USITT Electrical Workshop

Welcome

Basic Electricity 201

Copyright (C) 1997, 2001, 2013 USITT

Ken Vannice Leviton/NSI/Colortran USITT Representative NEC Panel 15 Electrical SME Chair Recognized ETCP Trainer

The National Electrical Code made simple Two separate principles A. Protection of equipment (objective) B. Reduction of the likelihood that equipment will be stressed (subjective) A. An objective approach to feeding dimmer packs Article 520 of the NEC requires: Three-phase, four-wire packs must have a double-size neutral bus, except if the pack is field-convertible to single-phase, three-wire, in which case it needs to be only large enough for the single-phase case.

Power supply cable neutral shall be sized considering the neutral as a current carrying conductor.

Supply cable shall be multi-conductor, and have ampacity suitable for the (total possible) load and not less than the rating of the connectors.

Supply receptacle outlet shall be protected for its rated ampacity.

Pack nameplate rating: 80A, 120/208V, 3Ph, 4Wire, 60Hz or 125A, 120/240V, 1Ph, 3Wire, 60Hz For the three-phase case: Our cable has four current-carrying conductors, 3 phase conductors and one neutral conductor, the grounding conductor hopefully doesn’t carry current NEC Section 400-5 states that we need to derate the 3-conductor values in Table 400-5(A) or (B) by 80%. We can either multiply the table values by 0.8

or our requirement value by 1.25. 80 x 1.25 = 100 Table 400-5(A) does not contain 100A so we must look in Table 400-5(B). The 60

o

C Column F requires 2AWG cable.

Finishing the discussion, we must have 80A connectors and an 80A circuit breaker.

Note that everything is protected for 80A including the conductors in the cable that inter-heat each other.

Size (AWG) 18 16 14 12 10 8 6 4 2

Table 400.5 (A)

15 20 25 35 45 A (3 Current Carrying Cond.) 7 10 60 80 18 25 30 40 55 B (2 Current Carrying Cond.) 10 13 70 95

B. The subjective approach to feeding dimmer packs The previous example is the most desirable situation and should be a goal, especially if the actual load is not known. Then reality sets in. Connectors with 80A ratings aren’t very available and we don’t have 80A available either.

NEC Section 220-10 (a) is entitled Ampacity and Computed Load. It continues by requiring the feeders to be large enough to supply the load served. It also requires that the computed load be no less than the sum of the loads of all the circuits supplied.

Here is the subjective part. Who calculates the loads? You.

Who agrees or disagrees with your calculations? The electrical inspector.

Hookup Production: The Sandbox Date: 4/1/97 Dimmer Location Unit# Type Use Color Ph-A Ph-B Ph-C 1 1st. Elect. 26 6” Fresnel Area 1L #009 1000 2 1st. Elect. 35 6” Fresnel Area 1R #003 1000 3 2nd. Elect. 48/49 2 6’ Strip Red Cyc #027 1200 4 1st. Elect. 30 6” Ellips UR Spec #328 1000 5 1st. Elect. 28 6” Fresnel Area 2L #009 1000 6 1st. Elect. 37 6” Fresnel Area 2R #003 1000 7 2nd. Elect. 48/49 2 6’ Strip Grn Cyc #090 1200 8 2nd. Elect. 51 8” Fresnel Sun Spl #021 2000 9 1st. Elect. 29 6” Fresnel Area 3L #009 1000 10 1st. Elect. 38 6” Fresnel Area 3R #003 1000 11 2nd. Elect. 48/49 2 6’ Strip Blue Cyc #068 1200 12 Floor 89 Lamp Pract. None 100 Total Watts 4200 5200 3300 Total Amps 35 43 28 Feeder Req’d: 50A, 120/208V, 3Ph, 4W 50A is kind of tight but 60A is quite available. Remember lamp bulbs have a tolerance of +/- 10% or so.

60A x 1.25 = 75A From Column A of Table 400-5(A) 2AWG is required.

The connector and circuit breaker must be rated for 60A.

Again note that the system is protected. If someone changes all the 1kW’s to 2 kW’s the breaker will protect the equipment and wiring.

Other ‘subjective’ thoughts: The NEC defines continuous load as one likely to be on continuously for three hours. In several places it talks about 100% non-continuous load plus 125% of continuous load. This essentially derates circuits to 80% or 1920W on a 20A branch circuit. In our application we rarely have all the dimmers on at full intensity for over 3 hours. We do need to derate circuits such as worklights, rehearsal lights, stage apron lecture lights, auditorium lecture/classroom house lights and the like. Furthermore, this 80% derating factor is on the use of the circuit and not the 80% derating factor applied to multiple conductors.

The subjective part of the NEC requirements sounds like a game between you and a mythical inspector, but it is not. If there is a problem and an inspector finds a violation after the fact, your insurance is as good as gone. Furthemore, if you undersize your circuits your lights will go off in the middle of the show.

The “big enough” rule NEC Section 520-9 essentially says that receptacle voltage and current ratings shall not be less than the circuit rating. Table 210-21(b)(2) shall not apply.

80A 80A 80A x 1.25 = 100A 80A Bus 80A 100A 80A x 1.25 = 100A 80A Bus 100A 100A 80A x 1.25 = 100A 80A Bus Do not assume that receptacles are protected at their rating. Check the rating of the circuit breaker or fuse.

Do not assume that the voltage available at the receptacle is the same as the rating of the receptacle. Check the voltage of the source.

Single-Phase, Three-Wire (Split Phase) Pack nameplate rating: 125A, 120/208V, 1Ph, 3Wire, 60Hz Our cable has three current-carrying conductors plus the grounding conductor. Three current-carrying conductors do not require the values in Tables 400-5(A) and (B) to be derated. Table 400-5(A) contains no values above 95A so we must look in the 60 o C Column F of Table 400-5(B). The table indicates that 1AWG is required.

125A 200A 125A 125A Bus Note that if the pack didn’t have the ability to be strapped for split phase, two of the three phases would have to be connected to one of the split phase legs.

For full use of the pack that leg would have to be rated for 160A. Since the neutral bus is only rated for 80A the main breaker could only be rated for 80A resulting in the pack loosing much of its capability.

Hookup Production: The Sandbox Date: 4/1/97 Dimmer Location Unit# Type Use Color Ph-A Ph-C 1 1st. Elect. 26 6” Fresnel Area 1L #009 1000 2 1st. Elect. 35 6” Fresnel Area 1R #003 1000 3 2nd. Elect. 48/49 2 6’ Strip Red Cyc #027 1200 4 1st. Elect. 30 6” Ellips UR Spec #328 1000 5 1st. Elect. 28 6” Fresnel Area 2L #009 1000 6 1st. Elect. 37 6” Fresnel Area 2R #003 1000 7 2nd. Elect. 48/49 2 6’ Strip Grn Cyc #090 1200 8 2nd. Elect. 51 8” Fresnel Sun Spl #021 2000 9 1st. Elect. 29 6” Fresnel Area 3L #009 1000 10 1st. Elect. 38 6” Fresnel Area 3R #003 1000 11 2nd. Elect. 48/49 2 6’ Strip Blue Cyc #068 1200 12 Floor 89 Lamp Pract. None 100 Total Watts 6200 6500 Total Amps 52 55 Feeder Reqd: 60A, 120/240V, 1Ph, 3W From Table 400-5(A) Column A 4AWG is appropriate.

In summary the breaker needs to be 60A and the connector needs to be at least 60A because the 4AWG cable is 60A.

“Tap” Rules 200A 200A 125A 125A Bus The general rule is that the overcurrent protection device must be located where the wiring changes size.

Section 520-53(h) of the NEC contains two exceptions which are known as the “tap” rules.

The first one states that if the conductors are not over 10ft. long between the supply and switchboard or subsequent overcurrent device the overcurrent device protecting the conductors may be 4 times larger providing the following conditions are met: 1. The supply conductors shall terminate in a single overcurrent device no larger than the rating of the conductors.

2 thru 6. The supply conductors shall meet other requirements that reduce the likelihood of damage.

This does not mean that the conductors may be used at 4 times their rating!!!

10ft. Max.

200A 200A 125A 125A 125A Bus 10ft. Max.

200A 200A 125A 125A 125A Bus The second rule is similar except the length is 20ft., the overcurrent device may be only twice the rating and the restrictions are greater.

That’s All Folks

Non-Resistive Loads

• In order to simplify the previous discussion we assumed that the load was resistive. What if that is not true?

• The formula is: E = I x Z.

• The Impedance (Z) is made up of a resistive component, a inductive component, and a capacitive component.

• Resistive loads: E = I x R. Resistive loads use energy and provide power to do useful work.

Non-Resistive Loads

•

Inductive loads: E = I x XL

•

XL, the inductive reactance

, is a function of frequency and the inductance of the load. Inductive loads store energy in a magnetic field.

•

Capacitive loads: E = I x XC

•

XC, the capacitive reactance

, is a function of frequency and the capacitance of the load. Capacitive loads store energy in an electrostatic field.

Effect of Loads

• In the case of Resistive loads, the current is in line with the voltage.

• In the case of Inductive loads, the current lags (occurs later than) the voltage.

• In the case of Capacitive loads, the current leads the voltage.

1.5

1 0.5

0 -0.5

-1 -1.5

Voltage and also Resistive Current Q Inductive Current (lagging)

Load Math

• When the current is not in line with the voltage, a more complex math involving the concept of vectors is required.

Power Factor = cos Q pf of 1 is resistive Q Power in Watts pf of .9 lagging is inductive Reactive Power in VARS + for Inductive - for Capacitive