Print Page   |   Contact Us   |   Sign In   |   Join
IAEI Featured Articles
Blog Home All Blogs
Search all posts for:   

 

View all (16) posts »
 

Article 240, Part 2 — Overcurrent Protection

Posted By Randy Hunter, Friday, March 01, 2013
Updated: Wednesday, February 13, 2013

In Part One of Article 240, we left off with a basic review of circuit protection. Now that we have an understanding of how it operates, let’s get back to Part II of Article 240 which starts at 240.21 Location in Circuit. Here we find a very simple statement which requires the overcurrent protection to be located at the point where ungrounded conductors receive their supply. As simple as this is, we naturally have exceptions which will allow taps to be made under certain conditions. For this section, a tap is a conductor which is connected to a system; however, it is not sized to handle the ampacity of the upstream overcurrent protective device. We will cover the limited allowable conditions. But first, the last sentence of 240.21 needs to be emphasized; it explains that under the tap allowances in 240.21(A) through (H), you may not have an additional tap made to any of these allowed taps, or as I used to tell inspectors, "You can’t tap a tap.”

Feeder Taps Rules

In 240.21(A) Branch-Circuit Conductors, we find a method of performing taps for branch circuits, but note that you are referred back to Article 210.19 and 210.20 for the exact applications. In subsection (B) we start to deal with the Feeder Taps, which are further broken down to five conditions. I’m not going into details on each of these sections, just a brief overview of the different rules; so again, I will challenge you to open your code book and review the specific conditions for each of these allowances. First, we have a tap rule not over 10 feet in length. Please note that when we have a length in this section it is the actual length of the conductor from connection point to connection point. The 10-foot rule has four conditions which must be met. Note that if these conductors leave the enclosure from which they are tapped, the conductors must have the minimum ampacity of 1/10th of the protective device from which the tap is being made.

Photo 1

The second tap rule is for a 25-foot long tap, and in my experience this is the most common tap rule that I have seen applied. Here the intent is to have the conductors extend from the tap location to another piece of equipment. There are three simple rules: the ampacity of the conductors is at least 1/3 of the protective device ahead of the tap; the conductors terminate in a single overcurrent device that limits the load they can carry; and the conductors are protected in an approved raceway or other means. As I mentioned, this is the most common tap used in my experience. It is commonly used in a case where the equipment has a load that is not close to its rating, but there is no more room for sub-feed devices. We had a situation years ago in a commercial laundry where a 1200-amp switchboard fed several pieces of equipment, and they needed to install some additional washers. The actual load on the 1200-amp board was only about 700 amps, but there was no remaining space for additional overcurrent units. The solution was to use the 25-foot tap rule and install a panelboard with a main breaker. The trick was that there was a door right next to the 1200-amp unit, so by the time we made the tap, ran the conduit, and set the new panel on the other side of the door, the 25-foot rule barely made it.

The third tap rule deals with transformer taps which include the primary and secondary conductors in the 25-foot allowance. The fourth rule provides an allowance for taps over 25 feet when applied in high bay manufacturing buildings where the walls are 35 feet tall, which would make overhead taps impossible under the other rules when the distance from the tap to a piece of equipment is over 25 feet. Again there are specific conditions to follow for each rule, so please review them. The last feeder tap rule is for outside taps, while the taps covered so far are assumed to be inside facilities. However, if the taps are located outside of a building or structure, the tap can be unlimited in length. Please read the details in 240.21(B)(5).

Photo 2. In this condition, if the feeders to this transformer were taps then according to 240.21(B)(3) the length of the transformer feeders and the secondaries together shall not be over 25 feet in length.

Rules for Protection

In 240.21(C) through (H), rules for protection of transformer secondaries, service conductors, busway taps, motor circuits, generators and battery conductors are covered. In keeping with the combination inspector direction of these articles, I have just detailed the most common cases. I’ve never tried to memorize any of the tap rules, as there are simply too many details and conditions, so please remember to utilize the code book and check each installation you are inspecting.

Locations and Accessibility

Next we will jump to 240.24 Location in or on Premises. We will hit the highlights here, the first being that the overcurrent devices have to be readily accessible and shall be installed so that the center of the grip of the operating handle is not more than 6 feet 7 inches in its highest position from the floor or working platform. This is an enforcement issue and is often caught in the field.

In (B) we discover that each occupant shall have ready access to the overcurrent devices protecting the conductors supplying that occupancy, and this applies to both residential and commercial. We have a couple of conditions which may modify this. One is a facility that has continuous electrical building maintenance and supervision, and this may apply to various facilities such as guest rooms or similar locations. The overcurrent devices shall be located where they will not be subject to physical damage. This may be a judgment call, but one that comes to mind is storage facilities which set their electrical equipment on the outside of the storage buildings where they are subject to damage by various vehicles. Here we usually use some type of bollard to provide the protection needed.

The last three conditions for locations requires the locations not to be near ignitible materials, not to be located in bathrooms of dwelling units, dormitories, guest rooms or guest suites, and the last condition to avoid is over steps or stairs. The thought here is that we want to provide a stable and level working area for anyone who may have to service or operate the overcurrent devices.

Photo 3. Here is an example of providing physical protection to this equipment by the use of bollard posts.

Plug Fuses, Fuseholders and Adapters

In Part V of Article 240 we have requirements for Plug Fuses, Fuseholders, and Adapters. We will cover this since many of the combination inspectors working in parts of the country that have older facilities may still find in-use plug fuses, more commonly referred to as screw-in fuses. First is a limitation that these will only be used on circuits not exceeding 125 volts between conductors or on a system having a neutral point where the line-to-neutral voltage doesn’t exceed 150 volts. This really applies only to branch circuits. Each fuse and, if used, adapter will be marked with its ampere rating. Further, if 15 amps or lower, they will be identified by a hexagonal configuration of the window or other prominent part to allow them to be identified from other fuses of higher ratings.

Next, we need to visit some screw-in fuse basics. First, they shall have no exposed energized parts after the fuses have been installed, and second, the screw shell they screw into shall be connected to the load side of the circuit. With this, let’s consider this further. If the screw shell was connected to the line side of a circuit and you screwed in a fuse, if your fingers contacted the outer portion of the fuse you would get shocked. So this is a critical requirement to make sure the screw shell is not energized until the fuse has been completely installed.

Photo 4. Edison-base fuses are shown in the bottom four examples; please note the hexagonal configuration of the 15 amp units. The top fuses are examples of Type S fuses with the appropriate adapter shown in the middle; note the spring tang which prevents the adapter from being unscrewed once installed.

Edison-base fuses are covered in 240.51; however, these are only to be used for replacements in existing installations where there is no evidence of overfusing or tampering. This is because Edison-base fuses are totally interchangeable, so you can replace a 15-amp fuse with a 20-amp fuse or a 30-amp fuse. That interchangeability leads to applications where misapplications may occur due to availability issues or ignorance of proper circuit protection.

So to solve this issue, Type S Fuses were invented to limit the size of the fuse that can be installed through unique physical dimensions to the screw portion of the device. This is covered in 240.53 and they have three ranges 0–15, 16–20 and then 21–30 amps. This makes it so they are not interchangeable; and if you have a circuit which is using 20-amp conductors, you can’t install a 30-amp fuse. Now to follow up with this additional safety provision, they made adaptors for Type S Fuses so they could be installed in standard Edison-base fuse holders. These adapters have a spring tang on them so that once an adapter has been installed it can’t be removed, thus providing the same limitations and additional safety of the Type S fuses in older installations. These adapters are covered in Article 240.54.

Photo 5. These are renewable fuses — on top is the interior of a 200-amp fuse and below is a 20-amp fuse, as mentioned in 240.60(D).

Cartridge Fuses and Fuseholders

In Part VI we cover Cartridge Fuses and Fuseholders. In 240.60 we have general rules for cartridge fuses which cover the voltage limitations and the need for unique sizing to make it difficult to install a fuse into a fuse holder which is designed for a lower current. Also, the marking requirements are covered, which should help both the contractors and inspectors to verify we have the proper fuse for the application.

Renewable fuses are cartridge fuses which disassemble and allow one to replace the internal fuse element. As I travel around, I often ask if anyone has seen these and only those who have been in the field for some time have usually heard of them and even fewer have seen them. The last item to cover in the fuse area is that fuses that are rated 600 volts nominal or less shall be permitted to be used for voltages at or below their ratings.

Circuit Breakers

Coming down to the end of overcurrent devices, we step into the circuit breakers in Part VII. Circuit breakers come in many sizes and options, and they have the ability to be reset once they are tripped if they have been installed in the proper application. Breakers shall be capable of being opened manually, and they shall clearly indicate whether they are in the open (off) or closed (on) position. If the handles operate in the vertical, then the "up” position of the handle shall be the "on” position. This is sometimes a challenge when doing service work, because some breakers have various orientations which can be used that may not provide the "on” position to be up. Breakers also have a requirement that they are not subject to tampering, or unauthorized alteration of the factory calibration, unless intended for adjustment.

Marking of breakers must show the amperage rating in a manner that will be durable and visible after installation. In some cases the marking may be under a trim or cover; however, generally the amperage is listed on the operating handle. Units made for 100 amps or less shall have this marking on the handle or escutcheon areas. The interrupting rating of breakers shall be marked on the breaker. It might be referred to as AIC (amps interrupting capacity) or IR (interrupting rating). If it is not marked, then it is assumed the breaker is only listed to be used in an installation which has an available fault current of 5000 amperes or less. This is a reminder to make sure the overcurrent devices used are listed for the available fault current at that point of the distribution system.

If a breaker is to be used as a switch, then it has to listed for this usage and the marking will either be a SWD (switched) or HID (high-intensity discharge). These applications usually happen in locations such as warehouses or small retail shops. So if the only method of turning on the lights is the breaker, then it would make sense that you have to enforce the switch rating requirement. If they are utilizing high-intensity discharge lighting, the breakers have to be marked with the HID rating.

Applications in 240.85, reminds us that we have to use breakers that are compatible with the system voltages in each installation. Please see this section of the code regarding the use of straight rated breakers and slash rated breakers. Most of us deal with solidly grounded systems, which require slash rated (120/240V or 480Y/277V) breakers.

Breaker Series Ratings

Breaker Series Ratings is found in 240.86, which allows breakers to be installed under some very special conditions. Series rating is a way to install breakers which will rely on the upstream device to protect it in the case of a high-fault current. This allows breakers that are not rated for the available fault current to be utilized only in two conditions.

One is under engineering supervision in existing installations, which at times happens when work is being done to older systems. Here the engineer who is qualified may apply a series rating if the system is properly documented and field marked to identify it is a series system.

The second is under the condition of 240.86(B), Tested Combinations, that allows series rating if the combination of devices have been tested and certified to operate together to provide the protection of the system from the available fault current. This is often done to lower the cost of the equipment package. This requires that the system be identified as a series system and the documentation is maintained on-site for the inspector and future electricians who may have to service the installation. This is a very complicated item because when it is used and any additions or modifications are made, the original requirements of the series rating still apply and must be followed.

In my jurisdiction, we had a strip mall constructed using a series rated system using brand X gear. As each individual suite had their tenant improvements done, each contractor performing the work for the new tenant had to match the exact requirements and brand of the series system. One contractor who installed brand Y sub-panels learned the hard way and had to change out two panels to keep the system installed within its series rating. If a system has a motor load connected on the load side of the higher rated device and on the line side of the lower rated device, or if the motor load is more than 1 percent of the rating of the lower rated device then a series system is not allowed to be used. So if you have a series rated system, review the code and insure the conditions are met.

Noninstantaneous Trip

The last subject to cover in this article on overcurrent protection is fairly new to the code. In 240.87, Noninstantaneous Trip, a circuit breaker may be used that doesn’t have an instantaneous trip feature or has that feature turned off. The instantaneous portion of an overcurrent device gives the device the ability to react as fast as possible to a short-circuit condition within the system. The instantaneous portion of the overcurrent devices also reduces the incident energy at a point of fault within the system.

You might ask why would we do this; in cases where breakers are used to selectively coordinate a system, the upstream device may have the instantaneous set higher or turned off so the downstream device has the time needed to operate. So as the upstream device waits for the downstream unit to do its job, the current will not be limited and will continue to flow for an extended period of time increasing the incident energy at the fault location. Under these conditions we must provide one of three equivalent means, (1) zone-selective interlocking, (2) differential relaying, or (3) energy-reducing maintenance switching with local status indicator. The purpose of these three options is to reduce the stress on the equipment but most of all for the protection of personnel who may be working on the system.

These requirements are unique to breakers and do not apply to fused systems. From my experience when we dealt with systems like this, we asked for third party verification that the electrical system was installed properly and that any adjustments to breakers were done properly and coordinated to function together. After the third party report was done, the engineer of record would review it and then submit it to the AHJ for inclusion in the approved set of plans for the project.

This concludes Article 240. I hope you have had your code book open as we continued, due to the fact that I cover only the issues which I feel you need to have a basic knowledge of as a combination inspector. It is always good for you to review the portions I skipped; this will help you when a situation may occur in your area and you will recall that you heard or read something about that, and you’ll be able to find it that much easier in the code book.


Read more by Randy Hunter

Tags:  Featured  March-April 2013 

Share |
Permalink | Comments (0)