These 10 exam questions show where the 2017 vs. 2020 NEC answers split, and on a 2017 NEC exam only the 2017 answer scores. Tennessee, Nevada, Arizona and Kansas still grade on the 2017 edition (see the full list of 2017 NEC exam states), where GFCI covers only unfinished basements and 125-volt, 15- and 20-amp receptacles, there is no 210.8(F), no 230.67 surge rule and no 230.85 emergency disconnect, and six service disconnects can share one enclosure. Section numbers differ too. Work the 10 questions below with your 2017 book open. Want the whole list of edits rather than a quiz? Read 2020 NEC changes vs. 2017. Prefer straight practice first? Try these five 2017 NEC practice questions with answers.
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2017 vs. 2020 NEC: the rules that flip exam answers
| Topic | 2017 NEC (scored) | 2020 NEC habit |
|---|---|---|
| Dwelling GFCI voltage | 210.8(A): 125-volt, 15- and 20-amp receptacles only | 125- through 250-volt receptacles |
| Basements | 210.8(A)(5): unfinished portions only | 210.8(A)(5): all basements |
| Outdoor outlets | No 210.8(F) | 210.8(F): dwelling outdoor outlets |
| Service disconnects | 230.71(A): up to six, single enclosure allowed | 230.71(B): separate enclosures or compartments |
| Dwelling surge protection | No service requirement | 230.67 |
| Outdoor emergency disconnect | No requirement | 230.85 |
| Dwelling 83% rule | 310.15(B)(7) | 310.12 |
| Ampacity table | Table 310.15(B)(16) | Table 310.16 |
| Ambient correction | Table 310.15(B)(2)(a) | Table 310.15(B)(1) |
| More than 3 conductors | Table 310.15(B)(3)(a) | Table 310.15(C)(1) |
| 8 ft rod in soil | 250.53(G) | 250.53(A)(4) |
Worked example: same answer, different section
A 200 A, 120/240-volt service supplies a one-family dwelling. Under 2017 310.15(B)(7), the conductors need 83% of 200 A: 200 × 0.83 = 166 A. In the 75°C column of Table 310.15(B)(16), 1/0 copper is 150 A (too small) and 2/0 copper is 175 A, so 2/0 Cu. Aluminum 3/0 is 155 A (too small) and 4/0 is 180 A, so 4/0 Al.
The 2020 NEC gives the same wire, but from 310.12 and Table 310.16. If the answer choices pair the size with a section number, only the 2017 numbers are right on your exam.
Exam traps to unlearn
- Reading 250 volts into 210.8(A). A 2017 GFCI question about a dryer, range or 240-volt receptacle almost always ends at 125 V, 15 and 20 A.
- Answering from the new sections. 210.8(F), 230.67 and 230.85 don't exist in the 2017 book. If you can't find a section in your 2017 NEC, it isn't the answer.
- Tabbing the 2020 numbers. Label your tabs with Table 310.15(B)(16), (B)(2)(a) and (B)(3)(a).
10 2017 NEC practice questions with a 2020 trap
Each question has one 2017 answer and one tempting 2020 answer. Write yours down, then open the explanation.
Question 1
A 125-volt, 15-amp receptacle is installed in a finished basement family room of a dwelling, not near a sink. What protection does the 2017 NEC require?
- GFCI only
- AFCI only
- Both GFCI and AFCI
- Neither
Question 2
A 30-amp, 125/250-volt dryer receptacle is installed in a dwelling laundry room, 4 ft from a laundry sink. Is GFCI protection required by 2017 210.8(A)?
- No, 210.8(A) covers only 125-volt, 15- and 20-amp receptacles
- Yes, laundry areas, 210.8(A)(10)
- Yes, within 6 ft of a sink, 210.8(A)(7)
- Yes, but only on a 4-wire circuit
Question 3
A hardwired 240-volt, 30-amp outlet supplies an air-conditioning condenser outdoors at a one-family dwelling. Does the 2017 NEC require GFCI protection for it?
- No, the 2017 outdoor rule, 210.8(A)(3), covers 125-volt, 15- and 20-amp receptacles
- Yes, outdoor outlets, 210.8(F)
- Yes, 210.8(A)(3) covers all outdoor outlets
- Yes, if it is within 25 ft of the meter
Question 4
What is the minimum copper conductor for a 200 A, 120/240-volt single-phase dwelling service using the 83% rule, and which 2017 section allows it?
- 2/0 Cu, 310.15(B)(7)
- 2/0 Cu, 310.12
- 3/0 Cu, Table 310.15(B)(16)
- 4/0 Cu, 310.15(B)(7)
Question 5
How much of a rod electrode must be in contact with the soil, and where is that rule in the 2017 NEC?
- 8 ft, 250.53(A)(4)
- 8 ft, 250.53(G)
- 10 ft, 250.53(G)
- 6 ft, 250.52(A)(5)
Question 6
What is the ampacity of #3 THWN copper, three current-carrying conductors in a raceway, 30°C ambient, and which 2017 table gives it?
- 100 A, Table 310.16
- 100 A, Table 310.15(B)(16)
- 85 A, Table 310.15(B)(16)
- 75 A, Table 310.15(B)(16)
Question 7
Four current-carrying #6 THW copper conductors run in one raceway at 120°F ambient. What is the adjusted ampacity, and which 2017 tables supply the factors?
- 39 A, Tables 310.15(B)(2)(a) and 310.15(B)(3)(a)
- 39 A, Tables 310.15(B)(1) and 310.15(C)(1)
- 48.8 A, Table 310.15(B)(2)(a) only
- 52 A, Table 310.15(B)(3)(a) only
Question 8
Under the 2017 NEC, may a service have six circuit breakers as its service disconnecting means, all mounted in a single enclosure?
- Yes, 230.71(A) allows up to six in a single enclosure
- No, each must be in a separate enclosure or compartment
- No, a dwelling service is limited to one disconnect
- Yes, but only up to two
Question 9
A new 200 A service is installed for a one-family dwelling. What surge protection does Article 230 of the 2017 NEC require at the service?
- None
- A Type 1 or Type 2 SPD
- A Type 3 SPD at the meter
- An SPD only if the service is overhead
Question 10
A one-family dwelling has its meter outside and its main breaker inside, at the nearest point of entrance of the service conductors. Does the 2017 NEC require an additional outdoor emergency disconnect?
- No, 230.70(A)(1) allows the disconnect inside, nearest the point of entrance
- Yes, 230.85
- Yes, every dwelling disconnect must be outdoors
- Only if the meter is more than 10 ft from the panel
Answer key: 2017 vs. 2020 sections
Show the answer to question 1Hide the answer to question 1
1. B, AFCI only (2017 210.8(A)(5), 210.12(A))
2017 210.8(A)(5) requires GFCI only in unfinished portions of basements, and a family room is on the 210.12(A) AFCI list. 2020 210.8(A)(5) covers all basements, which is why a 2020-trained candidate picks C.
Show the answer to question 2Hide the answer to question 2
2. A, not required (2017 210.8(A))
In 2017, 210.8(A) applies to 125-volt, 15- and 20-amp receptacles, so neither (A)(7) nor (A)(10) reaches a 30-amp dryer receptacle. 2020 210.8(A) covers 125- through 250-volt receptacles without the 15- and 20-amp limit, so a 2020-trained candidate picks B or C.
Show the answer to question 3Hide the answer to question 3
3. A, not required (2017 210.8(A)(3))
The 2017 edition has no 210.8(F). Its outdoor rule, 210.8(A)(3), is limited to 125-volt, 15- and 20-amp receptacles. 2020 added 210.8(F) for dwelling outdoor outlets, the trap in B.
Show the answer to question 4Hide the answer to question 4
4. A, 2/0 Cu (2017 310.15(B)(7))
200 × 0.83 = 166 A. 2/0 Cu is 175 A at 75°C in Table 310.15(B)(16). The size is the same in 2020, but the 83% rule moved to 310.12, so B has the right wire and the wrong section.
Show the answer to question 5Hide the answer to question 5
5. B, 8 ft (2017 250.53(G))
2017 250.53(G) requires at least 8 ft of the rod in contact with the soil. The 2020 NEC moved it to 250.53(A)(4), choice A.
Show the answer to question 6Hide the answer to question 6
6. B, 100 A (2017 Table 310.15(B)(16))
THWN is a 75°C conductor, and #3 Cu is 100 A in that column. 85 A is the 60°C value, the trap for anyone who skips the insulation type, and no column lists 75 A for #3 copper. 2020 renumbered the table to 310.16, choice A.
Show the answer to question 7Hide the answer to question 7
7. A, 39 A (2017 Tables 310.15(B)(2)(a) and 310.15(B)(3)(a))
#6 THW Cu is 65 A at 75°C. At 120°F, the 75°C column of Table 310.15(B)(2)(a) gives 0.75, and 4 to 6 conductors take 80%: 65 × 0.75 × 0.80 = 39 A. Don't read 0.82 from the 90°C column. 2020 numbers those tables 310.15(B)(1) and 310.15(C)(1), choice B.
Show the answer to question 8Hide the answer to question 8
8. A, yes (2017 230.71(A))
2017 230.71(A) allows not more than six switches or sets of circuit breakers, mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard or switchgear. 2020 230.71(B) requires separate enclosures, panelboards or compartments, the trap in B.
Show the answer to question 9Hide the answer to question 9
9. A, none (2017 Article 230)
The 2017 NEC has no dwelling service surge requirement. The Type 1 or Type 2 SPD in B comes from 2020 230.67.
Show the answer to question 10Hide the answer to question 10
10. A, no (2017 230.70(A)(1))
2017 230.70(A)(1) permits the service disconnect outside or inside nearest the point of entrance of the service conductors. The outdoor emergency disconnect for one- and two-family dwellings is 2020 230.85, choice B.
2017 vs. 2020 NEC trap question FAQ
Which electrical exams still use the 2017 NEC?
Tennessee's LLE and CE through Prov, Nevada's C-2 and Arizona's R-11, C-11 and CR-11 through PSI, and Kansas KGD, KGH and KGX through ICC and Pearson VUE.
Will a 2020 answer be marked right on a 2017 NEC exam?
Only when the rule and section are the same in both editions. Where they differ, the exam scores the 2017 answer, including the 2017 section number.
Does the 2017 NEC require GFCI for a 240-volt receptacle in a dwelling?
Not under 210.8(A). The 2017 dwelling list covers 125-volt, 15- and 20-amp receptacles. The 2020 edition expanded it to 250 volts.
Is the free 2017 NEC mock exam the full 100 questions?
No. The free mock is 25 questions with instant feedback and an NEC reference for every answer. The full 100-question timed mock is in the $495 package.
Do journeyman and master exams test the 2017 vs. 2020 NEC differences?
They test the 2017 rules, so the differences matter. Kansas KGH journeyman and KGD master candidates, and Tennessee, Nevada and Arizona contractor candidates, are scored on the 2017 section numbers and the 2017 GFCI, disconnect and surge rules.
Drill the 2017 answers before exam day
Start with the free 25-question mock, then lock in 2017 numbers with the full package.
Pick your state's package: Tennessee, Nevada C-2, Arizona or Kansas ($495 each). Another 2017 NEC exam? See the 2017 NEC self-study package.
Sources
- NFPA 70, National Electrical Code, 2017 edition: 210.8(A), 210.12(A), 230.70(A)(1), 230.71(A), 250.53(G), 310.15(B)(7), Tables 310.15(B)(2)(a), 310.15(B)(3)(a) and 310.15(B)(16)
- NFPA 70, 2020 edition: 210.8(A), 210.8(F), 230.67, 230.71(B), 230.85, 250.53(A)(4), 310.12, Tables 310.15(B)(1), 310.15(C)(1) and 310.16
Reviewed November 6, 2026. NEC references are to the 2017 edition.
