Overview

Probably one of the more confusing tasks in using the 1996 (and now the 1999) NEC is applying Table 310-16, accompanying notes, and code sections. There have been several articles written on this, and new code books and video tapes have tried to address the issue. I have reviewed these and more and found using Table 310-16 to be a complex task. I finally wrote an excel spreadsheet computer program to make the task easier for all of us .

The selection of the correct conductor size and overcurrent protection requires four independent processes for selecting four independent overcurrent devices (OCPD's), then the four overcurrent devices must be tested to find which set is correct, and if they do not pass the test then the next higher conductor size must be selected and three of the processes must be repeated until the four independently selected OCPD's pass the test. The correct conductor size is a by-product of this overall process.

Find the the Low Load and Select the Low Load OCPD

This load is going to be called the Low Load. So keep in mind that throughout this article the low load is the Low Load and the units is amperes. The definition of this load varies. If the final selected OCPD is listed for use at its rating at 100 per cent continuous load the Low Load is the sum of the Noncontinuous load and the Continuous load. If the OCPD is not listed for use at 100 per cent of its rating for a continuous load then the Low Load is equal to the sum of the noncontinuous load and 125 per cent of the continuous load. A continuous load is defined in the NEC as a load that continues for three hours or more. The Low Load is used to select the OCPD from Section 240-6. The OCPD that is equal to the Low Load or the next Higher OCPD is selected. This OCPD is call the Low Load OCPD.

Find the Conductor Size and the maximum OCPD-1 for this Conductor

This is the more complex of the four processes. The conductor size is required before we can perform the fourth process, selecting the Maximum OCPD for the terminal temperature.

First we must find another load current. In this case we will call this Load our Conductor Load Current that may be equal to or less than the Low Load current that we found previously. The Conductor Load Current is equal to the sum of the noncontinuous load plus the continuous load. Next we use the Conductor Load Current to select the wire size from Table 310-16.

To select the correct conductor size we must know the insulation temperature and the total derating factor. The total derating factor is the product of the ambient derating factor from the bottom of Table 310-16 and the derating factor from Note 8 to table 310-16, the derating factor for over three current carrying conductors in a raceway, cable, or trench. The Note 8 derating factor is one for up to three current carrying conductors in the same raceway, cable, or trench. Likewise, the derating factor for an ambient temperature of 26 to 30 degrees C. is one. We must multiply the ampacities in Table 310-16 for the type of conductor insulation by the total derating factor and find the conductor size that has a derated ampacity at or above the Conductor Load Current.

There is more to this than meets the eye. Since we can have a conductor with a 60, 75, or 90 degree C. insulation terminated on a 60, 75 or 90 degree C. termination there are nine ampacity tables instead of three. When a higher temperature conductor insulation is used on a lower temperature conductor termination, the conductor size is selected by choosing the the size of conductor having the lower temperature ampacity that is greater than or equal to the conductor load current and having its own higher temperature ampacity multiplied by the total derating factor greater than or equal to the conductor load current. When a conductor is used at its insulation temperature on a termination at or above its insulation temperature the same selection process is used, but only the conductor's ampacity at its rated insulation temperature multiplied by the total derating factor is checked against the conductor load current to make sure that the conductor selected has a derated ampacity greater than or equal to the conductor load current.

Next we must find the OCPD for this conductor size. The OCPD selection is done using the derated ampacity for the size and temperature of the conductor that was selected above. We multiply the total derating factor times the Table 310-16 ampacity for the conductor selected above to find the conductor derated ampacity. The selection of this OCPD depends on one other variable, whether or not the circuit is a branch circuit supplying multiple receptacles and outlets. If the circuit is of this type then the OCPD is selected using section 240-6 so that the OCPD is equal to or is the next lower OCPD below the derated conductor ampacity. Otherwise, the OCPD is selected such that it is equal to or is the next higher OCPD above the derated conductor ampacity using section 240-6. The OCPD selected in this process is called OCPD-1.

Find OCPD-2 for Table 310-16 oblisks and Table 210-24 conditions

There are some other conditions that apply to the OCPD. Table 310-16 has several obelisks notes at the bottom. If the wire size selected above is No. 14 copper then the OCPD must not exceed 15 amperes, and for No. 12, 20 amperes, and for No. 10, 30 amperes. Also if the circuit is for multiple outlets or receptacles Table 210-24 limits the OCPD to 40 amperes for No. 8 and 50 amperes for No. 6 copper wire sizes. We must perform these checks and limit the OCPD to these conditions provided that OCPD-1 is equal to or greater than the selected OCPD using this process. We finally can select the OCPD that complies with these conditions or we select OCPD-1 if these conditions do not apply. We will call this OCPD, OCPD-2. Now we are ready for selecting the fourth OCPD for the terminal temperature.

Select OCPD-3 using the Wire Size and the Terminal Temperature

The terminal temperature places a limitation on the circuit because of section 110-14(c). This requirement was implied in the NEC for many years by section 110-3(b) that requires that listed equipment be used according to the listing instructions. The terminal temperature and Size of the conductor is used to select the maximum terminal ampacity using table 310-16, and this ampacity, that is not derated, is used to select the maximum OCPD for the maximum terminal ampacity using section 240-6. The OCPD that equals the terminal ampacity or the next higher OCPD is selected. We will call this OCPD, OCPD-3.

Compare OCPD-1, OCPD-2, and OCPD-3 and find the minimum and compare this to the Low Load OCPD.

We next compare OCPD-1, OCPD-2, and OCPD-3 and select the one that is the minimum. We will call this our High Load OCPD. We next compare our High Load OCPD to see if it is equal to or greater than our Low Load OCPD. If it is, the High Load OCPD becomes our final maximum OCPD and the Low Load OCPD becomes our minimum OCPD. If the High Load OCPD is not equal to or greater than our Low Load OCPD we must select the next higher wire size and reselect OCPD-1, OCPD-2, and OCPD-3, select the minimum, and compare the new High Load OCPD to the Low Load OCPD and continue this process until the High OCPD is equal to or exceeds the the Low Load OCPD. When this condition is satisfied the High Load OCPD becomes the Maximum OCPD and the Low Load OCPD becomes the Minimum OCPD and the respective conductor size for the High Load OCPD becomes the conductor size that satisfies all the conditions placed on it by the sections of the NEC used in this process. We must remember, of course, there are other conditions in the NEC for the other types of loads, but, in general, this process, is the most common and can be used most of the time.

In some extreme cases, the Minimum OCPD may exceed the Maximum OCPD for larger wire sizes, and where the circuit is a branch-circuit and is for multiple receptacle outlets, a very unlikely situation.