With the popularity of Solar, battery backup systems, and additions to commercial and residential properties, line-side and load-side taps to existing switchgear have become common. Tapping into existing switchgear is considered a modification of the original UL listed product. Because such a modification could invalidate the original UL listing, city inspection agencies and utilities alike are requiring third-party approval or re-certification of modified switchgear.
In order to pull off a successful switchgear tap, there could be three or more parties having requirements, all of which need to be satisfied in order to operate the new system. The main three parties are: the local building department, the utility, and the field evaluation body. Finding where requirements from all parties are clarified can be difficult. This blog is being written as a three-part series to clarify these requirements and hopefully help avoid multiple utility shut-downs and/or failed attempts to satisfy all parties.
As a Field Evaluation Body (FEB), we will start this series with the equipment safety requirements. The main purpose of a field evaluation body here is to ascertain whether modifications made comply with the UL standard for switchboards (UL 891). Here is a list of common items that we look for when evaluating bus taps:
Clearance Distances and Wire Routing:
An air gap must be maintained from the new taps to:
- Bare parts of opposite polarity
- Chassis
As a rule of thumb, plan on a minimum of 1” spacing. Distance requirements could be a little more or less, but in most cases 1 inch will suffice and can typically be achieved given some planning.
DO – check the spacing distance from the tap to the dead front cover. This distance can sometimes be forgotten as the work is generally done with the cover removed.
DO – preplan the layout of the taps given the space to work with before scheduling a shut-down. Routing of PV conductors from the penetration point to the entrance angle of lugs should allow for sufficient conductor bending space and separation of conductors from bare live parts.
DON’T – allow turning or rotation of lugs or bus flags 30° in any direction to reduce spacings less than the minimum.
Tap Hardware:
When fastening taps to existing bussing we highly recommend using standard hardware:
- Grade 5 or 8 bolts, and
- Conical (Belleville) washers
- Lugs (UL listed for CU and AL)
Other hardware may also be acceptable, but during the installation power is generally shut down to the building and time is of the essence. This isn’t an opportune time to ponder if other methods of connection will be acceptable.
DON’T – If possible, use split-ring lock washers in place of conical (Bellville) washers
DO – Use existing bus holes to connect the tap when possible
Depending on the installation, you may also need:
- Bus Flags
- Barriers
Make sure these items are purchased from a commercial electrical supply distributor. These are not parts to be looking for at Home Depot.
Tap Connection Point:
The surface contact area of any current-carrying joint should be at least 200A/in2. The amount of potential current in a tap joint will be based on the fuse size in the disconnect ahead of the tap. For mechanical (chair) lugs this contact surface area may be less than 200A/ in2. Here is how to determine if your chair lug is making adequate contact with bussing: The contact area shall not be less than the area that would result when two of the same type connectors are bolted together back-to-back with wires leaving in opposite directions or at right angles, whichever results in the smallest area.
DO – Use lugs with larger tangs to gain additional contact area if necessary
Neutral Disconnect Link:
In accordance with UL 891, the neutral disconnect link shall be located on the load side of all neutral grounding connections. The disconnect link is intended to separate the ungrounded neutral portion supplying the facility from the grounded portion coming from the utility. When tying in solar or other parallel power sources the neutrals coming from the source inverters are typically grounded at the source. Since these neutrals have a ground reference they should be tapped on the line-side of the neutral disconnect link (if there is one) in the existing switchgear. This way disconnecting the link will still separate the grounded side from the un-grounded side. This link comes in handy when testing for ground-fault issues.
Torque Values:
The hardware used for the tap must be torqued to the switchgear manufacturer’s specification. Some torque charts can be confusing. For bus taps, the torque values that we are looking to confirm are connections from the lug to the bus or bus flag to bus. In addition, when using mechanical lugs, the torque of the lug screw securing the conductor must also be verified. The bus connection point utilizes bolts or screws, and the torque values are based on bolt/screw diameter. The conductor connection point for mechanical (Chair) lugs is based on the hex wrench size that fits the lug screw. The following values are generally accepted by switchgear manufacturer’s and can be used if the gear is not marked to indicate torque values:
Bolt Connections
| Bolt Diameter (inches) | Torque Values (ftlbs) |
|---|---|
| 1/4 | 7 |
| 5/16 | 12 |
| 3/8 | 20 |
| 1/2 | 50 |
Lug Screw Connections
| Hex Head Size (inches) | Torque Values (ftlbs) |
|---|---|
| 1/4 | 17 |
| 5/16 | 23 |
| 3/8 | 31 |
| 1/2 | 42 |
| 9/16 | 50 |
If the torque tables on the gear identify values for “Socket Size Across Flats”, this is referring to the hex head size of the lug screw. If this is mistaken for the socket size of bolt connections, it can result in significant over-torqueing.
DO – Make sure you are applying the correct torque to each connection
Wrap Up:
A third-party evaluation of a bus-tap modification must be conducted at the time of installation. As illustrated in this blog, several different aspects of compliance are at play during the inspection, requiring more than simply showing up to witness torque values. Because a significant amount of work goes into setting up for this type of modification, we want to make sure that there are no surprises on the day of inspection. In order to avoid disappointing results, G&M sets up a preplanning meeting to prepare for issues before installation. This early communication of expectations leads to a smoother inspection and a higher likelihood of favorable results.
To see our Part II article regarding NEC requirements applicable to Switchgear, click on Part II (NEC Requirements). For more information on how we can assist with Field Evaluations click on Field Evaluation Service.
For a quotation for Field Evaluation Services click Quotation
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