The longer the length the greater the impedance. The fault current is determined by the size of the conductors supplying the fault as well as other factors such as impedance on the wire. Article 250.122(B) requires that we must upsize the EGC proportionally to the upsize in the ungrounded conductors.Ī primary reason for the requirement of Article 250.122(B) is that when you upsize the ungrounded conductors, you increase the available fault current at the same time. But suppose we decide to upsize the ungrounded conductors to #3Thhn because of voltage drop on the circuit. Table 250.122 states the minimum size EGC would be #8 for 70 amps overcurrent devices. For this example let’s say we want to protect the panel at 70 amps so a #8 EGC is required. However, if we protect it at 70 amps, then we would need a #8 awg wire for the EGC. If we protect the panel at 60 amps then we only need a #10 wire for the EGC. So, back to the example, the calculated load is 55 amps and we are allowed to protect the wire at 70 amps using #6Thhn. As long as the final ampacity is not more than the weakest part of the circuit then we are good to go. So what good is 90C wire? It may be used for derating purposes such as excessive heat or conductor fill. Even when we have conductors rated 90C we cannot use the 90C rating of the wire for the final ampacity. Since there are basically no terminals rated 90C we must work with the weakest link in the circuit. Most all of today’s terminals and breakers are rated 75C. It states that if we know the rating of the terminals in the circuit then we may use that rating but if the terminal ratings are not know( old equipment may not have the info available), then we must use 60C for circuits 100 amps or less and 75C for circuits over 100 amps. First we need to look at article 110.14 (C) of the NEC. Let us explore why we can use the 75C rating of #6 Thhn and not the 60C or 90C rating. Article 240.4(B) allows us to use the next higher sized breaker as long as the calculated load does not exceed the 65 amp rating of the wire. Table 310.16 says that for 55 amps we could use #6 THHN copper and we could fuse it at 70 amps since the 75C rating of the wire is 65 amps. We will be using PVC conduit with Thhn wire. Suppose we need to feed a sub panel that will have a calculated load of 55 amps. Upsizing the Equipment Grounding Conductor (EGC) Motor Protection- Voltage Unbalance- Single Phasing by Cooper-BussmanĮxplanation of Range Tables with examples- by Dennis AlwonĬsst Bonding Requirements- by Dennis Alwon Sizing the equipment grounding conductor by Dennis Alwon w/ Mike Holt graphics Why grounds and neutrals are tied only at the main service- By Charlie Beck Navy Electricity & Electronic Training Series Be aware that code rules change and some info could be out of dateīasic Theory and Meter Usage - Just The Cowboy (Harvey)
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