Substation NESC 161.A

[RSChinn]

We just received a new substation switchgear and the control wiring to the feeder breakers (and generally everywhere) is #14awg. The control circuit is protected by a 30A breaker (per feeder breaker mfgr GE). The switchgear mfgr claims he is only obligated to follow ANSI C37.20.2 which says size the wire for the anticipated steady state load and overcurrent is not a consideration. However, our utility follows NESC and para.161.A says that a conductor and its insulation must be protected from overheating by overcurrent, alarms, indication or tripping devices. A #14awg wire cannot be protected by a 30A breaker. Question 1: Is there a grey area here?

Question 2: If the switchgear manufacturer is OK to do this, are we OK to install new circuitry into the switchgear using his requirements only? (We were going to add a completely new annunciator with #18awg wiring and protect it with a 20A panel breaker claiming the switchgear mfgr's logic. I would like to see us protect it with a 10A breaker instead. Our #18awg 90 degree C wire can handle 14A.)

 

[jghrist]

Is the 30A breaker in the switchgear or in a panelboard serving the switchgear? Usually, the internal wiring is protected by fuses in the switchgear and is sized to protect the internal wiring. Circuits feeding the switchgear need to have breakers suitably sized for the circuit conductor (or vice versa).

I would follow NEC ampacity tables for auxiliary power wiring even though you are exempt as a utility. Also, you can't size the breakers for the 90°C insulation rating if the terminals are rated for 75°C (normally the case). The NESC and NEC are installation standards, not equipment standards, and do not cover the manufacturer's internal equipment wiring.

 

[RSChinn]

To jghrist (and anyone else): The 30A breaker is in the DC panel in the switchgear across the aisle from the feeder compartments as it is delivered. The feeder breakers are GE ML-18 that have a coil rated to operate at 17A and the GE fuse size (per the GE breaker spec table) is 30A. GE uses #14 awg and I suspect GE is fabricating the feeder breaker per equipment standards and not installation standards, because that is what the switchgear manufacturer is doing (using #14 awg).

The switchgear manufacturer insists that he does not need to consider overcurrent concerns but will size the wire such that it will handle the steady-state anticipated loading of the wire (per ANSI C37.20.2). There is no further fusing downstream of the 30A breaker. He does run #10 awg from the DC panel to the first terminal point in the feeder compartments and then uses wiring is #14 awg.

However, we are going to add stuff in this switchgear and because we are under NESC, we cannot size breakers less than the wire ampacity I don't think.

We now have a switchgear where some of the circuits have a 30A breaker protecting a #14 awg wire by the switchgear vendor and then our 20A breaker that protects a #12 awg wire (per NESC) because we put it in. Does this seem contradictory?

 

[steelerfan28655]

You would be better served in the event of an incident to have followed the NEC. I work for a utility and even though we do not have to follow the recomendations of the NEC, we do. This gives us the ability to say that even though the minimum standards were all we had to achieve, we went above and beyond the minimum for the sake of safety.

Happiness is a way of travel, not a destination.

 

[apowerengr]

17 amp coil current for 3-5 cycles hardly overloads #14 wire and I would expect you have fuses (or provisions which may be slugged) on the trip, close and motor circuits which more closely protect the conductors in question. The #10 from the DC panel to the termination comparment is sized as much for voltage drop as anything. I don't see that you have an issue.

 

[RSChinn]

TO apowerengr: I agree that the wire used in the GE breaker is OK. Even a #12 awg can handle 300A for 1 second before damage to the wire starts. I am now adding circuits to this switchgear and I do not think I am allowed to follow C37.20.2 like manufacturers/ equipment can. I think I am under NESC and sec 161.A says I must protect conductors and insulation from overheating (not simply for steady-state like C37 says). Question: Can I ignore the NESC and claim C37 in the switchgear since this will be the same as the original basis for manufacturing? (i.e. use a 30A fuse to protect a #14 AWG WIRE??)

 

[dpc]

Factory-wiring inside the gear does not fall under NESC or the NEC. For field wiring outside the gear, we would size per NEC just to avoid any concerns. The cost difference is trivial for a new installation.

It is quite common to see larger field wires and smaller wires inside the equipment for the same current. Once you start dealing with installation codes, you must accept that the results are not always logical or the most cost-effective solution. Conductor sizing requirements in the NEC are only partially based on the physics of current-carrying capability of the conductors.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)