Sizing Electric Wires
Theoretical physics has many possibilities, but the basic concept is all material and matter are made of molecules. Copper(CU) and aluminum(AL) atom configurations can be found in Chemistry’s Periodic Table. Water molecules (H2O) are made of two hydrogen and one oxygen atom bound together. Distilled water is a poor conductor resistive to electrical flow (current). Most tap or rain water contain minerals making it more conductive.
Basic electrical theory assumes atoms are somewhat similar to solar systems. Stars create gravitational pull keeping planets in orbit. Modern telescopes allow solar systems to be seen at great distances. Atoms are inversely miniature, invisible to electron microscopes. Atoms behave similar to solar systems with a nucleus greater in mass like the sun keeping electrons comparable to planets in orbit.
Pluto might pull from sun’s gravitational field easier than Mercury by outside forces. This is advantageous to electricity. Movement of electrons from one atom to another presently explain electrical current. Some atoms have electrons more easily displaced resulting in less electrical resistance. Gold is better and silver good but are not used except in smaller electronic devises due to cost. Copper suits larger applications. Aluminum has more resistance although cheaper.
All metals have some resistance to electricity without being at absolute zero ambient temperature. Some substances have considerably more resistance and are capable of withstanding heat used intentionally for light bulbs, cooking appliances and space heaters. Heat is undesirable in wires supplying power. Larger wires create less heat with more electrons flowing through them resulting in most produced power going to resistive and reactive devices. Sounds simple but gets complicated considering competitive cost and space allowed when installing wires and cables
Three things are fundamentally considered while sizing wires: metals they’re made of, types of insulation covering them, and minimum amperage that will cause over current devices protecting them to trip.
Amperage and De-rating
Table 8 in Chapter 9 of the National Electric Code offer physical dimensions for trade sizes. American Wire Gauge (AWG) is used for smaller wires #18 up to #4/0 AWG. Circular millimeters are used instead of square inches by other tables. Larger wires use circular mils (MCM) to designate size, identical for copper as aluminum. From these tables, ohms (resistance) per thousand feet are greater for aluminum usually needing them sized bigger than copper for a given current. Aluminum wires exposed to air oxidize causing conductive loss when insulation is removed for termination and requires a prohibitive compound coating applied in the field.
Article 310, Table 310.104(A) describes types of insulations with abbreviated letter designations and temperature ratings. These can be found more specifically and for other wires later in Chapter 3. Higher insulation ratings allow more amperage without damage. NM Romex found in Article 334 used in residences can be de-rated at 194 degrees Fahrenheit provided current does not exceed 140F ratings. Aluminum Underground Service-Entrance cable (USE) often supply these dwelling units.
Special allowance of 83% in 310.12 is made for these structures with services not less than 100 amps. Heat-resistant thermoplastic THHN and THWN-2 typically pulled in conduit go to 194 degrees F. W stands for moisture resistant insulation required in some locations including underground conduits. Wires are generally rated both categories marked on reels and insulation with their size.
The 2020 National Electric Code now requires one and two-family dwelling units to have outdoor emergency disconnecting means for all service conductors by Article 230.85 to provide safety to fire fighters and other emergency responders such as those called to assess gas leaks. Article 242 has been added requiring overvoltage and surge protection for these buildings as well.
Excluding exceptions found in other articles, 310.16 through 310.21 are used to determine wire size from corresponding tables. The first table is usually used unless for particular installations. Footnotes below tables are crucial. De-rating means wires must sometimes be larger than shown in tables made mandatory by Article 310.15.
The first de-rating pertain to ambient temperature corrections in spaces conduit, raceways and cables are installed. The equation provided can be used, but Tables 310.15(B)(1) & (2) make it simpler. Latter tables use higher ambient temperatures as basis for adjustment, but Table 310.16 revolves around 86 degrees F (30 Celsius). Warmer air outdoors or in rooms result in wires getting hotter. The far-right table column provide temperatures in Fahrenheit from below 50 to 185 degrees F. Adjustment for 78-86 F degrees is “1” making no difference when multiplied to amperages in Allowable Ampacity Tables. Temperatures below have greater adjustment and higher current allowance, but above 86F the correction becomes a decimal. Numbers multiplied by fractions become smaller.
A 12AWG wire with insulation rated 140 degrees F in an ambient temperature of 100F drops allowed current from 20 to 16.4 amps. The intended load must now be smaller, or wire area increased. Under the same circumstance type NM can be de-rated from 90-degree Celsius (194F) column capable of supplying a load pulling 27.3 amps, but only allowed up to 20 amps by Article 334.80. Attics can get hot during summer.
Circuit breakers and fuses are not intended to operate at full capacity for long periods of time. Total load is calculated at 125% times continuous loads and duties operating for more than three hours plus 100% of non-continuous loads.
The next adjustment factor for derating can be found in Section 310.15(C)(1) and corresponding table where number of current-carrying conductors in raceways, such as conduit, exceed three. Same applies for single and multiconductor cables not properly spaced apart for more than 24 inches. Those installed in cable trays are subject to Article 392.80 and sizing becomes dependent on type and distances spaced apart.
Heat generated by wires next to each other add up. Correction is similar to ambient temperature but depends on number of wires involved. Wires exposed to sunlight raised less than 7/8” above rooftops require additional 60F added to outdoor temperatures.
Circuits fall into three main categories. Service conductors are from the serving utility or other source to premise wiring systems. Section 90.2(B)(5) excuse installations by electric and communication utilities from adhering to the National Electric Code. Feeders are conductors between service equipment and final branch-circuit over current devices, commonly a panel with smaller breakers or fuses. Branch circuits go from there to outlets or devices to be operated. In no case can circuits be rated less than maximum load served, although 240.4(B) allows next higher standard ampere ratings found in Table 240.6(A) for services and feeders.
Not the case for branch circuits. Not only do wires need be at or above protection, but 240.4(D) requires them lower for #18 to #10AWG even if more amperage is allowed by higher temperature insulation. This does not mean higher values cannot be used for de-rating.
Dedicated branch circuits with less loads than wires are capable, like smoke detectors and alarms, are already de-rated.
Voltage-Drop
Beginning of Article 310.14 (Ampacities for Conductors 0-2,000 Volts), Informational Note No. 1 states voltage-drop is not taken into consideration, but references other Notes where recommendations are made. Insulation temperatures or number of wires are not considered when calculating voltage-drop. Three percent in either feeders or branch-circuits provide reasonable efficiency if their sum does not exceed five percent.
Voltage-drop is desirable in light bulb dimmers but results in poor efficiency for conductors feeding devices. It can be measured with instruments at the last device on a circuit provided others are operating at full capacity. Not a good idea to check for VD after wires are installed. Better to calculate it beforehand. Equations for it are not provided in the NEC.
Basic voltage drop formula is VD = 2 X Resistance (R) X Length one-way (L) X Amperage full load (I) / Circular Mils. Straight forward for direct-current but inductive and capacitive reactance in alternating circuits have an effect. Resistance(R) + Reactance(X) = Impedance(Z) now used instead of resistance. For close approximations, 12 can be used for copper wire and 18 for aluminum for both AC & DC, but Tables 8 & 9 in Chapter 9 get more specific. For a balanced load on a single-phase system with a common neutral wire the voltage-drop between it and load wires allow the number calculated to be divided by half. Not the case for three-phase systems but voltage-drop is now multiplied by 86.6%. Inductive motors and incandescent lighting cause current to lag voltage creating a power factor (PF), maybe 85% so voltage-drop would be divided by .85 making it higher. To determine percentage, voltage-drops are divided by total voltage available at the source X 100.
Many vacuum cleaners now require 15 amps at 120 volts which should be considered when installing 20-amp circuits to distant rooms. Continuous loads require breakers and wires be increased by 125%, or loads limited by 80% with 16 continuous amps. Wires normally being #12 over 100 feet might be increased to #10AWG to the first outlet ensuring more efficient operation.
Wires increased to compensate for either higher insulation temperature or voltage-drop are also de-rated for the other.
Neutrals
Neutral wires are also referred to as grounded conductors in the electrical code. In two-wire DC or AC circuits conductors are sized the same as load wires. Generators and inverters produce sine waves better explained in trigonometry or seen slow motion on an oscilloscope. Voltages produced increase and decrease, sixty times a second in North America, creating alternating-current. How these waves are positioned at and above or below zero potential determine size of grounded conductors.
Single phase generators and transformers produce one sine wave every 360-degree cycle. A three-wire circuit delivering power splits the wave in half by a grounded neutral being zero volts. Upper half of the wave is considered positive with electron flow going in one direction and lower half negative making electrons go the other way. Voltage potential between the two peaks still add up. In balanced loads power consumption is identical for upper half of the wave as the lower. Positive peaks require current going one way and negative peaks the other in common grounded conductors, so electrons do little in neutrals now not needed.
Completely balanced loads are uncommon in 240/120-volt branch circuit breaker panels and equipment. The intent is to make them so, but stands to reason not all devices operate at once. Large cooking appliances, clothes dryers, hot water tanks, baseboard heaters and motors rated 240 volts are for the most part balanced between the top and bottom part of the phase. 120-volt devices use either upper or lower part to ground. Hard to determine when all will be used at once making these combined loads impossible to balance.
Common grounded wires are allowed smaller than load wires to compensate cost and space in raceways for single phase service and feeder circuits. They are not allowed smaller than maximum unbalanced loads (Art. 220.61). Minimum size for feeders are found in Table 250.122 based on overcurrent ratings by Article 215.4 and Section 215.2(A)(2). Minimum sizes for common grounded conductors in services are provided by Table 250.102(C)(1) based on size of the largest ungrounded conductor required by Articles 230.42 & 250.24(C) but not smaller than 1/0 AWG for 100 amp or greater loads. Exceptions are made for parallel circuits.
Parallel circuits are when two or more smaller wires used in place of a bigger one, provided square area or circular mils add up equal to or larger than the bigger one they replace. Sometimes an advantage for wires over 250 MCM since their reals are heavy and they are difficult to bend in panels or junction boxes. Parallel wires must be installed in the same raceway as their load wires but the same wire lengths are necessary if in parallel raceways. In many parallel circuits, common neutrals cannot be reduced from those carrying loads.
Three phase generators and transformers produce three sine waves per 360-degree cycle, common in 208/120-volt panels and devices. Calculations depend on square root of 3 (1.732) and natural trigonometric sine function (reactive factor) for vectors 120 degrees apart (.866). Harmonics are created and it is recommended and required neutrals in delta or wye connected systems be sized not less than load wires.
Grounding and Bonding
Ground wires are often referred to as grounding conductors by the NEC. Because they don’t conduct electricity under normal operating conditions they’re allowed even smaller than neutrals. Their purpose is to trip breakers or fuses on short circuit or ground fault conditions where load wires accidently conduct to metal and ground. Since they supply no loads over current happens quickly and smaller ground wires don’t have sufficient time to heat up and cause damage.
Unlike neutrals which are only allowed and required to be grounded to earth at the main disconnecting means, more grounding paths bonded together and taken to earth the better provided they also go back to the main disconnect. Return via metal raceways bonded correctly can also be used. A separate grounding conductor is not needed back to the utility supplying power since grounding and grounded circuits are connected where service is delivered to the main.
Sizing equipment and raceway grounding conductors are addressed in Article 250.122 with allowable minimum sizes shown in Table 250.122 based on rating or setting of circuit overcurrent devices but not required larger than conductors supplying loads. If ungrounded wires are increased in size to compensate for insulation temperature corrections or voltage-drop, grounding conductors must be increased proportionately.
Table 250.66 provide sizes for grounding electrode conductors for ac systems, but exceptions are made to buried electrodes such as ground rods earlier in this article by (A) thru (C). Electrodes are not intended carry fault current back to the service neutral but provide an equal potential to earth (0 volts). Electrode grounding conductors might be larger or smaller than shown in the table, but minimum sizes are given to withstand physical damage.
Bonding connects various metals and grounding systems together addressed by Part V in 250. Intent is to conduct fault current back to the service usually requiring bigger bonding wires than electrode conductors by Table 250.102(C)(1) based on circular mils of the largest ungrounded conductor or combined parallel conductors. This table also pertains to bonding jumpers. Piping systems and exposed structural steel are a safety risk when conducting fault current and must carry full load currents back to service disconnecting means.
Tap Conductors
Taps defined in Article 240.2 take exception to requiring conductors being sized for over current devices ahead of them. Often sub-panels with lower overload protection are used next to service panels or other sub-panels distributing power to smaller loads. Reduced feeder taps not only lessen costs but accommodate minimum conductor bending radius found in Article 300.34 and in Article 408.55 for panels. Problems can result in panels with several big wires. It can be difficult installing dead fronts and shutting outer panel doors completely when they’re full of wires.
It is often desired to install large wires protected by main breakers capable of handling entire loads to gutters and tap smaller wires to sub-panels using split-bolt, crimp or multi-tap connectors. Taps cannot be smaller than allowed by sub-breakers they feed with only raceways now protecting them. Reduction in size depends on length from where they are tapped by Article 240.21 or Article 368.17 from busways. Equipment grounding conductors are still determined by Table 250.122, but not required larger than tap conductors (Art. 250.122(G)).
Tap conductors can sometimes be used for branch circuits allowed by Sections 210.19(A)(3) & (4) for household ranges, cooking appliances and some other loads. Branch circuit taps to motors are addressed in Articles 430.28 & 430.53(D).
Transformers
Transformers increase or decrease voltage from the supply (primary) side to load (secondary) side. Amperage is adjusted proportionately by Power(KVA) / Voltage(V) = Current(I). Depending on physical size transformers are only capable of handling limited power as rated. In all cases, transformers must have adequate overcurrent protection on primary sides. When voltage is stepped-up by 2, available current delivered is divided by 2. Wires leaving these transformers then do not have to be as big as supplied. Voltage stepped-down causes the opposite effect.
Transformers are often considered a power source similar to utilities or generators. If grounded wires supplying them, which many don’t have, aren’t connected to neutrals leaving transformers then they are considered separately derived systems. Neutrals leaving must now be bonded to the grounding system.
Because transformers are protected on primary sides, overcurrent devices may not be needed for relatively short distances from the secondary considered tap wires discussed in Articles 240.4(F) and 240.21(C).
Motors
Motors have different rules. They take considerably more electricity to start than run. Starting happens quickly, and overcurrent protection is allowed greater for wires supplying motors than what they’re rated. Article 430 considers different types of motors and means of protection. Conductors must be increased 125% for continuous duty motors.
Tables 430.247 through 430.250 are used to determine full-load current based on horsepower ratings instead of nameplate values. Article 430.6(A) makes exception for motors with speeds less than 1,200 rpm, high torque, and multi-speed motors. Feeders and branch-circuit conductors are sized according to Article 310.15 provided they’re capable of carrying starting currents (Art. 430.52(B)).
Over-current protection for motor lead wires can be increased from requirements of Table 310.16 by Table 430.52. Non-time delay fuses and instantaneous circuit breakers can be much larger, but only for certain situations under Section 430.52(C)(3). Time-delay fuses and inverse time breakers have trip times inversely proportional to overcurrent. Large overcurrent trips them faster and wires do not need to be increased as much but still permit a 250% allowance for standard loads. Not much room in motor termination boxes for big supply wires.
Load Calculations
In all cases, protected wires must be capable providing current necessary to operate all devises. Load calculations for dwellings and residences are found In Article 220 and Informative Annex D. If arc welders, hot tubs, large chandeliers or electric vehicle charging systems addressed in Chapter 6, Special Equipment are desired, or might be, load calculations must be increased. Commercial and industrial complexes require panel schedules with sub-breaker ratings and intended loads (KVA). Intent is to balance breakers supplying loads most used by placing them on opposite sides of panels and phases.
Considerably more information is given by the 2020 National Electric Code. Table of Contents provide page numbers found at the bottom. The index uses article numbers easily accessed at top corners of pages.
Eberling@www.thndrsns.com
