The 10-foot Tap Rule, 240.21(B)(1) is as solid as a flimsy 2014 NEC Code Book

From HeatSpring.com Adv. PV Discussion Board

Q:

Hey Sean,

With the 10 foot tap rule, do you know where that came from? Do you see a lot of inspectors looking for this? It does not seem like a very solid rule, one of those that is more a of guideline.

A:

The 10-foot Tap Rule, 240.21(B)(1) is as solid as a flimsy 2014 NEC Code Book.

We have had the tap rules for plain old feeder taps for as long as I have been getting electrocuted (death by shock). I do not see the Tap Rules in the 1897 NEC here: https://www.dropbox.com/s/zzfgo5yfwp3bjwj/1897%20NEC.pdf?dl=0

The 2014 NEC decided to refer to the Tap Rules for interconnections in 705.12(D)(2)(2) in order to protect the tap conductors from the extra potential currents coming from the inverter connected to the feeder in addition to the original currents coming from the supply breaker.

Tap conductors are sized to protect conductors from fault currents rather than overcurrents. This means that a super skinny conductor can not be connected to a big feeder, because it needs to be large enough to clear a short circuit. In reality, the inverter due to its anti-island provisions will shut off and not have fault currents. It was a compromise put into the 2014 Code, so that we could connect to feeders and have a guideline. The reason that a feeder tap conductor has to be larger when the conductor is over 10 feet, is because there will be more resistance in the conductor itself and the conductor might heat up and catch fire before opening the overcurrent protection device. Longer wire means more resistance and lower resistance has a better chance at clearing the fault at the breaker.

Since the tap rules for solar were put in the Code in 2014 and most of the solar installed in the USA is still installed under the 2011 Code, most inspectors are not applying this rule. This is because half of the solar installed in the USA is installed in California, which will adopt the 2014 Code on 1/1/2017.

If you are looking to connect to a feeder using the 2011 NEC, you can apply the 120% Rule, which applies not only to busbars, but also conductors. With the 120% rule, you are typically more limited in what you can do.

Thanks for the good question!

Sean White

From HeatSpring.com Adv. PV Discussion Board