These NEC service and feeder practice questions cover Articles 230, 215 and 240 — conductor sizing, disconnects, overcurrent and taps. Services and feeders bring together several articles at once, which is why they show up so often on the master exam. Article 230 governs everything from the utility connection to the service disconnect. Past that point you are into feeders under Article 215 and overcurrent protection under Article 240, and the rules genuinely change at that boundary.
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Written by Kevin, a licensed
Michigan master electrician — 26 years in the trade and 17 of them as an electrical
inspector. Every answer below names the NEC section it comes from, so you learn where to
look it up rather than memorising it.
Exam tipWork out where the service ends and the feeder begins before you answer anything. A rule that is right for a service conductor is often wrong for a feeder, and the exam writes questions specifically to catch that.
Tap an answer to see whether you got it right, the code section it comes
from, and the reasoning worked out in full. Nothing to sign up for.
Question 1Service Conductors
A service has a service load of 6020 VA at 120/240 V. What is the total service load in amperes?
Correct answer: C. 25A
NEC 6020 VA ÷ 240 V
Rule — service load in amps
Amps = VA ÷ volts. For a 120/240 V single-phase service, divide by 240.
6,020 VA ÷ 240 V = 25 A
25 A
How to do it
Total VA over the service voltage. Use 240 V (line-to-line) for a 120/240 service.
Common mistakes
Dividing by 120 instead of 240.
Forgetting to total the VA first.
Question 2Service Conductors
What min size copper wire can be used for a 150A service on a single family dwelling?
Correct answer: C. #1
NEC Table 310.12(A)
From Table 310.12(A): a 150 A single-phase dwelling service = #1 copper.
Answer: #1 AWG.
Question 3Service Conductors
What is the minimum wire size for a 100A single-phase service on a one-family dwelling?
Correct answer: C. #4
NEC Table 310.12
From Table 310.12(A) (single-phase dwelling service): a 100 A service = #4 copper.
Answer: #4 AWG.
Question 4Service Conductors
A house demand load is 24,800 VA at 120/240 V single-phase. What minimum standard service size is required?
Correct answer: B. 110 A
NEC 310.12 — 83 % dwelling service (was 310.15(B)(7) in 2014)
Service size from the load
Step 1 — Load current
24,800 VA ÷ 240 V = 103 A
Step 2 — The service disconnect/breaker must carry the load, so round 103 A up to the next standard rating (240.6). A 100 A service is too small for a 103 A load, so the minimum is a 110 A service
Step 3 — The 83 % dwelling rule (310.12) does NOT shrink the service rating - it only lets the service CONDUCTORS be sized at 83 % of that rating
110 × 0.83 = 91 A, so #3 copper (100 A) conductors are permitted on the 110 A service
310.12 — 83 % dwelling service (was 310.15(B)(7) in 2014)
110 A
How to do it
Service current = total demand VA ÷ the service voltage (240 V for a 120/240 V service). Then round UP to the next standard overcurrent / disconnect size (240.6).
Common mistakes
Dividing by 120 instead of 240.
Rounding the service size down.
Using the connected load instead of the demand (calculated) load.
Question 5Taps
A 300 A breaker feeds a 9 ft tap. What is the minimum-rated breaker the tap may feed?
Correct answer: B. 30 A
NEC 240.21(B)(1) — 10-ft tap
Feeder taps (240.21(B))
A 9-ft tap uses the 10-ft tap rule (240.21(B)(1)): the tap must carry at least 1/10 of the feeder OCPD.
Step 1 — Feeder OCPD = 300 A
Step 2
300 A × 0.10 = 30 A
240.21(B)(1) — 10-ft tap
30 A
How to do it
The 10-ft tap must carry at least 1/10 of the feeder OCPD; the 25-ft tap must carry at least 1/3. Multiply the feeder's overcurrent rating by 0.10 or by 1/3 to get the minimum tap ampacity.
Common mistakes
Swapping the fractions — 10-ft = 1/10, 25-ft = 1/3.
Using the load instead of the feeder OCPD rating.
Ignoring the length and termination limits that come with each tap rule.
Question 6Service Conductors
Wire size for a 600 A service run as 3 parallel sets?
Correct answer: B. 3/0
NEC 310.16 / parallel conductors
Parallel conductor sets
Parallel sets share the load equally, so size each set for its share (310.10, parallel conductors).
Step 1 — Per-set current
600 A ÷ 3 sets = 200 A each
Step 2 — Pick a conductor rated 200 A (Table 310.16, 75 °C)
3/0 Cu = 200 A
310.16 / parallel conductors
3/0
How to do it
Divide the total load by the number of sets to get each set's share, then size one conductor for that share (minimum 1/0, 310.10(G)). Every set must be the same size, length, and material.
Common mistakes
Sizing each set for the full load instead of its share.
Using conductors smaller than 1/0 in parallel.
Unequal set lengths — they will not share current evenly.
Question 7Feeder Overcurrent & Sizing
What size OCPD is required for a feeder with 100 A continuous + 100 A non-continuous load?
Correct answer: B. 225 A
NEC 215.3 — 125 % of continuous + 100 % of non-continuous
Bump only the continuous load by 25% (it's the sustained heat that matters), leave the non-continuous alone, and add. The feeder conductor is sized the same way.
Common mistakes
Multiplying the whole load by 125%. Only the continuous part gets the bump.
Applying 125% and 80% both. 125% of the load and 80% of the breaker are two ways of saying the same limit — pick one.
Forgetting "continuous" = 3 hours or more.
Question 8Feeder Overcurrent & Sizing
What size THHN feeder conductor is required for a 200 A continuous load?
Correct answer: B. 250 kcmil
NEC 215.2 & 215.3 — continuous-load sizing
Continuous load — size at 125 %
A feeder carrying a continuous load is sized at 125 % of that load (215.2 / 215.3).
Step 1 — Required ampacity
200 A × 1.25 = 250 A
Step 2 — Find a conductor rated at least 250 A (Table 310.16, 75 °C)
250 kcmil Cu = 255 A
215.2 & 215.3 — continuous-load sizing
250 kcmil
How to do it
A continuous load (on 3 hours or more) is figured at 125 %. Multiply the load by 1.25 to get the minimum conductor ampacity and overcurrent size, then pick the wire and breaker that meet it.
Common mistakes
Forgetting the 125 % on the continuous part.
Applying 125 % of the load and 80 % of the breaker both — pick one.
Rounding the conductor down.
Question 9Feeder Overcurrent & Sizing
Minimum conductor for a 1,200 A OCPD using three sets per phase?
Correct answer: B. 600 kcmil
NEC 240.4 & Table 310.16 (parallel sets)
Parallel conductor sets
Split the load across the parallel sets.
1,200 A ÷ 3 = 400 A per set → 600 kcmil (rated 420 A).
Check: 420 × 3 = 1,260 A ✓
240.4 & Table 310.16 (parallel sets)
600 kcmil
How to do it
Divide the total load by the number of sets to get each set's share, then size one conductor for that share (minimum 1/0, 310.10(G)). Every set must be the same size, length, and material.
Common mistakes
Sizing each set for the full load instead of its share.
Using conductors smaller than 1/0 in parallel.
Unequal set lengths — they will not share current evenly.
Question 10Feeder Overcurrent & Sizing
What is the maximum OCPD that may protect a 1 AWG THWN copper conductor (75 C) serving a 104 A calculated load?
Correct answer: B. 150 A
NEC 240.4(B) — next standard size up
Overcurrent protecting a conductor (240.4)
Step 1 — look up the 1 AWG THWN ampacity in Table 310.16 (75 C column): 130 A. The load (104 A) does not exceed it
Step 2 — Rule 240.4(B): if the conductor ampacity does not match a standard OCPD size, you may go UP to the next standard size (allowed at 800 A and below)
Step 3 — 130 A is not a standard OCPD size, so the next standard size up is 150 A
Max OCPD = 150 A
240.4(B) — next standard size up
150 A
How to do it
Find the conductor's ampacity in the correct terminal column. If it lands between standard sizes and the load allows, 240.4(B) lets you round UP to the next standard fuse/breaker (up to 800 A).
Common mistakes
Rounding up when the load actually exceeds the conductor's ampacity.
Using the wrong temperature column.
Applying the round-up above 800 A (not permitted).
These are 10 of 803 questions
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