Guide de dimensionnement THHN/THWN-2 en conduit

Le dimensionnement THHN/THWN-2 devient délicat dès que le conduit rassemble plusieurs circuits, une longue distance et un environnement chaud ou humide. Un choix rapide en 12 AWG pour 20A peut manquer les limites de borne, le groupement, le remplissage et la chute de tension.

Ce guide s’adresse aux électriciens, ingénieurs et bricoleurs avertis qui utilisent des conducteurs individuels en EMT, PVC ou conduit métallique. Le calculateur aide, mais le résultat doit être confirmé avec NEC 310.16, NEC 110.14(C), NEC 310.15(C)(1), le chapitre 9 et NEC 250.122.

Lors de revues de petits commerces en 2026, la correction fréquente venait du deuxième contrôle: six à neuf conducteurs actifs, un plafond chaud ou 45 m de parcours.

En contexte IEC, la logique se transpose vers IEC 60364-5-52: mode de pose, groupement, température et chute de tension.

Termes clés

• THHN est un conducteur thermoplastique gainé nylon pour conduits secs, souvent doublement marqué THHN/THWN-2.
• THWN-2 est un marquage pour lieu humide avec température d isolation de 90C.
• L ampacité est le courant continu admissible sans dépasser la température du conducteur.
• Le remplissage du conduit mesure la place physique occupée et se vérifie séparément.
• La chute de tension est la perte due à la résistance du conducteur.

Méthode de calcul fiable

Suivez cette séquence dès que le conduit n est pas un petit tronçon simple.

1. Définir le courant et le type de charge; à partir de 3 heures, appliquer 125% selon NEC 210.19(A)(1), 210.20(A), 215.2(A)(1) ou 215.3.
2. Choisir un conducteur de départ dans NEC 310.16; la colonne 90C sert souvent au calcul de correction, pas au calibre final du disjoncteur.
3. Limiter par les bornes selon NEC 110.14(C): 60C ou 75C selon le marquage.
4. Appliquer le facteur de groupement NEC 310.15(C)(1): 80% pour 4 à 6, 70% pour 7 à 9.
5. Corriger la température ambiante en toiture, combles ou locaux chauds.
6. Vérifier le remplissage du conduit avec le chapitre 9 du NEC; 40% est le repère courant au-delà de deux conducteurs.
7. Calculer la chute de tension avec distance aller, courant, matériau et tension.
8. Contrôler le conducteur de protection avec NEC 250.122, y compris 250.122(B) si les phases sont surdimensionnées.
9. Valider le tirage: coudes, lubrifiant, diamètre et accès futur.

Pour un circuit 20A avec six conducteurs actifs, 12 AWG cuivre THHN part de 30A en colonne 90C pour le calcul; 30A x 80% donne 24A, puis la borne et le disjoncteur restent à vérifier.

Scénarios THHN/THWN-2 comparés

Le même disjoncteur peut mener à des sections différentes selon le conduit.

How NEC and IEC Checks Fit Together

The NEC workflow starts with conductor ampacity in Table 310.16 and then modifies that starting point for real installation conditions. The important nuance is that THHN/THWN-2 often has a 90C insulation rating, but equipment terminals may not. NEC 110.14(C) prevents the installer from treating the 90C number as the final ampacity when the termination is rated 60C or 75C. In practical terms, 12 AWG copper may be useful at 30A for derating math, but small-conductor rules and terminal limits still keep the ordinary branch-circuit breaker at 20A.

Conductor-count adjustment is where many conduit jobs drift away from simple charts. If three 20A circuits share a raceway as six current-carrying conductors, the 80% factor matters. If four 2-wire circuits share a raceway as eight current-carrying conductors, the 70% factor matters more. Multiwire branch circuits, neutrals carrying only unbalanced current, and nonlinear loads need careful counting rather than a blanket assumption.

Conduit fill is a physical space rule, not a thermal ampacity rule. NEC Chapter 9 tables limit the percentage of raceway area that conductors occupy. A run can have enough ampacity and still fail fill. It can also pass fill and still be a poor design if the pull has four bends, mixed conductor sizes, and no spare capacity for maintenance.

IEC projects reach similar decisions through IEC 60364-5-52. Instead of AWG names and NEC 310.16 columns, the designer checks conductor cross-sectional area, insulation temperature, installation method, grouping, ambient temperature, protective-device coordination, and voltage drop. That is why AWG-to-mm2 conversion is only a starting point; 12 AWG near 3.31 mm2 is not automatically a substitute for a locally selected 4 mm2 cable under IEC rules.

Je cherche toujours l hypothèse qui a changé: borne 75C, isolant 90C, 8 conducteurs actifs, ambiance 44C ou 55 m de longueur. Un détail peut faire passer de 8 AWG à 6 AWG.

Exemples chiffrés

Confirmez toujours avec le code local et les marquages réels.

Erreurs fréquentes

• Utiliser 90C comme ampacité finale.
• Oublier le groupement au-delà de trois conducteurs actifs.
• Confondre remplissage et ampacité.
• Oublier NEC 250.122(B) après surdimensionnement.
• Employer un conducteur non adapté aux lieux humides.
• Sous-estimer les coudes et le tirage.

Calculateurs utiles

Associez calcul, code et jugement terrain.

Sur 45 m à 120V, la chute de tension peut justifier 10 AWG même si 12 AWG reste légal avec un disjoncteur 20A.

FAQ

À retenir

Le choix THHN/THWN-2 combine charge, bornes, déclassement, remplissage, chute de tension et protection.

Calculez d abord, puis confirmez avec NEC 310.16, 110.14(C), 310.15(C)(1), chapitre 9 et 250.122, ou l équivalent IEC local.

Guide de dimensionnement THHN/THWN-2 en conduit: Field Verification Table

Before you close out guide de dimensionnement thhn/thwn-2 en conduit, it helps to cross-check the same five items that inspectors and experienced installers review in the field: load basis, breaker protection, voltage drop, derating, and grounding or enclosure space. The underlying logic is consistent across the National Electrical Code and the International Electrotechnical Commission, the American Wire Gauge system, and the UL safety ecosystem: use the actual load, verify the conductor against installation conditions, and only then lock in protection and layout details.

“If a circuit will run for 3 hours or more, I treat the 125% continuous-load check as non-negotiable. A 16A design current turning into a 20A conductor decision is exactly the kind of detail that prevents nuisance heat and callbacks.”

“Once branch-circuit voltage drop gets close to 3%, I stop debating and price the next conductor size. Moving from 12 AWG to 10 AWG on a 120V run is usually cheaper than troubleshooting low-voltage performance later.”

“The breaker, phase conductor, and equipment ground are related, but they are not the same calculation. I may upsize a 60A feeder to 4 AWG copper for distance and still keep the grounding conductor at 10 AWG copper because NEC 250.122 keys it to the overcurrent device.”

Guide de dimensionnement THHN/THWN-2 en conduit: Practical Number Checks

The easiest way to keep guide de dimensionnement thhn/thwn-2 en conduit practical is to sanity-check a few common field numbers before you order wire or close walls. On a 120V branch circuit carrying a 16A continuous load, the 125% rule pushes the conductor check to 20A. That is why 12 AWG copper becomes the real starting point instead of 14 AWG, even before you think about distance. If that same run stretches to 110 feet one way, voltage drop often pushes the design to 10 AWG while the breaker stays at 20A because the load has not changed.

The same logic shows up in larger work. A 7.5 HP, 460V three-phase motor with a full-load current around 11A does not mean you can stop at an 11A wire decision. Motor circuits, feeder calculations, and equipment grounding all apply their own code logic, and the conductor selected from ampacity tables still has to survive ambient temperature, rooftop heat, or bundling. That is why experienced electricians compare the load calculation against conductor ampacity, then against raceway or box space, and only then against the final breaker or fuse size.

Residential work needs the same discipline. A box-fill calculation that lands at 24.75 cubic inches on a 12 AWG two-gang box, or a detached garage feeder that picks up 3.6V of drop on a 120V leg, is already telling you the installation is too close to the edge. Use the long-distance wire guide when length is the problem, and cross-check enclosure constraints with the box fill guide or the conduit fill guide. Those second-pass checks are where most field rework gets avoided.

A good field habit is to compare at least two design options before material is ordered. For example, a 240V 32A EV charger on a 140-foot run may look acceptable on 8 AWG copper when you only review ampacity, but the same circuit may justify 6 AWG once you hold voltage drop close to a 3% design target. The same pattern shows up on pump circuits, detached-building feeders, and HVAC condensers. The circuit can be legal at one size and still perform better, start motors more reliably, and leave more inspection margin at the next size up.

Guide de dimensionnement THHN/THWN-2 en conduit: Fast Field Comparison

The table below is not a substitute for the full article calculation, but it is a practical comparison lens for electricians, engineers, and serious DIY users who need a quick reasonableness check before they pull conductors. The numbers show how the design conversation changes once duration, distance, and enclosure limits are reviewed together instead of as isolated problems.

• Short branch circuits usually pass on ampacity alone, but continuous loads above 16A often force the next larger conductor or breaker check under the 125% rule.
• Runs around 100 to 150 feet are where voltage drop starts changing otherwise normal residential and light commercial conductor picks.
• Feeders and service work often pass ampacity first, then fail on grounding, raceway fill, or box-space details if those follow-up checks are skipped.

When those conditions stack together, the cheapest installation is rarely the smallest conductor that barely passes one table. The better choice is usually the conductor that clears ampacity, keeps voltage drop inside the design target, and still leaves room for a normal termination and inspection workflow.

Guide de dimensionnement THHN/THWN-2 en conduit: Frequently Asked Questions

How do I know when guide de dimensionnement thhn/thwn-2 en conduit needs a larger conductor than a simple chart shows?

If the run is long, the load is continuous for 3 hours or more, or the conductors are bundled in hot ambient conditions, the simple chart is only the starting point. A 20A circuit may still need 10 AWG instead of 12 AWG once the 125% rule or a 3% voltage-drop target is applied.

Does the 125% continuous-load rule matter for guide de dimensionnement thhn/thwn-2 en conduit?

Yes, whenever the load is expected to run at maximum current for 3 hours or more. Under NEC 210.19(A)(1) and 215.2(A)(1), a 24A continuous load is treated as 30A for conductor sizing, which is why field calculations often move up one breaker and wire size from the first rough estimate.

What voltage-drop target is practical when planning guide de dimensionnement thhn/thwn-2 en conduit?

The common design target is about 3% on a branch circuit and 5% total for feeder plus branch circuit. That is not a mandatory blanket rule in every NEC application, but it is the benchmark many electricians use to decide when a 100-foot to 200-foot run should be upsized.

Can I upsize wire without increasing breaker size for guide de dimensionnement thhn/thwn-2 en conduit?

Yes. Upsizing for voltage drop or future durability does not automatically require a larger breaker. A common example is a 20A circuit that moves from 12 AWG to 10 AWG copper on a long run while the breaker remains 20A because the load and overcurrent protection have not changed.

Which code checks should I finish before calling guide de dimensionnement thhn/thwn-2 en conduit complete?

At minimum, verify conductor ampacity in NEC Table 310.16, breaker protection in NEC 240.4 and 240.6, voltage drop design assumptions, grounding in NEC 250.122, and enclosure or raceway space in NEC 314.16 or Chapter 9. For international work, align the same review with IEC-style conductor and protection practices.

When should I move from a chart lookup to a full calculation for guide de dimensionnement thhn/thwn-2 en conduit?

Move to a full calculation whenever the run exceeds roughly 75 to 100 feet, the load is motor-driven, the circuit is expected to operate for 3 hours or more, or the conductors share a hot raceway with more than three current-carrying conductors. Those are the situations where a simple chart is most likely to miss a required upsizing step.

What is the most common inspection failure tied to guide de dimensionnement thhn/thwn-2 en conduit?

The most common failures are not dramatic math mistakes. They are incomplete checks: a conductor that passes NEC Table 310.16 but ignores a 75 C termination, a long run that misses a 3% branch-circuit design review, or a feeder that works electrically but lands in an undersized box or raceway. Most red tags happen when one of those second-pass checks is skipped.

Next Steps

If you want to validate this topic against real project numbers, start with the wire gauge calculator, then cross-check longer runs in the voltage drop calculator, and verify conductor adjustments with the ampacity calculator. If you want us to add another worked example or application note, contact us here.