DC theory & Neutral current

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  1. It’s important to understand that in a series circuit, current is identical through (how many) circuit elements of the circuit.
    • ALL.  While voltage and resistance are additive in a DC series circuit.
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 36
  2. (how many wire)_____________ series circuits are often used to give a signal something has occurred. They can indicate that a door is open. process is operating, or that there’s fire or smoke, for example.
    Two-wire closed-loop accomplish this.  Note: If you hear “series” then that should automatically imply a closed loop system versus parallel which is an open loop system.
  3. Resistance opposes the flow of electrons. In a series circuit, the total resistance is equal to the sum of?
    In this type of circuit the the sum of all the series resistances. Resistance is additive.
  4. The internal wiring of many types of equipment is connected in series. example, an 115/230V dual-rated motor connected to a 230V circuit must have the internal windings connected in _______so that each winding will receive at least 115v
    Series is the connection for this type of 115/230V rated motor.
  5. opposition (resistance) to current flow (amperes) results in a drop of the circuit voltage (voltage drop), and is calculated by the formula ________ for a purely resistive DC circuit.
    E vd=IxR is the formula for this
  6. If the voltage is doubled, the power will ________.  If the voltage is decreases 50% the power will decrease to 25% of its original value.
    • This will increase 4 times in any if this is doubled. 
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 19
  7. What law states that in a series circuit, the sum of the voltages across all of the resistors (or “voltage drops”) in series circuit is equal to the applied voltage?
    • Kirchoff’s Voltage Law states this.
    • voltage is additive in a series circuit. (remember current is constant and resistance is additive) Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 36
  8. Kirchoff’s Current Law states?
    that the sum of currents flowing into a junction  (parallel is a junction) equals the sum of currents flowing away from the junction is stated by this law. Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 38
  9. The Law of Conservation of Energy states?
    The power supply (a battery, and so forth ) will only produce as much power as that consumed by the circuit… is stated by this law.
  10. Calculate the total resistance (Rt) of the circuit: What is the formula for a series circuit?

    What is the
    Rt=R1+R2+R3+R4 Is the additive formula for a series DC circuit.
  11. What is also called “open-loop systems,” and are used for fire alarm pull stations and smoke detectors?
    parallel circuits are also know as this type of loop system.
  12. There are three basic methods of calculating the total resistance of parallel circuit.  What are they?
    The Equal Resistance method, the Product-Over-Sum , and the Reciprocal method… are the 3 ways to solve for this in a parallel circuit, versus a series circuit where it is the only additive.
  13. What method can be used to determine the resistance total for more than two resistors in a parallel circuit, but what must be done?
    • only two resistors can be dealt with at a time using the product over sum to determine the resistance total.
    • Rt=(R1xR2)/(R1+R2)
    • Mike Holt’s Illustrated Guide to Electrical Exam Preparation, based on the 2014 NEC. PP 40
  14. In a parallel circuit, the total circuit resistance is always less than what?
    Total circuit resistance is the smallest, while individual resistance, is always more than the total resistance in a parallel circuit.
  15. The advantage of the Reciprocal method in determining the total resistance of a parallel circuit is that?
    this formula can be used for as many resistors as the parallel circuit contains.
  16. _______circuit has two or more paths through which current flows.
    A parallel circuit has this current path.
  17. When power supplies are connected in parallel, the voltage and current does what? In a DC circuit.
    voltage remains the same but the current increases. In a DC circuit.
  18. The neutral conductor of a 3-wire, 120/240V circuit will carry current only when?
    When the circuits are unbalanced, or when the ungrounded conductors don’t have identical current flow, the neutral will carry current in this type of system.
  19. The neutral of a 3-wire circuit from a 4-wire wye (system carries about the same current as?

    What is the formula for a 3 or 4 wire 120/208 and 277/480v system?  Use the Acronym AMAP to remember this.
    Image Upload
    The neutral of a 3-wire circuit from a 4-wire wye system carries about the same current as the phase conductors.

    • AMAP: Addition Minus the Added Product of each line.
    • In=√(L1+L2+L3)-
    • [(L1xL2)+(L2xL3)+(L1xL3)] is the formula for this type of system.
  20. A balanced 3-wire, single-phase or 4-wire, three-phase multiwire branch circuit has unbalanced current flowing only in the ungrounded circuit conductors. Therefore, the circuit voltage drop for the line-to-neutral only includes the voltage drop of one conductor. The line-to-line voltage  is found by?
    multiplying the line-to-neutral voltage drop by 1.732 will find the circuit voltage drop for the line to neutral in a both of these wire systems. PP49
  21. Because of the dangers associated with an open neutral conductor, the NEC specifies that the continuity of the neutral conductor of a multi wire branch circuit can’t be dependent upon any wiring device [300.13(B)] - In other words, the neutral conductors of a multiwire branch circuit must be?
    • spliced together, and a wire (pigtail) brought out to the device. s way, if the receptacle is removed, it doesn’t result in an open neutral conductor.
    • Also…Each muftiwire branch circuit must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B)].
  22. each multiwire branch circuit must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B)].  What mitigates this?
    individual single-pole breakers with handle ties identified for this purpose, or a breaker with a common internal trip, can be used for this application [240.15(B)1]. PP51

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Author:
johnbowens
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330431
Filename:
DC theory & Neutral current
Updated:
2017-05-02 04:46:01
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This is DC theory in preparation for the supervisor or master electricians licensing test. Electrical Exam Preparation based on the 2014 NEC, Mike Holts
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