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Marking of Available Short-Circuit Current

Posted By Daniel R. Neeser, Monday, November 01, 2010
Updated: Monday, January 28, 2013

Introduction

In the 2011 National Electrical Code®(NEC) Report on Proposals (ROP) process, two proposals were submitted and accepted that required the marking of the amount of available fault current on service equipment or enclosures that contain service or feeder overcurrent protective devices. The intent of the proposals was to provide a marking of the maximum available fault current to assist owners, designers, engineers, installers, inspectors and others concerned with the proper selection and application of equipment in accordance with the requirements of NEC 110.9 and 110.10. Section 110.9 covers the requirements for interrupting rating (IR) of equipment, such as overcurrent protective devices. Section 110.10 covers the requirements for short-circuit current rating (SCCR) of equipment. One of the proposals also required a date of the fault current calculation to be marked. The other proposal additionally identified the need to have this marking be reviewed and possibly changed if modifications were made to the electrical installation that affected the maximum available fault current at the service equipment. Figure 1 shows an example of a label that was intended to be required.

 

Figure 1. Label with maximum available fault current marking and date of calculation

 


Figure 2. Maximum available fault current marking

New Fault Current Marking Requirement

The modified and ROC accepted wording is shown below:

110.24 Available Fault Current.

(A) Marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved.

(B) Modifications. When modifications to the electrical installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment interrupting ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field markings(s) in (A) above shall be adjusted to reflect the new level of maximum available fault current.

Exception: The field marking requirements in (A) and (B) shall not be required in industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.


Part A of the requirement indicates the maximum available fault current marking is only required to be applied to the service equipment in other than dwelling units. In practice, electrical designers must calculate the fault current at downstream equipment to determine proper interrupting ratings of overcurrent protective devices and short-circuit current ratings of equipment. Figure 2 illustrates the required maximum available fault current marking at the service equipment per the new NEC requirement as well as additional calculations of short-circuit currents downstream of the service equipment. The requirement also indicates that the date of the fault current calculation be identified and durably marked to withstand the environment involved in order to assure the label is legible years after application. By including the date, those involved in the maintenance of the equipment or work at another point in time can verify if it is still accurate. If no system changes have occurred since its marking, then the maximum available fault current has not changed.
 
Figure 3. Circuit breaker and fuses applied above their marked interrupting rating

However, electrical distribution systems do not always remain unchanged, especially if upgrades or expansions are performed. If changes to the system have occurred, it is possible that the maximum available fault current has increased. Part B of the requirement identifies that the calculation of the maximum available fault current be verified or recalculated when changes that affect the maximum available fault current at the service occur. The service equipment is then required to be remarked with the new level of maximum available fault current and the date of determination or calculation. If the maximum available fault current has decreased, then there are no issues with the adequacy of the service equipment or downstream equipment.


Figure 4. Service equipment fault current when transformer is connected

However, if the fault current has increased, it is possible that the service equipment and downstream equipment are not adequate for the new maximum available fault current. In this case, the equipment must be replaced or modified to accommodate the higher fault current in order to prevent a potentially serious safety hazard. This requirement also correlates with the requirements in OSHA 29 CFR 1910 and NFPA 70E. OSHA 1910.303(b)(4) requires compliance with 110.9, and 1910.303(b)(5) requires compliance with 110.10. NFPA 70E 210.5 requires protective devices be maintained to adequately interrupt available fault current. Figure 3 shows examples of what can happen if an overcurrent protective device attempts to clear a fault current above its marked interrupting rating.

There is an exception to this requirement for industrial installations where only qualified persons service the equipment. In these installations, the user must still assure the service equipment is suitable in accordance with 110.9 and 110.10 for the maximum available fault current, but a label with the maximum available fault current at the service equipment is not required. Also, if changes are made that increase the maximum available fault current at the service equipment, the equipment must be verified for suitability and replaced or modified if needed.

How to Comply – New Installations

The service equipment can be supplied directly from the utility grid (network) or through a single transformer, owned by the utility or the user. Where the service equipment is supplied from the utility grid (network), such as in a large city with a downtown utility grid (network) system, the user must contact the utility to identify the maximum available fault current at the service point. If the service point is not at the terminals of the service equipment, the user can perform a quick calculation based on the length of service conductors to identify the maximum available fault current at the service-entrance equipment. If the conductors are larger sized conductors with multiple conductors per phase, the reduction in fault current is often limited.
Figure 5. Transformer calculation with unlimited primary fault current

Cooper Bussmann provides a point-to-point short-circuit calculation method in their electrical protection handbook — Selecting Protective Devices (SPD) — that can be used to determine the maximum (bolted) available fault current at the service equipment. This is available as a free download at www.cooperbussmann.com/spd. There is also a free short-circuit calculation program, based on the point-to-point short-circuit calculation method which can be utilized online or be downloaded athttp://www.cooperbussmann.com/7/SoftwarePrograms.html.

Whether hand calculation or software is used, there are three basic types of calculations:

  • Fault on the secondary of transformer with infinite primary fault current
  • Fault on the secondary of transformer with primary fault current known
  • Fault at the end of a run of conductor or bus

If the service equipment is supplied from a single transformer, the fault current must first be determined at the secondary of the transformer. If the transformer is owned by the utility, the user can contact the utility to identify the maximum available secondary fault current. Then a quick calculation can be performed by the user based on the length of service conductors. If the transformer is owned by the user, the user has two options. The user can either contact the utility for the actual fault current on the primary of the transformer or assume an unlimited primary. Most utilities provide the maximum available fault current values in their regulations or guideline documents. The maximum available fault current must be known to design an electrical installation and install equipment suitable for the amount of available fault current.

Figure 6. Conductor calculation – maximum available fault current at service equipment

An example is shown in figure 4 with the results of the calculations at both the transformer secondary and service equipment. In this case, the user owns the transformer and has elected to assume unlimited primary fault current to achieve the maximum available fault current.

The first calculation to be completed is a transformer calculation. As previously mentioned, unlimited primary fault current is chosen and the maximum available secondary fault current is calculated. A calculation is made using the Cooper Bussmann Short-Circuit Calculator Program as shown in figure 5.

Figure 7. Service equipment fault current increasing due to increase in transformer size (kVA)

After this calculation, another calculation is performed based on the service conductors from the transformer to the service equipment. The Cooper Bussmann Short-Circuit Calculator Program is used again to find the maximum available fault current at the service equipment as shown in figure 6.

How to Comply – Existing Installations

For existing systems, the fault current at the service equipment can increase if the transformer size (kVA) is increased or if the impedance (%Z) is decreased. This is not an uncommon situation due to increased load or providing more energy efficient transformers. See figure 7 for an example of the effect of increasing the transformer size and figure 8 for the effect of decreasing the transformer impedance.

Figure 8. Service equipment fault current increasing due to decrease in transformer impedance (%Z)

In both examples shown in figure 7 and figure 8 the interrupting rating of the overcurrent protective devices and short-circuit current rating of the equipment must be verified. If the interrupting rating and short-circuit current rating initially selected was 200,000 A — as is typical for Class R, J, and L fuses installed in a switchboard — the equipment would still be adequate and all that would need to be done is to apply a new label on the service equipment with the new maximum available fault current. However, if the interrupting rating and short-circuit current rating initially selected was 65,000-A circuit breakers in a switchboard, then the overcurrent protective devices and equipment would not be adequate and would have to be either replaced or modified to accommodate the higher fault current in addition to the new label indicating the maximum available fault current. If modification of the equipment is desired over replacement, the provisions of 240.86 Series Ratings, (A), Selected Under Engineering Supervision in Existing Installations, may be a possible option for the user.

Summary

The new rule only requires a marking that demonstrates compliance with 110.9 and 110.10, something that should already be happening. The new requirement for the marking of maximum available fault current on service equipment will facilitate the awareness and compliance of the existing requirements for interrupting ratings per 110.9 and short-circuit current ratings per 110.10 by electrical designers, installers, and inspectors. In order to comply with this requirement, the maximum available fault current at the service point must be provided by the serving utility or the maximum can be calculated.If the user owns or is informed of the transformer ratings, a conservative calculation can be done that does not require acquiring the maximum available fault current information from the utility. Using this data, another calculation, based on the service conductors, can be performed to find the maximum available fault current at the service equipment. If system modifications have been performed that can increase the maximum available fault current, the service equipment and downstream equipment must be verified to be adequate. If the short-circuit current rating is inadequate, the installation is in violation of the NEC whether the installation is existing or not. The equipment must be replaced or modified to accommodate the higher fault current.

Read more by Daniel R. Nesser

Tags:  Featured  November-December 2010 

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