Proper Working Drawing Sets: An Engineer's Perspective

On any given construction project, multiple parties work together toward common goals to complete the job on time, on budget, and without any accidents or injuries. From the engineer to the general contractor to numerous subcontractors, individual participants in the overall project must have a detailed road map before embarking on their journeys. Serving a number of different purposes, the drawings

On any given construction project, multiple parties work together toward common goals — to complete the job on time, on budget, and without any accidents or injuries. From the engineer to the general contractor to numerous subcontractors, individual participants in the overall project must have a detailed road map before embarking on their journeys. Serving a number of different purposes, the drawings prepared by engineering firms and distributed to key players provide a means for effective project navigation.

During the design phase, these drawings go through any number of alterations, helping the client to determine the most efficient installation plan. Next, they become the basis for the bid packages that are submitted by subcontractors, including electrical contractors. After the project is awarded, the drawings form the basis of the contract between the selected subcontractor and the client. At this point, all drawings must feature sufficient detail to assure the client and engineering firm that the project will be installed to all standards and codes — and that no questions will arise during the installation process.

As every construction project is unique, so are the working drawings. However, that doesn't mean subcontractors should adopt a “get-what-you-get” mentality for fear of losing a bid or appearing nit picky. As the electrical contractor, if you're not getting the essential information you need in these drawings, demand something better by asking for clarification on any gray areas. To help contractors know what to look for, following is a discussion of how engineers should prepare proper working drawings, including what should be incorporated on a typical electrical project.

Drawing list, key plan, and plant drawing

A good-quality drawing list should have a complete list of drawing numbers, titles, and current revision. When the engineering firm issues a revision to a drawing, it can issue the list again and update the drawing list sheet and the affected drawings. This drawing would typically contain a company logo, project title, and client's name.

Most plants are large enough that a key plan is needed to show where the work will be performed. If there is room on the index sheet, this is a good place to show this information. It is also beneficial to have an overall dimensioned plant drawing, showing how to get to the work area. Major landmarks in the plant should be shown and identified. Many times, this brings up discussions on proximity to high-voltage lines or pipelines, permits required, and the logistics of getting the equipment into the room or the plant.

General notes and legend

If there are a large number of drawings — and specific items that apply to all the drawings — it's a good idea to have a drawing that includes general notes, a legend, and symbols. Typical details can also go on this drawing. If the drawing package is small, this information can be placed on individual drawings.

Notes in any drawing package should always be brief. If it takes more than a couple of sentences to convey the requirement, the designer should consider re-wording or making a detail that will make his point more easily understood. New work on drawings is usually shown with a heavy line type to make it stand out. Various other line types are used to delineate the scope of work, such as the wiring to be done by a vendor or hidden lines to show underground conduit. Of course, schematics are shown symbolically. Equipment not dimensioned on the drawings is shown diagrammatically with other symbols.

One-line diagrams

The purpose of the one-line diagram is to show all of the electrical equipment items, protective devices, and switches on the system. Each piece of equipment should be shown with its appropriate rating (i.e., motors in horsepower, heating loads in kilowatts, transformers in kilo-volt-amperes, and capacitors in kilo-vars). You should be able to add up all connected load on the power system from the single-line diagram.

The one-line should also contain information about the protective devices (i.e., fuses, relays, and circuit breakers with their frame sizes and trip settings). Relay settings are usually not placed on the one-line, but should be referenced on the drawings as to where to find this information. A qualified engineer should have been involved to make sure all equipment has the proper short-circuit ratings — and that the system is adequately coordinated as per 110.9 and 110.10 of the NEC.

Information such as starter sizes, transformer impedances, transformer connection type, current transformers, and their corresponding ratios, bus ratings, and short-circuit bracing should also be shown. If the cable sizes and cable numbers are not shown elsewhere, it's good to show this on the one-line diagram.

In addition, it's important to give concise names to all the equipment and switches and a brief location. Equipment and switches should be marked in the field with identical names as those shown on the one-line diagram. This assures compliance with 110.22 of the NEC. The control of these devices is shown on schematics and should not be included on the one-line diagram.

Panel schedules

Like the one-line diagram, panel schedules are essential for showing all of the small electrical equipment in the system. The panel schedule drawings should contain a tally that shows the expected amount of volt-ampere usage for each circuit and the overall expected connected load for each phase on the panel. This will assure the designer, client, and electrical contractor that the load is evenly distributed between the phases, and that the panel is balanced. The panel schedules should also show the size, type, and number of poles of each circuit breaker or fuse.

Area classification drawings

If the new installation will reside in, or the cables route through, any electrically classified area, this drawing must be provided as well. Refer to NFPA 497 for recommended practices when dealing with gases and NFPA 499 for combustible dusts. For consistency, the area classification drawings should be drawn similar to the various examples shown in those standards. Documentation of an electrically classified area is required per 500.4(A) of the NEC.

Equipment schedules

When specifying any piece of electrical equipment that the installing contractor will have to purchase, the designer should include enough information that the catalog number given for the equipment can be reproduced, or an alternate item can be obtained. Equipment schedule drawings are generally included on large electrical projects with many different types of lighting, receptacles, or special pieces of electrical equipment. On smaller projects, or those with limited amounts of differing pieces of equipment, this information is typically placed on other drawings.

Motor control schematics

Typical schematics can be used, but it's generally better to prepare one schematic per motor. The schematic should be shown in a standard ladder-type arrangement and illustrate how the motor control circuit works. It should also correspond to the complete installation in the field, showing all internal components, including control transformers, control devices, pilot lights, protective devices, terminals with numbers, cable numbers, and any color coding of the cables.

Plan drawings

Plan drawings should contain a general arrangement of the new installation, showing adequate detail so that an accurate material takeoff can be made for the project.

Electrical equipment that requires working space per the NEC (110.26 or 110.32) should be fully dimensioned on the drawings. Reference dimensions should be shown on all plan drawings, such as between columns in a building.

All plan drawings must be drawn to scale. The scale used on the drawings should correspond to one of the standard scales found on an architect's or engineer's scale (¼ inch = 1 foot or 1 inch = 10 feet, for example). Because nowadays drawings can easily be reproduced on any size sheet, the drawing should include an actual physical scale, such as those shown on any road map. It should also include a north arrow.

The source and destination of every cable shown on the one-line or cable schedule should appear on a plan drawing. In general, routing of the cables are shown as home runs on this drawing, unless there is a reason it must go a certain route — then it would be shown in its entirety. The wiring method to be used must be called out on the drawing (i.e., cable tray, conduit, on a pole line, underground, etc.). Conduit corridors and cable tray elevations should be noted. Grade elevation should also be noted on all plan drawings, and the corresponding benchmark location noted.

Boundaries of the electrically classified areas should be shown to indicate where seal-offs are required [501.15(A)(4) or 501.15BB)(2) of the NEC].

Other plan drawings might include: grounding, lighting, instrumentation, cable tray, communications systems. Sometimes a designer might combine the different types of drawings onto a single drawing.

Details and other drawings

Although qualified electricians know how to install most pieces of commonly used electrical equipment, the engineer must provide details where the client has a specific preference or where unusual equipment or seldom-used wiring methods are required. Another case is when critical dimensions must be adhered to, such as installing embedded duct or equipment foundations. Wiring details of junction boxes and all other electrical devices are generally included or excluded, depending on the client preference.

It is not a good idea for an engineering firm to redraw vendor drawings that will have no or little potential for revision, such as a control scheme on the incoming main of a piece of switchgear.

Conduit and cable information

It should be the designer's responsibility to properly size the conduit and wire to every electrical load or control circuit. The designer should also specify all the cable types and assign a cable number to all cables. This information can be found on the drawings.

An alternative is the designer issues a separate cable schedule or list. If the cable schedule is done on a spreadsheet or database, it has the advantage of being sorted by cable type, area, load or other field, depending on how it is set up. However, this does create another document that has to be cross-referenced.

Either way, the information should be given only once on a set of drawings.

Scope of work

In addition to the drawings, a brief scope of work document should accompany the drawing set when it is issued. The scope should help tie all the drawings and intent of the project together and never repeat specific information shown on the drawings.

Because all projects differ in complexity and scope, detailed drawings should be issued for most electrical projects. If you're not getting this type of documentation on the projects you're working on, maybe it's time to push back a little bit on the electrical designer and demand a quality set of working drawings the first time around.

Nicholson is a senior power and controls engineer at C&I Engineering, Louisville, Ky.

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