Looping A Circuit and Line Loss

LOOPING A CIRCUIT –AKA-- “RING OF FIRE”

The practice of looping a circuit has been used for many years with DC (direct current) power as a means reducing line loss (voltage drop) and increasing the current carrying capacity of the cable, the same method applies to AC (alternating current) circuits.

The purpose of looping a circuit is to provide an easy way of paralleling cables in a rig, which in effect, doubles the capacity of the cable. (See chart).

When using this method of rigging you must follow certain basic safety procedures; do not hook up this system while energized (hot), do not disconnect under load, make sure the continuity is correct before completing the circuit, use the same size cable for the main feeder cable, and use listed devices made for this application.

U.L. (Underwriters Laboratory) insists that portable power distribution devices used in our industry carry the following message: “for use by qualified personnel only – the routing of portable supply conductors, the making and breaking of supply conductors and the energizing and de-energizing of supply services shall be performed by qualified personnel only – for use in areas not readily accessible by general public.”

The National Electrical Code defines “Qualified Person” as one who is familiar with the construction and operation of the equipment and the hazards involved.

U.L. requires the neutral buss bar on devices feeding electronic dimmers or electronic ballasts have the capacity of 130% of the largest phase conductor in order to accommodate the heat created by harmonics in the circuit.

In the past we used only linear loads; arcs, incandescent lights and resistance type dimmers. The neutral only carried the imbalance between the phases, therefore, the neutral could be half the size of the phase conductor.

Today we use non-linear loads such as electronic dimmers and electronic ballasts, the neutral now needs to be larger than the phase conductors. Harmonics occur when electronic devices alter the relationship of the current and voltage sine wave form causing heat.

MAIN FEEDER CABLE RUNS

Standard Circuit                                                                           Looped Circuit

Cable Size Paper Load  Volt Drop 100’ Length Paper Load  Volt Drop 100’ Length Paper Load Volt Drop 100’ Length
 4/0  400 AMPS  4 volts  400 AMPS  2 volts  800 AMPS  4 volts
 2/0  300 AMPS  5 volts  300 AMPS  3 volts  600 AMPS  4 volts
 #2  200 AMPS  6 volts  300 AMPS  4 volts  400 AMPS  

A looped circuit is the same as paralleling cables on a spider and decreases the voltage drop as indicated above. In order to be effective the splicing device (spider) must have the same amount or more of copper in the connecting buss (circular mills) as the cable being paralleled.

LINE LOSS – WHAT IS IT?

Line loss refers to the amount of voltage drop from the power source to the load, caused by resistance in the circuit.

The factors that cause resistance are; the amount of the load, the size of the wire, the connector, the length of the cable, the duration of the load, the control device (dimmer) and the ambient temperature.

The effect caused by line loss is; less output of the light, lower Kelvin temperature with incandescent lights, higher Kelvin temperature in HMI lights, as well as ballast failure, increased heat in the cable and connectors.

The method for calculating specific line loss is a complicated formula, as a rule of thumb we use the following values:

Cable Size Paper Load Actual Load Voltage Drop 100' Length
4/0 400 AMPS 333 AMPS 4V
2/0 300 AMPS 250 AMPS 5V
#2 200 AMPS 160 AMPS 6V
#4 100 AMPS 83 AMPS 4V
#6 50 AMPS 42 AMPS 4V
#12 20 AMPS 16 AMPS 5V

Longer runs increase line loss accordingly, and add heat to the cable; multi-cable such as SOCAPEX type cable or cable stacked on top of each other also increases line loss and adds heat to the cable. Severe bends in the cable such as running cable to the floor over beams up high adds resistance in the cable.

Solution: Use smaller loads on the cable or increase the size of the cable, also shut down the loads as much as possible. Most of the lighting equipment used in the motion picture industry is designed for intermittent use, loads used in excess of 3 hours are considered a continuous load and the cable must be de-rated accordingly; cable will continue to heat up the longer the load is applied; voltage drop and amperage increases.

Cable runs should be spread out as much possible in order to minimize heat build-up, cable typically run on the catwalks above the lights are subject to heat added by the lights. Another method of decreasing line loss is looping a circuit, (see looping article).

Dimming lights thru electronic type dimmers increases the load on the neutral and therefore adds heat to the cable, in the case the neutral must be increased in size by adding more cable (paralleling) to the circuit. The neutral buss must be rated for the additional load. When paralleling cables the spider must be rated for the additional cable in order to be effective. DADCO Super Cam-Thru Spiders are rated at 800 amps per leg and 1040 amps per neutral.