Charlie Trout, author of Code FAQs and Code Question of the Day, has retired. For the rest of 2013, enjoy these snippets from his daily responses.
Calculating conductors and breakers
We are about to wire a recreational vehicle campsite. The designing electrical engineer set up the loads using 2 AWG direct burial aluminum wire fed from a 90-ampere (A) breaker, which the electrical inspector specified. The pedestal is equipped with either 50- or 30A receptacles, including the proper breakers. The pedestals are fed in one and then out to the next, daisy-chained. A typical layout may feed into a 50–30 combination then out to a 30–30 combination using 2 AWG wire. The engineer used 4 AWG aluminum wire to feed the last 30–30 pedestal. The voltage drop calculations work out fine using 4 AWG wire. The electrical inspector says we have to use 2 AWG wire throughout because we are feeding the runs from a 90A breaker. I believe feeder conductors have to be big enough to service the calculated load and not be sized to the feeder breaker. I believe the 4 AWG conductor should be allowed. Am I wrong? Additionally, does the feeder breaker have to be 90A, or is a 100A breaker OK.
The overcurrent device is there to protect the circuit or feeder conductors. The designer used the 75-degree column of Table 310.15(B)(16) and used a 2 AWG aluminum conductor rated at 90A protected by a 90A rated circuit breaker. You appear to be suggesting that a 4 AWG aluminum conductor, which is rated at 65A, should be allowed when protected by a 90A circuit breaker. The feeder conductors must be big enough to service the calculated load, but the circuit breaker must be sized to properly protect the conductors. The electrical inspector is correct, and the 4 AWG aluminum conductor cannot be protected by a 90A rated circuit breaker. A 2 AWG aluminum conductor must be used throughout to match the 90A breaker. A 100A circuit breaker will not properly protect a 4 AWG aluminum conductor.
GFCI protection at in-ground swimming pool
An in-ground swimming pool with a spa, light, filter motor, circulating motor and disinfectant light is served by a computer- controlled subpanel (i.e., compool) with three two-pole 20A circuit breakers and two one-pole 20A circuit breakers. None of these are ground-fault circuit interrupters (GFCIs). GFCI protection is from the main panel by way of a 50A two-pole GFCI, thereby protecting the compool subpanel and associated overcurrent protection devices (OCPDs). Is this type of protection applicable due to the fact a two-pole 50A GFCI will trip between 4 to 6 milliamps (mA) as a single-pole GFCI breaker would trip between 4 to 6 mA?
The installation as described appears to meet the intent and GFCI protection requirements. The Code actually requires the branch circuits [680.21(C) and 680.22(A)(4)] to have GFCI protection, but a feeder protected by a two-pole, 50A Class A GFCI device provides the GFCI protection for all branch circuits in the panel supplying the equipment.
But it’s not a service
My question deals with replacing a panel that is fed from a 3-wire feeder in underground rigid conduit. The conduit run is longer than 100 feet, and the conduit has been buried so long it has begun to rust. Pulling new wires is not possible due to the condition of the raceway. Do I isolate the grounded conductor in the new panel and hope the rigid conduit provides an effective ground-fault path, or should I install a bonding jumper in the new panel and let the grounded conductor clear any faults? I realize the latter would possibly create a parallel path if the integrity of the conduit is adequate; however, it is perfectly legal to do so regarding services, and I would feel better knowing a fault would clear without question.
You say your intended installation is perfectly legal to do regarding services, but your installation is not a service. Installing a bonding jumper between the grounded conductor (neutral) and a grounding conductor on the load side of the service disconnecting means is prohibited by 250.142(B). This would, as you say, create a parallel path for neutral current and create a potentially dangerous condition. There is a proper repair for this defect, and it should be used.
Sections: 680.21(C), 680.22(A)(4)
Emergency circuits in healthcare
I am doing some work in a hospital. When lighting fixtures are installed in a suspended grid ceiling and connected to an emergency (generator) circuit, is it a violation to wire them with HCF-90 cable or flexible metallic conduit? I heard the entire circuit must remain in nonflexible metallic conduit, which would make installation difficult. Does the Code allow any exceptions for offices and corridors as opposed to patient care areas?
Emergency circuits in healthcare facilities must have mechanical protection in accordance with 517.30(C)(3)(3). Installation of metallic conduit cable has to meet these requirements in list item (3) as acceptable to the authority having jurisdiction. The key is that flexibility is necessary for the type of equipment being connected. Based on the question, this type of lighting could be wired using a conduit or electrical metallic tubing system, and flexibility is not required after installation. There are no exceptions in emergency mode.
Sections: 250.142(B), 250.24(A)(5)
Parallel conductors installed in multiple raceways
My application is a 2,000-kilowatt (kW), 480Y/277 generator with an integral 4,000/3 circuit breaker. There will be 11 sets of individual cables, 4/0 AWG Type W, between the portable generator and transfer switch. The transfer switch will pick up the entire building load (i.e., premise and equipment). I am trying to determine the size and number of the ground conductor(s) required between a portable generator and the transfer switch. Can you help me out please?
Section 250.122(F) requires, where conductors are installed in parallel in multiple raceways, the equipment grounding conductors shall be installed in parallel in each raceway. Note the last sentence of 250.122(F): “Each equipment grounding conductor shall be sized in compliance with 250.122.” The equipment grounding conductors required for this installation in accordance with Table 250.122 are 400 kcmil copper.
Grounds and neutrals
If a 200A generator transfer switch with a 200A breaker is installed outside between a residential meter and the existing inside main panel, can the existing 3-wire SEU cable be reused? The grounds and neutrals would stay connected in the main panel. Or does this 3-wire cable have to be changed to a 4-wire, and do the grounds and neutrals in the main panel have to be separated?
Based on the information provided in the question, the transfer equipment needs to be SUSE rated (suitable for use as service equipment) and contain a service disconnect. The feeder from that transfer equipment/service disconnect needs to include an insulated neutral and separate equipment grounding conductor. The existing service equipment panelboard on the dwelling becomes a panelboard on the load side of the service disconnecting means. You are required to separate the neutrals and equipment grounding conductors at that location. Check out these applicable Code sections: 250.24(A)(5) Load Side Grounding Connections; 250.142(B) Load Side Equipment; 702.5 Transfer Equipment; 408.40 Grounding of Panelboards; and 338.10(B)(2) Uses Permitted; 338.10(B)(2) Uses of Uninsulated Conductor.
Sections: 250.24(A)(5), 338.10(B)(2), 408.40
Locknuts and bonding bushings
Are standard rigid locknuts suitable for bonding when used with a conduit nipple installed between a meter socket and the service disconnecting means enclosure? There are bonding bushings installed on both ends of the nipple with a bonding jumper connected to each enclosure.
This is fairly straightforward, and it seems you have answered your own question. First, standard locknuts, as a general rule, are not a suitable method of bonding in the line side of the service disconnect. Second, based on the information provided in the question, it sounds like bonding bushings are installed on each end of the conduit between enclosures, which would negate the need for bonding locknuts. Bonding locknuts is one method for bonding on the supply side of the service, and they must be used when no other method of bonding is installed. Since bonding bushings are installed, the use of standard locknuts is acceptable.
Sections: 250.92(A), 250.92(B)(4)
Sizing the grounded service conductor
Can the grounded (neutral) conductor be smaller than the ungrounded service conductors when it is supplied a three-phase, 3-wire, corner-grounded delta system?
In a corner-grounded, three-phase, 3-wire system, there is no neutral. I’ll answer the question on that basis. The answer is the grounded service conductor for this system must be the same size as the largest ungrounded phase conductor. Remember that the grounded conductor must also meet the identification requirements in 200.6, and overcurrent protection is not permitted in accordance with 240.22.