The 175 A Conductor on a 200 A Service
A single-family dwelling gets a 200 A, 120/240 V single-phase service. The Article 220 load calculation totals 178 A. The crew pulls 2/0 AWG copper THWN-2 in 1 1/2 in. PVC from the meter socket to a 200 A main-breaker panel. All terminations are rated 75 C, the run is short and indoors, no adjustment or correction factors apply, and 2/0 copper carries 175 A in the 75 C column. The same crew then feeds a 200 A subpanel in a detached garage, 70 ft away, from a 200 A breaker in the house panel, again with 2/0 copper. They also route the house service conductors through an attic where summer ambient reaches 50 C.
1. The inspector approves the 2/0 copper for the 200 A service. Explain the rule that makes a 175 A conductor legal on a 200 A service, and list the conditions that must hold.
Read the worked answer
Rule applied: 310.12 (services and feeders supplying a dwelling unit).
- For a one-family dwelling, service conductors and feeder conductors that carry the entire load of the dwelling may be sized from Table 310.12 instead of 310.16. Table 310.12 assigns 2/0 copper to a 200 A service.
- The underlying requirement is that the conductor ampacity be at least 83% of the service rating: 0.83 x 200 A = 166 A. The conductor's 75 C value of 175 A exceeds that floor.
- The service rating must still be adequate for the load. The Article 220 result is 178 A, so the 200 A service is not overloaded.
Conditions that must all be true:
- The conductors serve the entire load of a one-family (or two-family) dwelling, not a portion of it.
- Terminations are rated for the conductor's ampacity; here everything is 75 C.
- Any adjustment or correction factors under 310.15 still apply on top of the Table 310.12 value; the allowance is not a waiver of ambient or bundling derating.
- Conduit fill, bending space, and 110.14(C) termination limits are satisfied.
If any of these fails, the conductor must comply with 310.16 and 240.4 like any other feeder.
2. The 2/0 copper feeding the 200 A garage subpanel is the same size used for the service. Is that acceptable? Describe what must change if it is not.
Read the worked answer
Not acceptable as installed.
The 310.12 allowance applies only to conductors that carry the entire load of the dwelling unit. A feeder to a detached garage carries only part of the load, so it is governed by 310.16 and 240.4: the conductor must have an ampacity of at least the rating of the overcurrent device protecting it.
- 2/0 copper at 75 C = 175 A, less than the 200 A breaker ahead of it. That is an unprotected feeder.
- Options: upsize to 3/0 copper (200 A at 75 C) or 4/0 aluminum, or reduce the breaker to 175 A, provided the calculated load for the garage does not exceed 175 A and the panel's main lugs and bus are rated for that configuration.
A common field error is applying the dwelling 83% table to every feeder on the property. It applies only where the conductor carries the whole dwelling load.
3. The service conductors pass through a 50 C attic for 12 ft. Does the 2/0 copper still work? Show the math and give the smallest compliant fix.
Read the worked answer
It no longer works.
Ambient correction per Table 310.15(B)(1), 75 C conductor at 50 C ambient: 0.75.
- Corrected ampacity: 175 A x 0.75 = 131 A
- Required floor: 0.83 x 200 A = 166 A
- 131 A < 166 A, so the installation fails 310.12 and 310.16.
Smallest practical fix: upsize the conductor. 4/0 copper at 75 C = 230 A; 230 A x 0.75 = 172.5 A, which clears the 166 A floor and is also within the 75 C termination limit of the equipment. 3/0 copper does not work (200 A x 0.75 = 150 A). Rerouting the run out of the attic, or providing a ventilated path that keeps ambient at 40 C (0.88 factor: 175 x 0.88 = 154 A - still short) also has to be checked.
Note: you cannot rescue this by using a 90 C conductor's tabular value, because the ampacity is still limited by the 75 C terminations per 110.14(C). The correction factor must be applied to the lowest-rated component in the chain.