Overcurrent Device Sizing, Transformers and More

If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to codefaqs@earthlink.net. Answers are based on the 2011 NEC.

Overcurrent device sizing
Why doesn’t the Code permit using the next highest size overcurrent device where the ampacity of the conductor does not match a standard overcurrent device, if the next higher standard rating exceeds 800 amperes (A)?
Overcurrent devices below 800A are available in ratings increments that don’t exceed roughly 18 percent and are generally much less than that. Overcurrent devices rated 800A and above are generally increased in size by a much greater degree to a point where allowing the next higher size is not practical. For instance, the next higher size over 700A is 800A, an increase of 100A. However, the next higher size over 800A is 1,000A, an increase of 200A, and the increments become progressively greater. Check out NEC 240.6.

Secondary sides of transformers
I have a 75 kilovolt-ampere (kVa), three-phase transformer, 480-volt (V) primary, 120/208V secondary. The transformer is being fed from a 600A bus duct system with a 200A bus switch fused at 125A. The transformer is about 20 feet away, and a 100A fusible disconnect fused at 90A was installed at the transformer. The secondary side of the transformer is feeding a 225A circuit breaker panel with a main breaker of 200A. The plant electrical engineer told me that, if the secondary side run is more than 25 feet away, which it is, a secondary disconnect is required at the transformer. Wouldn’t the 200A main breaker in the circuit breaker panel be sufficient? I couldn’t find any articles in the NEC that require a disconnect at the transformer secondary if the run is more than 25 feet away.
NEC 240.21(C)(3) and (6) permit secondary conductors to be connected, without overcurrent protection at the secondary, where the length of the secondary conductors does not exceed 25 feet.

Vending machine receptacles
Are the receptacles installed for vending machines in malls or other public areas required to be protected by ground-fault circuit interrupters (GFCIs)?
NEC 422.51 requires that cord-and-plug-connected vending machines manufactured after Jan. 1, 2005, shall include a GFCI as an integral part of the attachment plug or be located within 12 inches of the attachment plug. Vending machines manufactured prior to Jan. 1, 2005, must be connected to a ground-fault protected outlet.

Dealing with voltage drop
Why do I keep hearing that voltage drop is not required to be considered by the NEC? It’s hard to ignore voltage drop when making an installation.
The NEC is not intended as design specification [90.1(C)] and certainly does not recommend ignoring voltage drop. In fact, in Informational Note No. 4 to Section 210.19(A)(1) and Informational Note No. 2 to Section 215.2(A)(4), it recommends that voltage-drop considerations be made. However, these informational notes are not mandatory and are not enforceable. Voltage drop is a requirement in Section 647.4(D), Sensitive Electronic Equipment, and Section 695.7, Fire Pumps, where sufficient substantiation was provided to establish this as a necessary requirement. If you have a NEC 2011 Handbook (and you should), the basic formula for determining voltage drop is shown in the explanatory material after 215.2(A)(4).

Unspliced Ufer grounds
Does the unspliced Ufer ground have to connect directly to the main service, or can it be connected to a cold water line.
The concrete-encased grounding electrode (Ufer ground) is part of the grounding-electrode system. It can be bonded to the metal underground water pipe or any of the other grounding electrodes that are present to make up the grounding-electrode system (250.52), or it can be run directly to the grounded-neutral bus in the grounded main service panel enclosure or to the ground bus in the main service panel. If you are daisy-chaining the grounding electrodes, ensure the grounding-electrode conductors are sized properly to maintain proper conductor sizing from the electrodes to the grounded neutral in the main service panel.

Type GTO cable
What is GTO cable and where is it shown in the NEC?
Type GTO cable (gas tube sign and ignition cable) is used principally for neon sign installations. Underwriters Laboratories Inc. evaluated this cable in UL 814, Gas-Tube-Sign Cable, and it is listed as GTO-5, GTO-10 and GTO-15 kV to indicate the kilovolt designations of different cable sub-types. The cable generally consists of conductors that use thermoplastic insulation, enclosed in an outer thermoplastic jacket. It is a special-use cable construction and is not listed under NEC Article 310, which lists conductors for general wiring. The only place in the NEC that I’ve seen Type GTO mentioned is in Section 600.32(B).

Paralleling service?
I have a service to install. I calculated it at 495A. According to Table 310.15(B)(16), I will need to use 900 kcmil conductors. I want to parallel this service, so I am using three sets of 300 kcmil. Will the Code allow this?
Yes, the Code will, but I don’t think your budget will. When paralleling conductors, don’t divide the circular mils. Divide the amperage required. In this case, 495A divided by 3 is 165A. You can use 2/0 AWG copper conductors to accomplish your job, or if you want to put them in two raceways, 495 divided by 2 is 247A. You could use 250 kcmil copper conductors in two raceways. There is no direct proportional relationship between amperage and circular mils.

Auxiliary gutter or wireway?
Is the sheet metal trough I am installing under a panelboard to pick up all of the conduits run in the slab considered to be an auxiliary gutter or a wireway?
It is an auxiliary gutter used to supplement wiring spaces at distribution centers (366.2). The construction of auxiliary gutters and wireways is basically the same, but the field application determines the designation and the NEC requirements for use.

What does this mean?
What do they mean when Section 310.15(B)(5)(b) states, “In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, three-phase wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors, etc.” Is this common conductor different than the neutral conductor?
No. They are one and the same. However, in this instance where you are using two-phase conductors and the “neutral” of a 4-wire, three-phase wye-connected system, we can no longer call this conductor “neutral” because it is carrying the unbalanced current. It is never neutral. They are telling you that when you take a single phase, 3-wire circuit from a 4-wire, three-phase wye-connected system, you have a common conductor, not a neutral, and it is the conductor that is intentionally grounded in grounded systems [Section: 310.15(B)(5)(b)].

Motors and systems
Can I install a 230V motor on a 208V system?
Only if the motor has dual ratings marked on its nameplate. Otherwise, the requirements of Section 110.3(a) must be followed regarding suitability, heating effects and classification.

Duplex receptacles
After looking at Section 210.52 carefully, I find that nowhere does it state that I must use duplex receptacles. It only specifies receptacle outlets, and by definition, a single receptacle would be a receptacle outlet. In my next house, I’m going to put single receptacles everywhere. Will this pass the Code?
Sure, go ahead. Let me know if you find another job. This emphasizes what I have been trying to get across. Don’t use the Code as a basis for your design. Incorporate the requirements of the NEC into your design as the minimum requirements for safety. Your design requirements should reflect the needs of the user. I know you’re just funnin’ me, but I’ve heard this one before.

TROUT answers the Code Question of the Day on the NECA website. He can be reached at codefaqs@earthlink.net.

About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

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