NEC Questions Based on the 2008 NEC
February – Part 2
Here is the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent yesterday so you can see how you did.
By Mike Holt for EC&M Magazine
Q1. I'm selling my 30 year old home and the home inspector has asked me to replace the existing breakers with AFCI's in order to maintain code compliance. Is it required by law to update a structure to make it compliant with new codes?
Q2. Can we use flexible metal conduit for the emergency lights in a hospital.
A2. Generally speaking, only nonflexible metal raceways or Schedule 80 PVC conduit can be used for the mechanical protection of emergency system in a hospital [517.30(C)(3)]. Flexible metal conduit can be used, however if it is fished into an existing wall and is not subject to physical damage [517.30(C)(3)(c)].
Q3. We have a large electrical service that consists of twelve raceways in parallel. Can I use one conductor to bond them all, and if so, how do I size it?
A3. No, you may not use one conductor to bond multiple service raceways. Equipment bonding jumpers for service raceways are sized in accordance with Table 250.66, based on the largest ungrounded conductor within the raceway or cable. Where service conductors are paralleled in two or more raceways or cables, the bonding jumper for each raceway or cable must be sized in accordance with Table 250.66, based on the largest ungrounded conductors within the raceway or cable.
Example: What size equipment bonding jumper is required for each metal raceway containing 700 kcmil service conductors?
Answer: (c) 2/0 AWG [Table 250.66]
Q4. If I have 25 current carrying conductors in a wireway, do I have to derate them?
A4. No, only when more than 30 current-carrying conductors are installed in any cross-sectional area of the wireway, does the conductor ampacity, as listed in Table 310.16, need to be adjusted in accordance with Table 310.15(B)(2)(a) [376.22(B)]. Please note that signaling and motor control conductors between a motor and its starter used only for starting duty aren’t considered current carrying for conductor ampacity adjustment.
Q5. We recently sized a circuit to a commercial oven. The manufacturer says the total load per phase is 58 amps single phase 240V and it’s not capable of being used continuously. What size conductor and protection can I use?
A5. The overcurrent rule is as follows: For nonmotor operated appliances, the appliance overcurrent device must not exceed 150 percent of the appliance rated current if the overcurrent device rating isn’t marked, and the appliance is rated over 13.30A. Where 150 percent of the appliance rating doesn’t correspond to a standard overcurrent device ampere rating listed in 240.6(A), the next higher standard rating is permitted.
Protection Size: 58A x 150% = 87A, 90A next size up [240.6(A)].
Conductors are sized no less than 125 percent of the continuous loads, plus 100 percent of the noncontinuous loads, based on the terminal temperature rating ampacities as listed in Table 310.16, before any ampacity adjustment [110.14(C) and 210.19(A)(1)].
Conductor size: 4AWG, rated 85A at 75°C [Table 310.16].
Q6. In an industrial building, the old panels do not have complete panel schedules. Does the code require us to update them?
A6. The circuits should have been identified when they were installed, in addition circuits that are modified must be legibly identified as to their clear, evident, and specific purpose [408.3].
Identification must include sufficient detail to allow each circuit to be distinguished from all others, and the identification must be on a circuit directory located on the face or inside of the door of the panelboard.
Q7. We have a 400A three phase feeder that we want to wire with parallel 3/0 AWG conductors. Can we install them in the same raceway?
A7. Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor for the purpose of conductor ampacity adjustment, in accordance with Table 310.15(B)(2)(a) [310.4(D)].
Example if neutral is considered a current carrying conductor: What is the adjusted ampacity of 3/0 THHN/THWN-2 conductors if the raceway contains a total of eight current-carrying conductors? Figure 310–17
Ampacity 3/0 THHN is 225A [Table 310.16 at 90C)
Adjustment Factor [Table 310.15(B)(2)(a)] = 0.70
Adjusted Ampacity = 225A x 0.70
Adjusted Ampacity = 158A
For this scenario you could not use 3/0 THHN in parallel in the same raceway because the combine ampacity is only 316A (158A x 2), which is not permitted to be protected by a 400A overcurrent device [240.4].
Example if neutral is NOT considered a current carrying conductor: What is the adjusted ampacity of 3/0 THHN/THWN-2 conductors if the raceway contains a total of eight current-carrying conductors? Figure 310–17
Ampacity 3/0 THHN is 225A [Table 310.16 at 90C)
Adjustment Factor [Table 310.15(B)(2)(a)] = 0.80
Adjusted Ampacity = 225A x 0.80
Adjusted Ampacity = 180A
For this scenario you could use 3/0 THHN in parallel in the same raceway because the combine ampacity is 360A (180A x 2), which is permitted to be protected by a 400A overcurrent device [240.4(B)].
Q8. Do the metal studs of a building need to be bonded or grounded to anything?
A8. No, however “exposed structural metal that forms a metal building frame that is likely to become energized” must be bonded to the service equipment enclosure, service neutral conductor, grounding electrode conductor of sufficient size, or grounding electrode system [250.104(C)].
Q9. I am hearing differing opinions about bonding steel gas piping. What are the rules for this?
A9. Metal piping systems such as gas or air that are likely to become energized must be bonded, but they can be bonded to the equipment grounding conductor for the circuit that may energize the piping [250.104(B)].
Q10. We have transformer with 8 parallel 500kcmil phase and neutral conductors on the secondary side. We connected the transformer XO to the case correctly, but we are having an argument about how to size the grounding conductors inside each of those raceways.
A10. An equipment bonding jumper must be run to the secondary system disconnecting means, and where the secondary equipment grounding conductor is of the wire type, it must be sized in accordance with Table 250.66, based on the area of the largest ungrounded secondary conductor in each raceway or cable [250.30(A)(2) and 250.102(C)].
Example: What size equipment bonding jumper is required for each flexible metal conduit containing 500 kcmil secondary conductors?
Answer: (d) 2 AWG [Table 250.66]
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