I need to install a 200A panel about 100 feet from the meter base. I have 400A service coming to the building, broken out at the meter base into two 200A exterior disconnects. One disconnect is currently unused, leaving it available for this project. The loads on this panel will be: lighting/receptacles, HVAC units drawing 75A max, 2 clothes dryers, and 1 electric oven. (Gas cooktop and water heater will be used.)

I'm seeing conflicting information about the size and type of wire to use in this run.

According to the 2011 NEC sizing table, 4/0 aluminum is the correct size to use, but max distance is not discussed. Calculations show that there should be about 1.5% voltage drop across this distance at full current, which I believe is within the acceptable limits.

However, 2014 NEC and newer remove the sizing table, instead instituting the 83% size rule.

Cerrowire's ampacity chart shows 4/0 aluminum to be capable of 180A at 75*C, which does meet the 83% rule. However, no voltage drops or additional heat due to increased resistance of longer distance wires are mentioned.

Also, I'm finding varying information about whether it's OK to run SE inside a building. This page seems to indicate SE cables are fine, but if run through insulation, the ampacities at 60*C must be used. In my installation, no insulation would be present, and the run would be through engineered wood I-joists between lower and upper floors of a single-family residential dwelling, so I am assuming I can use the 75*C ampacity numbers.

I discussed this with the electrical inspector, who seemed to be fine with 4/0 SER in this application. However, I'm trying to understand whether I missed anything that can come back to bite me later.

I would appreciate your comments and suggestions on wire size and type in this application.



Calculators I checked online returned 2% voltage drop for 4/0 Al at 200A load. I wouldn't hesitate at all at that size. 240.4(B) allows the next larger breaker size when the ampacity doesn't correspond to a standard size. This makes qualifying for the 83% rule irrelevant.

The Informational Note in 2017 NEC 215.2 recommends a 3% drop maximum for Feeders, with a maximum Feeder + Branch circuit totaling 5%. You will have to evaluate your building size and panel location to determine if branch circuit voltage drop is an issue. Informational Notes are advisory in nature, may point you to code requirements, but notes themselves are not enforceable.

The 2017 NEC in section 338.10(B)(4) added "For Type SE cable with ungrounded conductor sizes 10 awg and smaller, where" to the sentence requiring 60°C ampacity, so you don't need to apply that.

  • I've also read that the 2020 NEC brings back the residential service table (310.12), and Table 310.15(B)(7) is moving back to 310.16. – NoSparksPlease Jan 3 at 23:59

The 83% derate only applies if it is all the service to a single dwelling unit. With 75A of A/C and two dryers, that sounds like something else. So let's ignore the 83% derate.

Suppose we use 4/0 wire

First, a couple common errors in sizing voltage drop are calculating ampacity on breaker trip - you should actually calculate it on practical daily load -- and presuming 3.000% is a hard number. (the voltage drop calcs online will hurt you there; they treat 3.01% as cause for a wire size bump!) 3.0% is a nonsense number not mentioned anywhere in Code. What the Code really wants is less than 8% from meter to final outlet.

4/0 (aluminum, of course) wire, indeed, has a rating of 180 amps. You will need to breaker this run for 180--- Oh, snap! Nobody makes a 180A breaker for that panel! You'll have go upward to the next breaker size available. It's 200A.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.