NEC Lunch and Learn

By Mike Ruff11/15/07

September, 2007 2

Conductor Sizing

Loads are calculated according to Article 220.√ Specialized applications-220.3

The ampacity of a conductor is determined by considering the load and the conditions of use.Operating temperatures must not be exceeded.

September, 2007 3

Conditions of Use

Operating Temperatures are determined by√ Ambient temperatures√ Heat generated by load current√ Rate of heat dissipation into ambient√ Adjacent load-carrying conductors

September, 2007 4

Conditions of Use

Table 310.16 is based on specified conditions:√Insulation rated 60°C, 75°C, or 90°C√Up to three current-carrying conductors together√Ambient temperature of 30°C (86°F)Correction factors apply in other ambient temperaturesAdjustment factors apply with over 3 current carrying conductors

September, 2007 5

Conditions of Use-Example

Allowable Ampacities:√ Ampacity from 90°C Column= 40A√ Apply correction factor-0.87 X

40A=34.8A√ Apply adjustment factor-0.80 X

34.8A= 27.8A√ Ampacity= 37.8A

September, 2007 6

Terminal Temperatures

The load applied to a conductor may not cause the conductor to operate at higher temperatures than the terminal ratingConductor insulation temperature ratings are not limitedConductor operating temperatures at terminals are limited

September, 2007 7

Terminal Temperatures

Default values are provided:√ 60°C for OCDs through 100A and wire

sizes 14 AWG through 1 AWG√ 75°C for larger OCDs or larger wires√ 75°C for motor terminals, NEMA

Design B,C, and D√ Equipment markings may modify the

default values

September, 2007 8

Example:6 AWG THHN Cu

Ampacity is 75A under standard conditionsTerminals are assumed to be rated at 60°CLoad on conductors is limited to 55A at terminalsLoad could be 65A with 75°C terminals75A ampacity (90°C) may be used for derating

September, 2007 9

Conductor CompatibilityWith OCD

A continuous load is a load whose maximum current is expected to continue for 3 hours or more.Most OCDs are not rated for continuous loading at 100% of ratings.

September, 2007 10

Conductor CompatibilityWith OCD

OCD ratings must be no less than 125% of the continuous load plus 100% of the non-continuous load unless OCD is rated for 100%Conductor sizes must be increased by the same proportions for the OCD operation to be reliableThis minimum conductor size may be determined without considering the conditions of useOther conductor sizing rules may require larger conductors

September, 2007 11

Example: 95A Continuous load

Given standard conditions of use and terminal ratings and THWN Cu conductors√ 95A X 1.25 = 118.75A√ Minimum OCD rating is 125A (75°C lugs)√ Minimum conductor size is 1 AWG

Conditions of use and terminal ratings may establish other minimum sizes

September, 2007 12

Standard Ampere Ratings

Many rules for overcurrent protection refer to standard sizes of overcurrent devices.There are sizes available that are not considered standard, bur a list of standard sizes is needed for code application.Standard ampere ratings for fuses and circuit breakers are considered to be 15,20,25,30,35….4000,5000, and 6000 amperes

September, 2007 13

Standard Ampere Ratings

Additional fuse sizes are 1,3,6,10, and 601 amperesNonstandard ampere-rated fuses and circuit breakers may be used

September, 2007 14

Adjustable-TripCircuit Breakers

The rating of adjustable-trip circuit breakers having external means for adjusting the current setting (long-time pickup setting), not meeting the requirements of 240.6(C), shall be the maximum setting possible

September, 2007 15

Restricted AccessAdjustable-Trip Circuit Breakers

The actual adjusted long-time (overload) setting is considered to be the rating if the adjustment is:√ behind removable or sealable covers

or√ behind bolted equipment enclosure

doors or√ behind locked doors accessible only

to qualified persons

September, 2007 16

Protection of Conductors

Generally, OCDs should match conductor ampacity. However;√ Only SC & GF protection is required in

certain cases√ The next larger standard size may be

used, with restrictions√ Special rules apply to most small

conductors

September, 2007 17

Protection of Conductors

Generally, OCDs should match conductor ampacity. However:√ Special rules apply to taps and

certain types of equipment√ Flexible cords and fixture wires are

covered separately

September, 2007 18

Power Loss Hazard

September, 2007 19

Individual Branch Circuit