Here is an interesting installation which demonstrates the consequences of certain violations. It is located in a plastic film factory in south Texas.
A chiller has a feeder consisting of three sets of 350MCM THHN conductors, which originate at an 800 amp breaker in a switchgear room, extend about 600 feet to the side of the building, and terminate at the outdoor chiller enclosure main breaker. Most of the feeder is in a cable tray, with the last 40' or so in rigid conduit and a short section of flexible watertight conduit outdoors.
By calculation, the voltage drop with a load of 300 amps should only be about 3.9 volts but the measured voltage drop in the wire is 15 volts. The excessive voltage drop is due to higher impedance (specifically inductive reactance) resulting from code violations in the installation.
Photo (below) --feeder termination at chiller main breaker:
Three raceways carry the three sets of conductors, but all the phase A conductors are in one raceway, all the B are together, and all the C are together.
1999 NEC references--
300-20(a)--"all phase conductors...shall be grouped together" (to minimize the inductive reactance portion of the impedance).
300-3(b)--"all conductors of the same circuit...shall be contained within the same raceway".
300-3(b)(1)--"The requirement to run all circuit conductors within the same raceway...shall apply separately to each portion of the paralleled
Additional apparent violations in photo:
1. Raceways not bonded to enclosure.
2. Equipment grounding conductor missing from two of the three raceways.
3. Small pump unit subfeeder conductors tapped onto line side of breaker are not protected against overcurrent.
4. Equipment grounding conductor size appears smaller than that required by Table 250-122.
Photo (below)--transition from conduit to cable tray
Apparent code violations in Photo:
1. Conduits not bonded to cable tray. 250-9(a)
2. Phase conductors not grouped together in cable tray. 318-8(d)
3. Equipment grounding conductor not grouped together with phase conductors. 300-20(a).
Along with excessive voltage drop and unbalanced current between phases, the most dramatic consequence observed is induction heating of the metal conduits.
Photo (below)--At a load current of only 300 amps, the side of the conduit is 52 deg. C (125 deg. F) and is too hot to hold your hand on.
Our electricians benefitted from observing this "textbook case" of isolated phase conductors, and I thought others might benefit from it also.
[This message has been edited by Webmaster (edited 07-22-2004).]