Conduit fill calculator.

Calculator

Millimetres Inches

Conduit run Nipple, 600 mm or shorter

Conduit type EMT (electrical metallic tubing) Rigid steel (RMC) Rigid PVC

Trade size (metric designator)

Group 1: conductor Enter OD below

Group 1: quantity

Group 1: OD (mm)

Group 2: conductor Enter OD below

Group 2: quantity

Group 2: OD (mm)

Group 3: conductor Enter OD below

Group 3: quantity

Group 3: OD (mm)

Group 4: conductor Enter OD below

Group 4: quantity

Group 4: OD (mm)

Group 5: conductor Enter OD below

Group 5: quantity

Group 5: OD (mm)

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01Why conduit fill is limited

A conduit is a fixed volume. Every conductor pulled into it takes up part of that volume, and the fill limit is the code's way of leaving enough of it empty. There are two reasons, and both are about the insulation.

The first is heat. A conductor carrying current warms up, and in a raceway it warms the conductors beside it. The air space around the conductors is what lets that heat move to the conduit wall and out. Pack the conduit tight and the air is gone, the conductors run hotter than their rating assumes, and the insulation ages faster than it should. The fill limit keeps a margin of air in the raceway.

The second is pulling damage. Conductors are pulled into a conduit after it is installed, around bends and through couplings, under tension. In a full conduit the insulation drags against the wall and against the other conductors, and at a bend it can be scraped, flattened or nicked. Three conductors of the same size in a conduit whose inside diameter is close to three times their outside diameter can also jam: they line up side by side at a bend and wedge. A conduit that is only 40% full has room for the conductors to move past each other.

Fill is one check among several on a raceway. The ampacity of the conductors inside it is a separate question, with its own correction when more than three current-carrying conductors share a raceway (Section 4 of the code), and circuit loading has its own rule, Rule 8-104. A conduit that passes fill can still hold conductors that are too small for the load, and the other way round. Do the fill check, then do the others.

02The percentages and where they are in the code

The Canadian Electrical Code, Part I (CSA C22.1) sets the maximum fill for conduit and tubing in Rule 12-910, in Section 12, Wiring methods, with the percentages in Table 8. In words, the limits are:

• 53% of the conduit's internal cross-sectional area for one conductor.
• 31% for two conductors.
• 40% for three or more conductors.
• 60% for a nipple, a short length of conduit or tubing of 600 mm or less between enclosures. Read the subrule in your edition for the exact conditions that attach to it.

The percentages are the same as the ones in Chapter 9, Table 1 of the American NEC, and they have the same reasoning behind them. One conductor can fill more than half the conduit because it has nothing to pass on the way in. Two conductors get the lowest figure because they lie side by side, touching the wall on both sides, and that arrangement pulls worst. Three or more nest into a rounder bundle and get 40%.

Rule 12-910 points to a set of tables, and knowing what each one holds is most of the skill. Table 6 (with its lettered subtables by conductor type) gives the maximum number of conductors of one size and type that a given trade size may hold, which settles the common case without any arithmetic. When the conductors are of different sizes or types, you calculate: Table 8 gives the fill percentages above, Table 9 (9A to 9J, one per conduit or tubing type) gives the internal cross-sectional areas of each trade size, and Table 10 (10A to 10D, by insulation type) gives the dimensions and areas of insulated conductors. This page cites those tables and reproduces none of them; the internal diameters and conductor diameters used here are manufacturers' published figures instead, which is why the answers can differ slightly from the code's.

The exam expects you to find these figures in the code book, not to remember them. What is worth remembering is the shape of the problem: which percentage applies, which table gives the conduit area, which gives the conductor area, and that the comparison is between areas. If you know that, the code book supplies the rest. If a Canadian Electrical Code book is provided at your sitting is not settled by the Red Seal Program's published material, as our 309A exam guide explains, so ask your provincial or territorial apprenticeship office when you book.

03The arithmetic: areas, not diameters

Fill is a comparison of areas. The area of a circle of diameter d is A = π/4 × d² (the same thing as πr²), so every step is that one formula applied to a diameter, then a sum, then a division. The sequence is:

1. Find the conduit's internal diameter for the type and trade size and work out its internal area: π/4 × ID².
2. For each conductor, find the outside diameter over the insulation and work out its area: π/4 × OD². Multiply by how many of that size there are.
3. Add the conductor areas together.
4. Count the conductors, pick the percentage (53, 31 or 40, or 60 for a nipple), and multiply the conduit area by it. That is the allowed area.
5. Compare. The total conductor area must not be more than the allowed area. The fill percentage is total conductor area divided by conduit area, times 100.

Areas in square millimetres and diameters in millimetres, or areas in square inches and diameters in inches. Never one of each.

Worked example: the calculator's default

Question. Three RW90 copper conductors with an outside diameter of 5.9 mm and one with an outside diameter of 4.4 mm are to be run in EMT. The manufacturer gives the internal diameter of trade size 21 (3/4 in) EMT as 20.93 mm. Does the run meet the fill limit in trade size 21, and what is the fill?

Solution. Conduit area: π/4 × 20.93² = 0.7854 × 438.06 = 344.04 mm². The 5.9 mm conductors: π/4 × 5.9² = 27.34 mm² each, so three of them are 82.02 mm². The 4.4 mm conductor: π/4 × 4.4² = 15.21 mm². Total conductor area: 82.02 + 15.21 = 97.22 mm². There are four conductors, so the limit is 40% and the allowed area is 344.04 × 0.40 = 137.62 mm². 97.22 is less than 137.62, so the run passes. The fill is 97.22 / 344.04 × 100 = 28.26%. Trade size 16 (1/2 in) would not do: its internal area is π/4 × 15.80² = 196.04 mm², 40% of that is 78.41 mm², and 97.22 mm² of conductor would fill it to 49.59%.

Worked example: two conductors and the 31% limit

Question. Two 1/0 AWG RW90 copper conductors, outside diameter 11.8 mm, are to be run in trade size 27 (1 in) EMT with an internal diameter of 26.64 mm. Do they fit?

Solution. Conduit area: π/4 × 26.64² = 557.58 mm². Each conductor: π/4 × 11.8² = 109.36 mm², so two are 218.72 mm². Two conductors, so the limit is 31%: 557.58 × 0.31 = 172.85 mm². 218.72 is more than 172.85, so trade size 27 fails at a fill of 39.23%. Notice that 39.23% would pass if there were three or more conductors. The two-conductor limit is the one candidates forget. Trade size 35 (1-1/4 in), internal diameter 35.05 mm, has an area of 964.97 mm², allows 299.14 mm² at 31%, and passes at 22.67%.

Enter either example into the calculator and the working printed under the results follows these steps line by line.

04Finding the conductor outside diameter

The diameter that matters is the one over the insulation, not the diameter of the copper or aluminium. It comes from one of two places.

The manufacturer's specification sheet. Every building wire product page lists a nominal outside diameter (Nexans calls it the cable diameter) for each size. The presets in the calculator are Nexans Canada's published figures for RW90 copper, a 600 V cross-linked polyethylene (XLPE) insulated wire with compressed stranding: for example 3.8 mm for 12 AWG, 4.6 mm for 10 AWG, 5.9 mm for 8 AWG and 15.6 mm for 4/0 AWG. They are that manufacturer's nominal figures for that product. A different maker's RW90 will be close but not identical, and a spec sheet dated a few years apart can differ in the second decimal.

Table 10 in the code book. On the exam, and whenever you do not have the spec sheet for the wire actually being pulled, the code's Table 10 series gives the dimensions and cross-sectional areas of insulated conductors by size and insulation type, and Table 10 is what a code-based answer is expected to use. The tables are lettered by insulation type because the outside diameter depends on it.

That is the point that catches people: the same AWG size has different outside diameters in different insulations. Insulation thickness is set by the conductor size and the insulation type, and it is not small. A T90 nylon or TWN75 conductor has a thin thermoplastic wall with a nylon jacket and is noticeably slimmer than the same size in RW90 XLPE, so more of them fit in a conduit. Insulation also gets thicker as the conductor gets bigger: on the Nexans sheet the RW90 wall is 0.76 mm on 14 to 10 AWG, 1.14 mm on 8 to 2 AWG, 1.4 mm on 1 AWG to 4/0, and thicker again on kcmil sizes. A bare bonding conductor has no insulation at all and is smaller than an insulated one of the same size, but it still counts (see the common mistakes below). Compact-stranded conductors are smaller than compressed or concentric ones of the same size. When a question names an insulation type, that is the table you use.

If you have neither a spec sheet nor a code book, you do not have the number. Do not guess it from the conductor size.

05Conduit types and why the inside diameters differ

A trade size is a name, not a measurement. Trade size 21, which the trade still calls 3/4 inch, is not 0.75 in inside; it is about 0.824 in (20.9 mm) in EMT, about 0.836 in (21.2 mm) in rigid steel, and about 0.824 in (20.9 mm) in rigid PVC by IPEX's figures. The metric designator (16, 21, 27, 35, 41, 53, 63, 78, 91, 103) is closer to the truth: it is roughly the nominal internal diameter in millimetres, which is why it was chosen. Both names are shown in the calculator so that a question in either form can be matched.

The differences come from the wall. EMT (electrical metallic tubing) is thin-walled steel with an outside diameter of about 0.922 in in trade size 21, so it has the largest inside for its outside. Rigid steel conduit is heavy-wall pipe with an outside diameter of 1.050 in in the same trade size and a threaded end; its inside is a little larger than EMT's because the outside is so much larger. Rigid PVC conduit is made to the same outside diameter as rigid steel so that it shares fittings dimensions, with a wall thickness set by its own standard, and its inside falls close to EMT's in the small sizes and below rigid steel's in the large ones. The larger the trade size, the more the three types diverge: at trade size 103 (4 in) the internal diameters used here are 4.334 in for EMT, 4.050 in for rigid steel and 4.026 in for rigid PVC.

The calculator's internal diameters, in millimetres with inches in brackets, are:

These are nominal figures from the manufacturers named in the sources: Wheatland Tube for EMT and rigid steel (Wheatland marks its EMT inside diameter as for information only, not a requirement of the product standard) and IPEX for rigid PVC. Another maker's product will vary a little, and the code's Table 9 series carries its own figures for each type, which is what the exam uses. The difference is usually under 1% of the area and changes the fill percentage in the first decimal, not the pass or fail, but when a question gives you the code figures, use them.

Other raceways are outside the tool. Flexible metal conduit, liquid-tight flexible conduit, intermediate metal conduit, rigid aluminium and electrical nonmetallic tubing each have their own Table 9 subtable in the code and their own internal diameters, and the same arithmetic applies once you have the area.

06Nipples and short runs

A nipple is a short piece of conduit, 600 mm or less, joining two enclosures: a panel to a wireway, a box to a box. Nothing is pulled through it in the usual sense, there are no bends, and heat has a short path to the enclosures at each end. For that reason the fill allowance rises to 60% of the internal area regardless of the number of conductors. The calculator's nipple setting applies the 60% figure; everything else in the arithmetic is unchanged.

The allowance is for the fill only, and it is conditional on the length. A run of 700 mm is a run, not a nipple, and gets the ordinary percentage. Read the subrule under Rule 12-910 in your edition for the conditions, and do not extend the allowance to any other rule; what a nipple does or does not change about ampacity correction is a separate question answered in Section 4, not here.

Worked example. Five 6 AWG RW90 copper conductors, outside diameter 6.8 mm, between a panel and a wireway 450 mm apart, in trade size 21 (3/4 in) EMT, internal diameter 20.93 mm. Conduit area is 344.04 mm² (from the first example). Each conductor is π/4 × 6.8² = 36.32 mm², so five are 181.58 mm². As a run, the limit is 40%, the allowed area is 137.62 mm², and 181.58 mm² is over it: the fill is 52.78% and it fails. As a nipple of 450 mm, the limit is 60%, the allowed area is 344.04 × 0.60 = 206.43 mm², and the same conductors pass at 52.78%. Same conductors, same conduit, different answer, and the only thing that changed was the length.

07The reverse problem: smallest conduit for a set of conductors

On the job, and on the exam, the question is more often the other way round: here are the conductors, what size conduit do they need? The arithmetic is the same with one extra step. Work out the total conductor area and the percentage first. Then the conduit must have an internal area of at least total conductor area ÷ the percentage, and you pick the smallest trade size whose internal area is at least that. The calculator does this for every result, listing each trade size it tested.

For a set of conductors that are all one size and type, Table 6 in the code answers the question directly by count, with no arithmetic. The calculation is for mixed sizes, mixed insulation types, or when a question makes you show the method.

Worked example

Question. A feeder consists of four 4/0 AWG RW90 copper conductors, outside diameter 15.6 mm, and one 2 AWG RW90 copper bonding conductor, outside diameter 9.5 mm. What is the smallest trade size of rigid steel conduit for the run? Use the manufacturer's internal diameters: trade size 41 (1-1/2 in) 41.25 mm, trade size 53 (2 in) 52.91 mm.

Solution. Conductor areas: π/4 × 15.6² = 191.13 mm² each, four of them 764.54 mm²; π/4 × 9.5² = 70.88 mm² for the bond. Total 835.42 mm². Five conductors, so 40%. The conduit needs an internal area of at least 835.42 / 0.40 = 2,088.6 mm². Trade size 41: π/4 × 41.25² = 1,336.38 mm², too small (the conductors would fill it to 62.51%). Trade size 53: π/4 × 52.91² = 2,198.55 mm², which is at least 2,088.6, so it passes; the fill is 835.42 / 2,198.55 × 100 = 38.00%. The answer is trade size 53 (2 in) rigid steel. Leave out the bonding conductor and the total is 764.54 mm², needing 1,911.4 mm²; trade size 53 is still the answer here, but on a tighter set of numbers the bond is exactly what pushes a run into the next size, so it is always counted.

A check worth doing on the exam: the fill in the size you chose should be under the limit, and the fill in the size below it should be over. If both pass, you went one size too big; if both fail, one too small.

08Common mistakes

• Using the trade size as the inside diameter. A 3/4 in conduit is not 0.75 in inside, and a trade size 21 is not exactly 21 mm. Look the internal diameter or area up for the conduit type in Table 9 or on the manufacturer's sheet. Using 0.75 in understates the area of trade size 21 EMT by about 17%.
• Forgetting that 40% is for three or more. Two conductors get 31%, one gets 53%. A pair of conductors that would sit at 35% in a given conduit passes the 40% rule that does not apply to it and fails the 31% rule that does.
• Adding diameters instead of areas. Three conductors of 5.9 mm are not one conductor of 17.7 mm. Their combined area is 3 × 27.34 = 82.02 mm²; a 17.7 mm circle would be 246.06 mm², three times too much. Square each diameter, then add.
• Mixing millimetres and inches. A conduit area in square inches against conductor areas in square millimetres gives a percentage that is 645 times too big or too small. Convert every diameter to one unit before you square anything; 1 in = 25.4 mm.
• Leaving out the bonding conductor. An equipment bonding conductor pulled in the conduit takes up space like any other conductor and counts in the fill, insulated or bare. So does a neutral, and so does every conductor of a spare circuit pulled for later. The count for the percentage includes them too.
• Using the wrong insulation type. The outside diameter of 8 AWG in RW90 XLPE is not the outside diameter of 8 AWG in T90 nylon. Use the Table 10 subtable, or the spec sheet, for the insulation named in the question.
• Comparing the fill with the percentage the wrong way. Fill must be at or below the limit. A fill of 40.4% with a 40% limit fails; there is no rounding allowance in the rule.
• Applying the nipple allowance to a run. 60% is for 600 mm or less between enclosures. Anything longer is a run at 53, 31 or 40%.

09What the 309A exam expects about conduit fill

Conduit fill sits in the Construction Electrician standard's largest block. Our 309A exam guide sets out the Red Seal Program's breakdown: block C, wiring systems, is 30 of the 100 questions, and its task on raceways, conductors, cables and enclosures gets 9 of them, covering conductors and cables, conduit and fittings, raceways, boxes and enclosures. Selecting a raceway for a set of conductors is exactly the kind of sub-task those questions are written to.

The program says 60 to 70% of 309A questions are procedural and application questions, which its preparation guide says can include calculations and the interpretation of code books, and 20 to 30% are critical thinking questions that may take more than one step. The guide's own sample of a critical thinking question for this trade is a box fill problem: given the size of a box and what is already in it, how many more conductors of a given size are permitted. A conduit fill question has the same shape. It will give you the conductors and either ask whether they fit a named trade size or ask for the smallest size that takes them, and it may hand you the areas or make you find them in the code tables.

Three practical points follow. First, the preparation guide warns that a question will not announce that it is a code question; if a stem lists conductors and a raceway, Rule 12-910 and its tables are in play whether or not the code is named. Second, the percentages in this page are the code's and there are no others; if an answer option depends on a figure like 45% or 50%, that option is wrong. Third, the guide says the exam covers the trade as practised across Canada, so expect metric trade sizes and dimensions in millimetres, with the inch names alongside at most.

Practise until the sequence is automatic: conduit area from Table 9, conductor areas from Table 10, add, count, pick the percentage from Table 8, compare, and for the reverse problem divide the total by the percentage and go to the next size up. The free 309A questions include raceway and code items in the same format as the exam.

10Sources

• Red Seal Program: Construction Electrician trade page the trade the 309A exam certifies, the occupational standard editions, and links to the exam information page with the block breakdown.
• CSA Group: Canadian Electrical Code, Part I (CSA C22.1) the publisher's page for the code that contains Rule 12-910 and Tables 6, 8, 9 and 10. The code is copyrighted; this page cites it and reproduces none of its tables or rule text.
• Wheatland Tube: EMT and Conduit Brochure (PDF) the nominal inside diameters used here for EMT (marked by Wheatland as for information only) and for rigid metal conduit, trade sizes 1/2 to 4 in. The same rigid steel figures appear on Wheatland's Rigid Metal Conduit flyer.
• IPEX: Electrical Products Catalogue, Scepter Rigid PVC Conduit product selection chart and dimensions (PDF) the nominal inside diameters used here for rigid PVC conduit, trade sizes 16 to 103.
• Nexans Canada: RW90 Copper, 600 V XLPE insulated wire the nominal cable diameter for each size, used for the calculator's RW90 presets, and the insulation thickness by size quoted above.
• TicketPrep: 309A Electrician Red Seal exam guide the block and task question counts, the question-type mix, the sample box fill question and the open question of the code book at the sitting.

11Questions people ask

What is the maximum conduit fill under the Canadian Electrical Code?

Rule 12-910 and Table 8 of the Canadian Electrical Code, Part I limit the conductors to a share of the conduit's internal cross-sectional area: 53% for one conductor, 31% for two, and 40% for three or more. A nipple of 600 mm or less between enclosures may be filled to 60%. The exam expects you to find these in the code book, along with the conduit areas in Table 9 and the conductor dimensions in Table 10.

How do I calculate conduit fill?

Work out the area of each conductor from its outside diameter over the insulation (area = pi/4 x d squared), multiply by the quantity, and add the groups together. Work out the conduit's internal area the same way from its internal diameter. Divide the total conductor area by the conduit area and multiply by 100 for the fill percentage, then compare it with 53, 31 or 40% depending on the number of conductors. Keep every diameter in the same unit.

Where do I find the outside diameter of a conductor?

On the manufacturer's specification sheet for the wire, where it is listed per size as the nominal outside or cable diameter, or in the Table 10 series of the Canadian Electrical Code, which gives insulated conductor dimensions by size and insulation type. The diameter depends on the insulation, so 8 AWG in RW90 is larger than 8 AWG in T90 nylon. On the exam, use the code table for the insulation type the question names.

Does the bonding conductor count toward conduit fill?

Yes. Every conductor in the conduit takes up area and counts, including the equipment bonding conductor, whether it is insulated or bare, the neutral, and any spare conductors pulled for later. It also counts in the number of conductors used to pick the percentage.

Why does the calculator give a slightly different fill from the code tables?

Because it uses manufacturers' published nominal internal diameters for the conduit and, if you use the presets, one manufacturer's nominal conductor diameters, while the code's Table 9 and Table 10 carry their own figures. The difference is usually a fraction of a percentage point of fill and rarely changes a pass or fail. When a question gives you code figures, use those.

12Practice for this exam

Conduit fill is one raceway calculation among the code items on the 309A exam. TicketPrep 309A practice tests follow the Red Seal Program's block-by-block breakdown, and every answer is explained, including why each wrong option is wrong. You can try free sample questions with no account.

Ten free questions per trade, no account: free practice questions. How our questions are written and checked: how our questions are made.

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