After attending a seminar recently I was confused. Article 110.14c. The instructor said thhn should only be used for derating, since a breaker is only rated for 75c. The issue was that thhn transferred heat somehow? Can anyone elaborate on this so I can understand?

In general, if we installed Conductors with Insulation having a 90ºC rating, we use the Maximum Ampacity listed in Table 310-16 under the 90ºC column as a "Base Figure", then apply adjustment factors accordingly to end up with a "Derated Value". We still Terminate according to Article 110.14(c)(1) - meaning the highest Over Current Protection Device we will use is based on that shown in 110.14(c)(1).

**** Exception: Motor Circuits: OCPD for Ground fault and Short Circuit is based on Table 430.22(b), and Conductor load is based on 110.14(c)(1)... more on this later.

Here are some simple Derating examples - each having an Ambient Temperature of 30ºC, and using a common Raceway of greater than 24" length: ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ...

Example #1: System = 120/240V 1 Phase 3 Wire.

Four (4) 3 wire Multiwire Circuits in the same Raceway. Twelve # 12 THHN cu. Eight Current Carrying Conductors (if you have questions why 8 CCCs instead of 12, please let me know)

Maximum Amperage for # 12 THHN cu per 310.16 is 30 Amps

Adjustment Factor for 7-9 Current Carrying Conductors ("CCCs") per Article 310.15(b)(2)(a) = 70%

30 Amps x 0.7 = 21 Amps

Maximum Over Current Protection Device (OCPD) = 20 Amps.

If there were Five 3 wire Multiwire Circuits in the same Raceway, we would either Terminate the # 12s to a 15 Amp OCPD, or use #10s on a 20 Amp OCPD, as the adjustment factor would now be 50% for 10 to 20 CCCs in the same Raceway.

Two (2) 4 wire Multiwire Circuits in the same Raceway. Eight # 12 THHN cu. Eight Current Carrying Conductors (if you have questions why 8 CCCs instead of 6, please let me know)

Maximum Amperage for # 12 THHN cu per 310.16 is 30 Amps

Adjustment Factor for 7-9 Current Carrying Conductors ("CCCs") per Article 310.15(b)(2)(a) = 70%

30 Amps x 0.7 = 21 Amps

Maximum Over Current Protection Device (OCPD) = 20 Amps.

Here, if we add just a single 2 wire circuit with the multiwire circuits, the total will be 10 CCCs. Adjustment factors = 50% for 10-20 CCCs, so once again either we Terminate the #12s at 15 Amps, or use #10s Terminated at 20 Amps.

Parallel Feeders in common Raceway (Conduit). 400 Amp Panelboard Feeder

Eight #250 Kcmil THHN cu in same raceway.

250 Kcmil @ 90ºC = 290 Amps 290 x 0.7 (7-9 CCCs in raceway) = 203 Amps per Conductor 203 Amps x 2 = 406 total Amps

If we used Eight 4/0 THHN cu in the same raceway, the maximum OCPD would be 350, as the "Derated Value" would be 182 Amps per Conductor, or 364 Amps combined across two Parallel Conductors.

Parallel Panelboard Feeders - in separate raceways:

400 Amp Panelboard Feeder, fed with Two (2) 2-1/2" Conduits - each containing Four (4) 4/0 THHN cu + One (1) #3 EGC

Maximum Amperage of 4/0 THHN cu = 260 Amps

4-6 CCCs in same raceway = 80% adjustment factor

260 Amps x 0.8 = 208 Amps Maximum.

Two separate runs = 416 Max Amps.

Maximum OCPD for these Feeders = 400 Amps (Since the Conductors are larger than #1, &/or are for a Circuit of 100 Amps or larger, then we may Terminate using the 75ºC column of 310-16)

The total MCA computed for a motor Circuit may be applied to the size of the Motor Circuit's Conductors, yet we may protect that Circuit with an OCPD that is much larger than what is "typically used" per 310.16 and 110.14

For example, a Single Motor might calc out to have a total Amperage value of 16 Amps, so we use #12 THHN cu for the Circuit Conductors.

This Motor will be started Across-The-Line, and so we adjust our Branch Circuit's "Ground Fault & Short Circuit Protection" (the Circuit Breaker in the Panelboard feeding that Branch Circuit) to be 250% of the Minimum Circuit Ampacity we have calculated for the Motor - in this case 16 x 2.5 = 40 Amps, so we Terminate the #12s to a 40 Amp Circuit Breaker.

If we were using Motor Circuit Breaker (Instantaneous Trip Breaker) we could rate that as high as 800% of the MCA - which results in the Breaker being 125 Amp rated.

Nevertheless, the actual Circuit Conductors must be sized to carry the maximum amperage value calculated for the Motor Circuit. In these cases, the actual Amperage values in Table 310.16 may be used, if no adjustment factors are required to derate the conductors. Multiple Circuits in the same raceway, &/or higher ambient temperatures will determine what - if any adjustments are needed.

Table 310.15(b)(2)(a) determines the adjustment factors for multiple CCCs in a raceway, and the Ambient Temperature Correction Factors in Table 310.16 (found at the bottom), determine the adjustment for Ambient Temperature. Both of these factors combined determine the overall Conductor's "Derating Figure", and are applied to the appropriate column of 310.16 for maximum Amperage.

Hope this clears things up.

Let me know if you have additional questions.

Scott

Last edited by Scott35; 03/08/0811:41 PM. Reason: left out "80%" adjustment in example #4

Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!

I understand the derating part. Let say for instance, I have a 200amp 240v service on a house. 3cc, no need to derate. My breaker have a terminal rating of 75c. Is this a code violation to use 3/0cu thhn

...The issue was that thhn transferred heat somehow? Can anyone elaborate on this so I can understand?

Basically, if you load conductors to their 90 degree ampacities, terminations rated less than 90 degrees will act as a heat sink. Overtime, this will cause the connection / termination to fail. This is the purpose of 110.14 of the NEC. It is to prevent any part or component of the circuit to be exposed to temperatures greater than what they are rated for.

Originally Posted by sid123456

Let say for instance, I have a 200amp 240v service on a house. 3cc, no need to derate. My breaker have a terminal rating of 75c. Is this a code violation to use 3/0cu thhn

No. As a matter of fact you are permitted to use 2/0cu conductors per Table 310.15(B)(6).

The way you read Table 310.16 is like this:

FInd your conductor insulation type in one of the three temperature columns. Next, scroll down the table to your conductor size. The ampacity listed is the maximum load that can be applied to that size conductor with that type of insulation without exceeding its temperature.

So for example, If you were to load #6 awg TW conductors to 55 amperes, the temperature of that conductor will be 60 degrees C. 110.14 tells us that no other equipment can have a rating less than that value...

Bryan P. Holland, ECO. Secretary - IAEI Florida Chapter

I have a 200amp 240v service on a house. 3cc, no need to derate. My breaker have a terminal rating of 75c. Is this a code violation to use 3/0cu thhn

As Bryan (BPHgravity) pointed out, it is Code Compliant on that Residential Feeder to use 2/0 THHN cu for a 200 Amp Circuit. This is due to the fact that the Feeder will rarely draw anything close to the 200 Amp level, and extremely rare to draw that much of a load for 3 Hours or more.

Even if the example 200 Amp Circuit / Service Feeder was in a Commercial Environment, if there are only 3 CCCs, the Maximum OCPD for the 3/0s may be selected from the 75º column - or Terminated at 200 Amps.

sparkyinak:

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That was a very good write up. Scott

Thanks!!!

It has been covered here a few times over the years, so the writing gets easier each time

SolarPowered:

*** Per: (if you have questions why 8 CCCs instead of 6, please let me know) ***

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Yes, why? You haven't mentioned that harmonics are an issue, so presumably that's not the reason.

The reason behind this is that most of the time the Common Grounded Conductor will be carrying >= 70% of the highest L-N Load Current value - and more commonly, 100% the highest L-N value.

If the MWBCs are driving Loads which are nearly Pure Resistance, or >.95 PF with very low Harmonic Distortion, then the Common Grounded Conductor will carry a much lower Load Current. This is not too common - especially in my line of work, so I always consider the Common Grounded Conductor to carry 100% of the highest L-N Load value - when applied to MWBCs (4 Wire Multiwire Branch Circuits).

Of course, any 2 wire L-N Circuit does not have a "Common Neutral", only an Ungrounded Conductor + a Grounded Conductor - so each Conductor is carrying 100% Load Current, and figured as such.

Similar results are encountered with 1 Phase 3 Wire MWBCs from a 3 Phase 4 Wire Wye System. An L-L-N Circuit is more like a 3 Phase 3 Wire "Open Wye" Circuit - even though it contains a Grounded Conductor from the Star Point of the Secondary Windings.

These MWBCs typically have the higher Load Current value of one given Ungrounded Conductor flowing on the Grounded Conductor.

I can post more information later.

Scott

Last edited by Scott35; 03/16/0805:53 PM. Reason: inconsistencies

Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!

So the issue was harmonics, even though you didn't originally state it.

Sorry if I mislead anyone. Actually, it may or may not be the results of added harmonic Currents. It is more towards how Linear the Load is - and by "Linear" I am referring to the transferring of True Power, rather than the wave characteristics.

Like an Induction Motor driving an Axial Fan - that would be Linear; whereas an Induction Motor driving a Piston-type Air Compressor would not be very Linear (too many changing amplitudes across the peaks, and valleys which change randomly).

Discharge Lighting is not very Linear, as there will be a considerable level of Capacitive Reactance at the Lamp, and the Lamp's Load will vary greatly with changes in System Voltage + Lamp Temperature & age.

Computers will only be steady when they are at a "stable idle" - meaning there is no Disk Cache work going on, the NIC is not Polling the LAN, and all the "TSRs" are idling. Otherwise the SMPS (Switch-Mode Power Supply) will be drawing transient bursts of input Volt-Amps, which vary in intensity - making the Load very "Non-Linear"

All the above examples do not take into consideration any Harmonic Currents reflected back against the Supply (Transformer's Secondary) from the Loads which created the Distortion. If those Currents are large and Zero Sequencing, the extra Load Currents reflected back will become additive in the Circuit Conductors.

On the subject of Harmonic Reflections (Harmonic Currents), I have only seen excessive Currents with very old Technologies. Back when the first Prototypical "Electronic Ballasts" came into the Market (circa 1988 - 1990), the Hybrid Ballastry first introduced was very noisy. The total Reactance between the reduced size Reactor Coil and the "Pseudo Inductor" IC + Control Circuitry, resulted in a Tank Circuit which reflected a considerably high level of 3rd, 9th and 27th level Currents back against the Transformer's Secondary Windings.

This only happened in a few rare cases, and the issue was resolved by 1991.

Other than this, the "Harmonics Scare" per "Electronic Ballasts" is more propaganda than fact. Sure, there will be some levels of reflected Currents, and there certainly is Harmonic Distortion, but there always has been Harmonic Distortion + reflected Currents with Reactive Loads, so it's nothing new! Motors, Standard "Magnetic" Ballasts, and similar Loads will create Harmonic Distortion, and reflect Currents back against the Supply (Transformer, in most cases).

So, to wrap this up, Loads with Reactance tend to cause the Common Grounded Conductor of a 4 Wire Wye System to carry >=70% of the highest L-N Load Current. Harmonic Distortion may result in additional Current flowing - however, the overall sum is what Currents are Zero Sequenced across whatever Harmonic Frequencies (orders of the fundamental HZ - such as 3rd, 5th, 7th, 9th order).

If the Zero Sequenced Harmonics tend to resemble something Linear, these Currents may balance across the Ungrounded Conductors more than accumulate in the Common Grounded Conductor.

All this stuff is null & void, when applied to a 1 Phase 3 Wire 120/240V System - either from a single center tapped Transformer, or the "Base" Center Tapped Transformer of a 4 Wire Delta System. While there will always be Harmonic Distortion produced by the Reactive Loads, the issues of overloading a Common Grounded Conductor does not occur in the 1Ø 3W 120/240V System.

Scott

Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!