Sample calcs are going to be short examples of how to do calculate something that might help you out. They are geared towards issues that have come up in the field. If you are an electrician, you can probably skip this post. This is geared towards an estimate an engineer could use for pricing (aka RSMeans), or review of a schedule. As with all posts, this is for educational use only.
On paper, changing wire schedules is fairly cheap. After that, wiring modifications are far more costly. Typically, signal wiring will come in via hardwire or network cable.
If you are reviewing a conduit schedule, make sure...
AC and DC wires are in separate conduit (as AC will induce waves on the DC)
Signal cables are only in conduit with DC wires
Conduit fill is not excessive (usually 40% by cross sectional area)
Box fill is not excessive
Calculating conduit fill is fairly easy. First, check your local electrical codes! In the USA, this would be Table 9 of the NEC. Roughly, you sum up the cross-sectional area of the wires, divide by 0.4 and round up to the next size for your type of conduit. I recommend you actually go up two sizes or divide by 0.3, just to save time when signals are added later.
Pull box sizing is a little more complex. Table 314.16 goes into details. I attached a little spreadsheet to help out. Please note that this is for educational uses only- you still need to check this against the NEC and verify it yourself! (I did add the notes of where to look for exceptions and additions. Grounded conductors can be key, hence the separate column.)
Let's go ahead and try to size conduits! Imagine you have three conduits coming from three local control panels into a junction box, which then feeds out via on conduit to your PLC cabinet. C-xx will stand for the conduits and JB-yy for the junction box.
C-01 4x 10 AWG THHN ungrounded conductors
1x 12 AWG THHN grounded conductor
C-02 16x 12 AWG THHN ungrounded conductors
3x 14 AWG THHN grounded conductors
C-03 12x 18 AWG TSP ungrounded conductors
1x 14 AWG THHN grounded conductor
JB-01 All wires above
C-04 All wires above
But alas, if you are not an electrician, you probably need some reference.
AWG- American Wire Gauge, basically the size of the copper
THHN- Insulation type, heat resistant thermoplastic for dry and damp applications
TSP- Twisted shielded pair, commonly used for signal wires. Can come with a variety of shields, so we will assume it is sized as TFN for this calculation. (You really want to check this assumption before you get in the field!)
Some other common abbreviations for the controls industry include...
EMT- Conduit type, Electrical Metallic Tubing
FMC- Conduit type, Flexible Metal Conduit
MTW- Insulation type, machine tool wiring, heat resistant thermoplastic. Very flexible, I like to use inside of cabinets when possible, but expensive
THWN- Insulation type, heat resistant thermoplastic for dry and damp applications, slightly cheaper than THHN
UF- Insulation type, underground feeder and branch circuit
The first thing that would strike me is the 18 AWG TSP wire for the ungrounded conductors. Typically, this means you would have a DC signal (perhaps 4-20 mA). You need to check that beforehand, as putting everything in JB-01 would allow for cross talk (the AC wires inducing a voltage on the DC wires) and will certainly do so for C-04.
For the purposes of the demonstration, let's calculate the size of JB-01 and C-04 assuming they are all AC. I will leave coming back to do the same as an exercise for the reader.
We need the cross-sectional area of each of the wires.
THHN 10 AWG 0.0211 in2 13.61 cm2
12 AWG 0.0133 in2 8.581 cm2
14 AWG 0.0097 6.258
TFN 18 AWG 0.0055 3.548
The sum of areas inside would be...
C-01: 4 * 0.0211 + 1 * 0.0133 = 0.0977 in2
C-02: 16 * 0.0133 + 3 * 0.0097 = 0.2419 in2
C-03: 12 * 0.0055 + 1 * 0.0097 = 0.1417 in2
C-04: 0.0977 + 0.2419 + 0.1417 = 0.4813 in2
For more than two wires in a conduit, 40% is a standard fill. Let's assume EMT for the conduit. Going to Chapter 9, you can find Table 4, Article 358 and compare to the minimum areas you calculate with the fill...
C-01: 0.0977 / 0.4 = 0.24425 in2 => 1/2" (metric 16)
C-02: 0.2419 / 0.4 = 0.60475 => 1" (metric 27)
C-03: 0.1417 / 0.4 = 0.35425 => 3/4" (metric 21)
C-04: 0.4813 / 0.4 = 1.20325 => 1 1/4" (metric 35)
While this calculation will do that, I strongly encourage you to oversize any conduit with signal. It is way cheaper to put in an oversized conduit than to add a second one in.
Now, for the junction box size! Check out NEC 314.16. You will note Table 314.16(B) gives volume per conductor.
Size in3 cm3 # in JB1 in3 sum
18 1.50 24.6 12 18
16 1.75 28.7 12 21
14 2.00 32.8 4 8
12 2.25 36.9 17 38.25
10 2.50 41.0 4 40
Simply sum the contributions of each size and you get a 125.25 in3 box, which sounds about right. Five inches per side would make plumbing these conduits in fairly easy.
Note: This is slightly different than what your electrician should do. Grounded and ungrounded connectors do make a difference and you can get a discount on your box. This is outside of our scope but something to be aware of.
Are there shortcuts you could make? Well, if you had wires of the same size, you may be able to make use of the Annex C tables in NEC.
Typically signal wires are oversized, but plentiful. Mistakes don't have to be. Make your own spreadsheet (or modify this one), oversize things to save money in the long run and always double check your calculations!
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