Welcome back to the series on branch circuits and feeders. Last month we went over how branch circuits are sized and rated, including the correct rating of the overcurrent protective device (OCPD) and the accurate ampacity of the branch circuit conductors. We ended our discussion with the general requirement in Section 210.19 for selecting a conductor with an ampacity that meets either 210.19(A)(1) or (A)(2), whichever is larger. Of course, there are exceptions to this rule, but they are not as common since they require specialized equipment rated for continuous operation at 100% of its rating or special connectors at the supply and load end of the conductor.

In addition to the general rule of 210.19, there are also specific requirements for branch circuit conductor sizes when the branch circuit supplies more than one receptacle, household ranges or cooking appliances, or loads other than the specific branch circuits listed in Table 210.3. These additional conductor sizing requirements essentially state that the conductor must be sized to be sufficient for the load, or in the case of a branch circuit supplying multiple receptacles for cord-and-plug-connected loads where the load might not be known, the conductors must have an ampacity equal to or greater than the rating of the branch circuit. In other words, if a 20A OCPD supplies more than one receptacle, the circuit conductors must have an ampacity of at least 20A.

Example: Level 2 EV charger

Going back to the sizing requirements for branch circuit conductors in Section 210.19(A), it might be helpful to look at an example to clarify how this works. Take a branch circuit installed for supplying an electric vehicle charging station. In this particular instance, the EV charger is a Level 2 one with an input amperage of 32A at 208V, single-phase. 

To start, we need to know whether this charger is going to be a continuous load. Since it is reasonable to assume that the charger will be used for long durations, it is worth checking if Article 625 has a statement requiring EVSE to be considered a continuous load or not. What we find is that, regardless of continuous load status, the OCPD must be sized at a minimum of 125% of the input current of the charger. This means that 32A × 125% = 40A. Since 40A corresponds to a standard size OCPD per Article 240, we can use this rating for the branch circuit. 

Next, we need to determine the conductor size in accordance with 210.19(A). We will look at installing this EV charger by itself and then at different conditions that might alter our original answer. The branch-­circuit wiring will be run underground in a PVC raceway that will include the two ungrounded conductors and an equipment grounding conductor. The conductor insulation will be THHW and, because the raceway is underground, the interior will be considered a wet location per Article 300. 

Because the raceway will be underground and these will be the only current-carrying conductors in the raceway, there seems to be no adjustment or correction factors to apply here. In this case, 210.19(A)(1) will require the larger conductor size if the EV charging station is considered a continuous load as it multiplies 32A by 125%. If we can verify this is not a continuous load, then 210.19(A)(1) and (A)(2) provide the same minimum ampacity need of 32A. 

To select the corresponding conductor size, we must consult Table 310.16, where we see that THHW insulation is in the 75°C and 90°C columns. However, because the raceway is underground, and this is considered a wet location, the conductor’s ampacity must come from the 75°C column. 

But wait, there’s more! Remember that 210.19 referenced Section 110.14(C), where we can see that due to termination temperature limitations, for terminations marked for 1 AWG or 100A and less, the ampacity can be adjusted or corrected from the 75°C column, but the final ampacity is not permitted to exceed that listed in the 60°C column unless the terminations are marked for 75°C temperature ratings. Since we don’t know, we must use the 60°C column. There we can see that an 8-AWG copper conductor is good for 40A in the 60°C column. This satisfies the requirement in 210.19(A) whether we consider the load noncontinuous or continuous, since the next size down is only rated for 30A, which is not enough to satisfy the 32A load.

Adjustment and correction factors

Let’s explore what happens when we add adjustment and correction factors to the mix. Continuing with our example, our EV charging station is being installed as one of four in a bank of spots in the parking lot of a hotel. To save labor, the branch-circuit wires are installed in a single PVC raceway, which means there are to be eight current-carrying conductors in the raceway. Table 310.15(C)(1) gives us an ampacity adjustment factor of 70% for seven to nine current-carrying conductors in a raceway. To apply the 70% factor to the load of 32A, we divide by 0.7, resulting in the conductor needing a minimum ampacity of 45.71A. The 75°C column is still an 8-AWG copper conductor. 

One last step is to verify the conductor is protected from overcurrent in accordance with its ampacity. In other words, is an 8-AWG with eight current-carrying conductors in the raceway protected by a 40A circuit breaker? Applying the 70% factor to 50A gives us an adjusted ampacity of 35A. Unfortunately, 35A is a standard OCPD rating per Table 240.6(A), and therefore, a 40A OCPD is not able to protect an 8-AWG copper conductor in a raceway with eight current-carrying conductors. We must use a 6-AWG copper conductor, which has an ampacity after 70% of 45.5A and is protected by the 40A OCPD.

The common “double derating” mistake that folks make in this process is to apply the continuous load calculation of 125% stated in 210.19(A) to a load adjusted for the current-carrying conductors. In our example, that would mean the conductor would need an ampacity of 32A ÷ 0.70 = 45.71A, which multiplied by 125% gives us 57.14A. In this example, it happens to work out the same, but only because we had the issue with OCPD protection. If we wanted to avoid the OCPD issue, the simple solution would be to use two or more raceways to supply the EV chargers.

Understanding how to properly size branch-circuit conductors and OCPDs based on the load being supplied is paramount to installing safe electrical systems. 

Speaking of safety, next month’s article will begin the discussion on two important safety concepts for branch circuits: AFCI and GFCI protection.

Until next time, stay safe and remember to always test before you touch!

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