Generator Disconnects

[RAYMONDK]

The previous thread for this subject is closed so I guess I'm restarting it. Article 445.18 clearly states that if an installation complies with the two conditions that a disconnect is not necessary. This to me means no disconnect required and all the rules for feeders, SE rating, location, and visibility are moot. Why then is there such a deliberation of this topic. A generator which is connected to a system downstream of a main through a double throw automatic transfer switch (condition 2) with an engine kill switch (condition 1) is safe for any situation that can occur naturally. Additional protection for one who mistrusts automatic controls can be gained by simply removing the starting battery negative cable, which is advised by practically all service literature, or one can remove the battery control circuit fuse which is usually located in the vicinity of the starting battery positive terminal. Every enclosed unit I have ever installed has and E-Stop switch which totally disables the generator instantly. I have installed over 100 generators as the Engineer of record, from 20 KW to 2,000 KW and repaired probably 25 or more and have as yet to ever see a disconnect on a generator installed in this way, as most are. Most have circuit breakers to get around the over-current and short circuit requirement, but that is a useless waste since most generators will not sustain the required current to trip one. A generator will do 300% current for a second then it falls down to a lesser number depending on the engine and exciter system. A breaker needs 125% current to trip long time and a normal generator can't provide it unless the current is reactive, which is not typical. I know I'll get bombarded but as a Mechanical engineer with over 40 years of Electrical experience and many rounds with inspections I'll be interested to hear your thoughts. And Yes, it just popped up again.

 

[don_resqcapt19]

The rule in 445 has nothing to do with the other rules that may require disconnects for the feed from the generator. The scope limits the application of Article 445 to generator itself and has no effect on rules that cover the conductors supplied by the generator.
Can you supply additional details on the circuit and where a disconnect is being required?

 

[RAYMONDK]

I'm not sure I follow your logic. 445.18 specifically describes the feeder circuits fed by the generator. What else would a disconnect be for? The purpose of all the NEC rules is to insure absolute safety for anyone who may come into contact with the subject equipment. If the only device fed by a circuit is cold and the ends of the connection are to an open switch, which can't be closed into an energized circuit, why would anything else matter? Why would it be even reasonable to disconnect dead wires from a dead machine? Article 225 is for outside branch circuits and feeders to outside structures for the supply of power to a piece of equipment or structure. An outdoor generator isn't a structure; its electrically and logically the same as an outdoor motor except that power is not supplied to it. A disconnect is not required there, if the controller has a lockable disconnect at the source. Since the generator is the source, disabling the driver eliminates the need for a disconnect.
The subject generator is a typical enclosed (lockable) 150 KW diesel, 480 V. about 25 feet outside of an elevated (100') water tank that has the base enclosed ( a 60' diameter concrete cylinder) all the way up to the tank. It has an automatic transfer switch after the Main, feeding the entire electrical system and an electronic controller that provides all electrical and mechanical protection on the generator and has an E-stop switch which positively stops the engine and will not allow it to start even if someone jumped the starter. It came with a 250 amp circuit breaker mainly because that is standard and supposedly provides overload and short circuit protection. The installation is physically identical to the last 50 or so I designed except that it's a new (actually very old) client. The site is totally fenced (chain link) with access only to their operations and maintenance personnel.

 

[don_resqcapt19]

I'm not sure I follow your logic. 445.18 specifically describes the feeder circuits fed by the generator.
What else would a disconnect be for? The purpose of all the NEC rules is to insure absolute safety for anyone who may come into contact with the subject equipment. If the only device fed by a circuit is cold and the ends of the connection are to an open switch, which can't be closed into an energized circuit, why would anything else matter? Why would it be even reasonable to disconnect dead wires from a dead machine?

The scope of Article 445 limits the rules in that article to the generator. There are many cases where the code requires a local means of disconnect and this is one of them.

Article 225 is for outside branch circuits and feeders to outside structures for the supply of power to a piece of equipment or structure. An outdoor generator isn't a structure;

There is no question that an outside generator is a structure. See Article 100-Structure.

its electrically and logically the same as an outdoor motor except that power is not supplied to it. A disconnect is not required there, if the controller has a lockable disconnect at the source.

The general rule for motors requires a disconnect within sight of the motor. The exception permits remote lockable disconnects, but the exception is limited to industrial applications or the very rare cases where a local disconnect would create an additional hazard.

Since the generator is the source, disabling the driver eliminates the need for a disconnect.

The section you cited provides for that.

The subject generator is a typical enclosed (lockable) 150 KW diesel, 480 V. about 25 feet outside of an elevated (100') water tank that has the base enclosed ( a 60' diameter concrete cylinder) all the way up to the tank. It has an automatic transfer switch after the Main, feeding the entire electrical system and an electronic controller that provides all electrical and mechanical protection on the generator and has an E-stop switch which positively stops the engine and will not allow it to start even if someone jumped the starter. It came with a 250 amp circuit breaker mainly because that is standard and supposedly provides overload and short circuit protection. The installation is physically identical to the last 50 or so I designed except that it's a new (actually very old) client. The site is totally fenced (chain link) with access only to their operations and maintenance personnel.

The code clearly requires a disconnect at the building that is served by the generator since the generator breaker is inside the enclosure and not visible from the structure that is served by the generator.

 

[ceb58]

The rule in 445 has nothing to do with the other rules that may require disconnects for the feed from the generator. The scope limits the application of Article 445 to generator itself and has no effect on rules that cover the conductors supplied by the generator.
Can you supply additional details on the circuit and where a disconnect is being required?

I agree with Don, look at 702.3 it tells you that "all applicable articles of this code shall apply". 702.11 directs you to 225.36. When you get to 225.36 it then spreads it out to other code articles. In this case you cannot take one specific code article such as 445.18 and apply it to the complete situation.

 

[iwire]

445.18 specifically describes the feeder circuits fed by the generator. What else would a disconnect be for?

The disconnect would be for the building or structure supplied by the generator.

If an emergency responder wants to kill the power to a building or structure it makes sense for that disconnecting means to be at that building or structure.

You have a feeder from a structure (the generator) that is supplying either another structure or building.

Article 225 is for outside branch circuits and feeders to outside structures for the supply of power to a piece of equipment or structure. An outdoor generator isn't a structure;

Actually to the NEC it is without a doubt a structure.

From Article 100.

ARTICLE 100
Definitions

Structure. That which is built or constructed.

I have yet to see a generator that was not built or constructed.


All the rules found in part II of Article 225 apply to generator feeders / branch circuits ........ unless this generator happened to be located inside the building or sturcture served.