nec 15th november[1]
TRANSCRIPT
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
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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
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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
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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
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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
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Power Loss Hazard
September, 2007 19
Individual Branch Circuit