrode Conductor. A grounding electrode conductor, sized in accordance with 250.66 for the derived phase conductors, shall be used to connect the metal enclosures of the derived system to the grounding electrode as specified in 250.30(B)(2). This connection shall be made at any point on the separately derived system from the source to the first system disconnecting means. In a separately derived system that is ungrounded, a bonding jumper must be installed to connect the disconnect enclosure and equipment grounding conductors to the grounding electrode system.
which now brings up this nifty little ex a tad back in A.... Of course this does not apply, being in A, and having rid the nuetral from the system, yet it is of interest in that 647 may in fact provide for the 'fix' for it......
Exception No. 1: A bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded circuit conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.
To add to this little 'code loop' here, pursuant to the 'PQ' angle, we now delve into 'listings'....
The practice of grounding the secondary of an isolating transformer to a ground rod or running the grounding electrode conductor back to the service ground (usually to reduce electrical noise on data processing systems) is prohibited if an item in (1) or (2) of 250.30(A)(4) is available. However, an isolation transformer that is part of a listed power supply for a data processing room is not required to be grounded in accordance with 250.30(A)(4), but it must be grounded in accordance with the manufacturer's instructions.
somebody here's had to have been there..... ( speak up, don't be shy...)
[This message has been edited by sparky (edited 07-28-2002).]
Re: Voltage Drop Requirements in 2002 NEC#81234 07/28/0207:18 PM07/28/0207:18 PM
A bit off-topic, but the center-grounded 60/120V configuration had another interesting application in the disco days of the late 1970s. It became a cost-effective solution to 4.8kW of medium-based sign lamps strung up through many feet of wiremold and what seemed to be miles of 14THHN. The idea behind the tapped grounding allowed sets of 16 SCRs on an 8x8 matrix to control three groups of 64 lamps in various patterns, giving control of individual rows and columns of colored lamps. We used a 10kVA, 480-120/240V dry-type transformer, powered from a dedicated 240V circuit, providing 60/120V for the SCR/lamp matrix. This arrangement allowed for switching of both conductors to each lamp, and limited the lampholder voltage to below 150V. [A microprocessor and keypad controlled the SCR-gating patterns.]
It looked great when it was completed, but we didn’t want to think about a sign lamp for a long time. Disco still sucks.