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Kansas Electrical Prep > Conductor ampacity and sizing > Flashcards

Conductor ampacity and sizing Flashcards

1
Q

True or False: The definition of ampacity is the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its watts rating.

A

False: True or False: The definition of ampacity is the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its TEMPERATURE rating.

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2
Q

When determining the ampacity of a conductor, or sizing of a conductor, there are several factors that needed to be taken into consideration?

A

The ambient temperature
Insulation rating
The # of current carrying conductors contained in a raceway
Rating of the terminations
The environment the conductor will be subject to and voltage drop

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3
Q

What is the table that lists various types of insulated conductors, allowable ampacitites?

A

Table 310.104

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4
Q

What table lists the ampacity values for copper, aluminum conductors.

A

Table 310.15

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5
Q

What is normal operating temperature for conductors?

A

Temps between 30’ C or 86’F

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6
Q

The left half of Table 310.15 is copper conductors or copper clad or aluminum conductors?

A

Left = Copper.

Right is the copper clad or aluminum conductors

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7
Q

What happens when conductors are subject to ambient temperatures above 86 dec F?

A

The resistance of the conductors increase proportionately. The greater the elevated ambient temperature, the greater the resistance.

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8
Q

True or False: Some conductors are dual rated?

A

True: THHW is rated 75 deg. C. for wet locations and 90 deg. C for dry locations.

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9
Q

True or False: The greater the elevated ambient temperature, the greater the resistance.

A

True

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10
Q

True or False: When resistance of a conductor increases the ampacity of the conductor decreases.

A

True

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11
Q

When a size 3 AWG copper conductor, with THW insulation, in installed in an area where the ambient temperature in 114 deg. F, the wire has an allowable ampacity of ________

100 amperes
75 amperes
82 amperes
58 amperes

A

75 amperes

100 amperes x .75 amperes

To solve this problem, locate the wire size on the left side of the table. (Table 310) and look to the right under the copper THW column, with a temperate rating of 75’C, Note the ampacity of the wire is 100 amperes at normal operating temperature. Then, we apply the values as shown in table 310. Look to the right side of the table where the temperature is show ‘F and find the ambient temperature. Next we follow that line to the left where the line intersects with the 75’ C col. Finally, to find the allowable ampacity of the conductor, multiply the two values together.

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12
Q

When a size 1/0 AWG THWN aluminum conductor is installed in an ambient temperature 45’C, the conductor has an allowable ampacity of ______.

100
90
98
104

A

=98

98 amperes
1/0 THWN aluminum ampacity before derating =120 amperes
120 amperes x .82 (correction factor) = 98.4 amperes

First, we find the wire size which has an ampacity of 120 amperes, Table 310.15 Then we find the ambient temperature, on the left portion of Table 310.15, and go to the right under the THWN aluminium col. to find the correction factor .82. Next, multiply the two values together.

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13
Q

What formula should be used when sizing conductors for a given load located in an area with an elevated ambient temperature?

A

Required ampacity = load/correction factor

Table 310.15

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14
Q

Where a 100 ampere load is to be supplied with THWN copper conductors in an area where the ambient temperature will reach 110 deg’F, size THWN conductors are required to serve the load.

1 AWG
2 AWG
3 AWG
1/0 AWG

A

= 1 AWG

Required ampacity= 100amps/.82 = 122 amperes

Table 310.15 indicates 1 AWG copper conductors with an ampacity of 130 amperes, should be selected. To cross reference:

130 amperes x .82 (correction factor) = 107 amperes

Conductors must be sized to the lowest temperature rating of the wire, circuit breakers, terminal or device.

Today, most terminations are rated at 60 deg. C or 75 deg. C

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15
Q

The load on a size 6 AWG THHN copper conductor is limited to _____ where connected to a circuit breaker with a termination rated at 60’C

75 amperes
65 amperes
60 amperes
55 amperes

A

= 55 amperes

Table 310.15 lists the ampacity of size 6 AWG, copper, 60’C rated conductors to be 55 amperes.

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16
Q

The load, in amperes, on a size 6 AWG THHN copper conductor is permitted to be no more than ______ where connected to a fusible disconnect switch with terminals rated at 75’C.

75 amperes
65 amperes
60 amperes
55 amperes

A

=65 amperes

Table 310.15 lists the ampacity of size 6 AWG copper 75’C rated conductors to be 65 amperes.

Many circuit breakers and disconnect switches now have dual ratings marked 60/75 deg. C

Many conductors now are dual rated such as THWN/THHN. The ampacity rating is based on the THWN 75 deg. C values, but the derating factor(s) may be based on the THHN 90 deg. C values.

The allowable ampacity of types NM, NMC, NMS, cable shall be in accordance with the 60 deg. C conductor temperature rating. The 90’ deg. C. rating shall be permitted to be used for ampacity derating purpoes, provide the final ampacity does not exceed that for a 60’ deg C rated conductor 334.80

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17
Q

A size 12/2 AWG w/ ground copper NM cable is to be installed in an attic of a dwelling where the ambient temperature is 125’ F. Determine the ampacity of the cable

  1. 8 amperes
  2. 5 amperes
  3. 6 amperes
  4. 4 amperes
A

=22.8 amperes

Ampacity of 12 AWG at 90 deg. C before derating = 30 amperes

30 amperes x .76 (temperature correction) = 22.8 amperes

Under this condition, the ampacity of 22.8 amperes is NOT permitted, because this value is more than the 20 ampere rating of the 12 AWG conductors at the 60 deg. C temperature rating

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18
Q

A size 10/3 AGW w/ground copper NM cable is to be installed in the crawl space of a dwelling where the ambient temperature is 100 deg. F. Determine the ampacity of the cable.

  1. 6 amperes
  2. 4 amperes
  3. 3 amperes
  4. 0 amperes
A

=30 amperes

Ampacity of 10AWG at 90 deg. C before derating = 40 amperes
40 amperes x .91 (temp correction) =36.4 amperes

Under this condition the ampacity of 36.4 amperes is NOT permitted, because this value is more than the 30 ampere rating of 10 AWG conductors at the 60 deg. C rating.

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19
Q

True or False: When there are more than 3 current-carrying conductors in a raceway or cable, the ability of the conductors to dissipate heat is increased.

A

False: When there are more than 3 current-carrying conductors in a raceway or cable, the ability of the conductors to dissipate heat is REDUCED.

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20
Q

True or False: When a 3 current carrying conductor in a raceway or cable is longer than 24 inches, the allowable ampacity of each conductors shall be reduced.

A

True: When there are more than 3 current-carrying conductors in a raceway or cable, the ability of the conductors to dissipate heat is increased.

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21
Q

What is the allowable ampacity of a size 1/0 AWG THW copper conductor installed in a conduit longer than 24 inches, when there are three 3 other current - carrying conductors contained in the same pipe?

150 amperes
120 amperes
105 amperes
140 amperes

A

=120 amperes

150 x .8 (adjustment factor) =120 amperes

First find the ampacity of the conductor, before derating, as shown Table 310.15 Then apply the appropriate adjustment factor Table 310.15

When there are four or more current-carrying conductors in a raceway or cable, longer than 24 inches, install in an area with an elevated temperature, BOTH the temperature correction and the adjustment factor for the number of current-carrying conductors MUST be applied.

22
Q

True or False: When there are 4 or more current-carrying conductors in a raceway or cable, longer than 24 inches, ONLY the temperature correction needs to be applied.

A

False: When there are four or more current-carrying conductors in a raceway or cable, longer than 24 inches, install in an area with an elevated temperature, BOTH the temperature correction AND the adjustment factor for the number of current-carrying conductors MUST be applied.

23
Q

Determine the allowable ampacity of a size 2 AWG THWN aluminum conductor installed in a 20 foot long conduit with 7 other current-carrying conductors of the same size and insulation, where ambient temperature is 99 deg. F.

59 amperes
71 amperes
55 amperes
63 amperes

A

= 55 amperes

Size 2 AWG THWN AL ampacity before derating = 90 amperes
90 amperes x . 88 (temp correction) x .7 (adj. factor) = 55 amps

First find the ampacity of the conductor as shown table 310.15 before derating. Then, apply the temperature correction. Next apply the adjustment factor for the multiple current carrying conductors as shown in Table 310.15 and multiply the three values together.

24
Q

When sizing conductors for a given load in an area with an elevated ambient temperature, installed in a raceway or cable more than 24 inches in length, what formula may be used?

A

Required ampacity = load/ temp correction x adj. factor

25
Q

A 100 ampere load is located in an enviornment where the temperature reaches 115 deg F, and is to be supplied with 4 75 deg C rated copper current carrying conductors. Determine the proper size wire to serve the load. All terminations are rated at 75 deg. C.

1/0 AWG
2/0 AWG
1 AWG
3 AWG

A

= 2/0 AWG

ampacity = 100 amperes (load) / .76 (temp) x .8(adj)
=100/.6
=167 amperes

  • Note Size 2/0 AWG copper conductors rated at 75 deg C with an ampacity of 175 amperes are required.

When considering current-carrying conductors, the neutral conductor is not counted when the neutral carries only an UNBALANCED CURRENT of the circuit.

26
Q

True or False: In a 120/240 volt, single phase system, the neutral carries current, but is NOT required to be counted

A

True

27
Q

True or False: The neutral is NOT considered a current carrying conductor on a 4 wire, 3 phase wye circuit where the major portion of the load consists of nonlinear loads.

A

False. The neutral IS considered a current carrying conductor on a 4 wire, 3 phase wye circuit where the major portion of the load consists of nonlinear loads.

28
Q

What are examples of non linear loads?

A

high intensity discharge lighting (HID) and florescent lighting, information technology equipment, computers, and data processing equipment.

29
Q

A one family dwelling is supplied with a 120/240 volt, single phase electrical system from the local utility company and has a demand load of 200 amperes. What is the MINIMUM size THWN/THHN aluminum service entrance conductors required?

250 kcmil
4/0 AWG
3/0 AWG
2/0 AWG

A

= 4/0 AWG

200 amperes x.83 =166 amperes

Size 4/0 AWG THWN/THHN aluminum service-entrance conductors with an ampacity of 180 amperes should be selected from Table 310.15

30
Q

True or False: When single insulated conductors are installed in free- air they can accumulate heat, caused by the current flowing in the conductors, more readily than when installed in raceways

A

False: When single insulated conductors are installed in free- air they can DISSIPATE heat, caused by the current flowing in the conductors, more readily than when installed in raceways

31
Q

When ambient temperature is not a consideration, when installed in free air, a size 6 AWG THWN copper conductor has a MAXIMUM allowable ampacity of _____.

65 amperes
75 amperes
105 amperes
95 amperes

A

= 95 amperes

Table 310.15 shows size 6 AWG THWN copper conductors to have an allowable ampacity of 95 amperes.

32
Q

A conductor having a length of fifty (50) feet contains 9 conductors, 6 conductors are considered to be current-carrying. What is the derating factor that must be applied to the ampacity of the current-carrying conductors?

80
70
60
50

A

= 80

Table 310.15

33
Q

Type XHHW insulated conductors may be used in _____

Dry locations only
Wet locations only
Dry or damp locations only
Dry, damp or wet locations

A

Dry, Damp or wet locations

Table 310.15

34
Q

A twenty five feet run of electrical metallic tubing encloses three current carrying size 2 AWG THW copper conductors in an area where the ambient temperature is 125 deg. F. The correction factor that must be applied for the elevated ambient temperature is _____.

.67
.82
.75
.76

A

.67

Table 310.15

35
Q

What is the MINIMUM size copper SE cable with Type XHHW insulation that may be used as ungrounded service-entrance conductors for 150 ampere, 120/240 volt, single phase residential service?

1/0 AWG
1 AWG
2 AWG
3AWG

A

1 AWG

310.15

150 amperes x .83 = 124.5 amperes

Size 1 AWG XHHW copper conductor with an ampcity of 130 amperes should be selected from the 75’ C col. of Table 310.15

36
Q

When ambient temperature is not a factor, a size 8 AWG single copper conductors with Type FEPB insulation, installed in free air, we have a MAXIMUM allowable ampacity of _____.

80 amperes
55 amperes
45 amperes
83 amperes

A

=80 amperes

Table 310.15

37
Q

What is the MAXIMUM allowable ampacity of a size 8 AWG THWN copper conductor installed in a conduit with 2 other current-carrying conductors where the ambient temperature is 86 deg. F?

40 amperes
45 amperes
50 amperes
55 amperes

A

=50 amperes

Table 310.15

38
Q

Three size 1/0 AWG THW copper current-carrying conductors are contained within a raceway where the ambient temperature is 120 deg. F. What is the allowable ampacity of the individual conductors?

113 amperes
130 amperes
146 amperes
150 amperes

A

A

Table 310.15

Size 1/0 AWG THW ampacity before derating = 150 amperes
150 amperes x .75 temperature correction = 112.5 amperes

39
Q

Determine the MAXIMUM allowable current-carrying capacity of a size 4 AWG copper conductor with THWN insulation installed in a conduit 10 feet in length with five other current-carrying conductors

60 amperes
52 amperes
85 amperes
68 amperes

A

=68 amperes

Table 310.15

Size 4 AWG THWN ampacity (before derating) =85 amperes
85 amperes x .8 (adjustment factor) = 68 amperes

40
Q

You are to serve a 240 volt, single phase, 3 wire, 200 ampere non-continuous load in an area where the expected ambient temperature is 110 deg. F. Using 75 deg. C. rated aluminum conductors are considering terminations are rated at 75 deg. C, determine the MINIMUM size conductors required to supply the load.

250 kcmil
300 kcmil
350 kcmil
400 kcmil

A

350 kcmil

Table 310.15

Required ampacity = 200 amperes/.82 (temp. correction) =243.9 amperes

Table 310.15 requires size 350 kcmil aluminum conductors rated for 75 deg. C with an ampacity of 250 amperes

41
Q

When a 60 deg. C rated size 6 AWG copper conductor is terminated on a circuit breaker having a marked temperature rating of 60/70 deg. C., the conductor has an allowable ampacity of

40 amperes
55 amperes
50 amperes
65 amperes

A

=55 amperes

Table 310.15

42
Q

The allowable ampacity of a size 3/0 AWG THHN copper conductor installed in an environment with an ambient temperature of 50 deg C is ______

225 amperes
200 amperes
164 amperes
185 amperes

A

185 amperes

Table 310.15

3/0 AWG THHN ampacity (before adjusting) = 225 amperes
225 amperes x .82 (temperature correction) = 184.5 amperes

43
Q

The allowable ampacity of a size 2 AWG copper conductor with a dual rated insulation of THWN/THHN enclosed in a rigid metal conduct (RMC) when installed in a wet location is _______

115 amperes
130 amperes
95 amperes
100 amperes

A

115 amperes

Table 310.104
Table 310.15

44
Q

After all demand factors have been taken into consideration for a one-family dwelling the demand load is determine to be 45,000VA. The house is to be supplied with a 120/240 volt, single phase under ground installed electrical system. What MINIMUM size USE cable with aluminum conductors is required for the underground supplied service?

2/0 AWG
3/0 AWG
4/0 AWG
250 kcmil

A

4/0 AWG

Single-Phase Current Formula
I=P/E
I = 45000VA/ 240 volts
=187.5 amperes x 83% 
= 155.625 amperes

Table 310.15 requires a size 4/0 AWG aluminum USE cable.

45
Q

Adjustment factors for ambient temperature correction and more than 3 current-carrying conductors in a raceway, shall not apply to conductors rated 0 to 2000 volts, in a raceway having a length NOT exceeding

48 in
36 in
30 in
24 in

A

24in

310.15

46
Q

What MINIMUM size THHN copper conductors are required for a 240 - volt, single phase, 70 ampere branch circuit if the temperature on the circuit breaker and the equipment are listed for 75 deg. C when the demand load is 65 amperes?

10 AWG
8 AWG
6 AWG
4 AWG

A

6 AWG

110.14
Table 310.15
240.4

47
Q

The ampacity of 8 current-carrying size 10 AWG THW copper conductors enclosed in an electrical metallic tubing (EMT) 18 inches long is _____.

30 amperes
35 amperes
28 amperes
24 amperes

A

35 amperes

310.15
Table 310.15

48
Q

Under which, if any, of the following conditions is the neutral NOT to be counted as a current-carrying conductor?
1-When it is only carrying the unbalanced current of a single phase system.
2- When it is the neutral of a 3 phase, wye connected system where the major portion of the load consists of nonlinear loads

1 only
2 only
both 1 and 2
neither 1 nor 2

A

1 only

310.15

49
Q

A conductor with ____ insulation has a different ampacity and temperature rating when used in a wet location compared to when used in a dry location.

THWN
THHN
THW
XHHW

A

XHHW

Table 310.104

50
Q

A 3 phase, 4 wire, 208/120 volt feeder is to supply a panelboard of a department store with a 104 ampere continuous florescent lighting load. Where using 75 deg. C rated aluminum feeder conductors, determine the MINIMUM size required.

250 kcmil
4/0 AWG
3/0 AWG
2/0 AWG

A

310.15
215.2
Table 310.15

=4/0

310.15
215.2
Table 310.15

Required ampacity= 104 amperes x 125% / .8 (adjustment factor)
=130/.8
=162.5 amperes

51
Q

8 75 deg C rated current-carrying branch circuit conductors are to be installed in a common raceway having a length of 50 fee where located in an attic having an ambient temperature of 100 deg. F. Each conductor is to supply a continuous lighting load of 15 amperes and terminate on a 75 deg. C. circuit breaker. Each conductor is required to have an ampacity of at least

  1. 3 amperes
  2. 8 amperes
  3. 4 amperes
  4. 6 amperes
A

30.4 amperes

Required ampacity = 15 amps x 125% / .7 (adj factor) x .88 (temp cor)
=18.75/.616
=30.4 amepres

Kansas Electrical Prep (6 decks)

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