Double tapped terminal blocks

[Lordzorak 0]

I have recently started to learn how to design control panels. Everything has been in CAD so far but eventually will try my hand at building them. Regardless, I would like some input on how other designers/builders handle their terminal blocks. When you figure how many terminal blocks you will need, do you figure each wire with its own terminal block (or two wires, one on each side of the block with center jumper), or do you use two wires under each terminal block screw to cut down the number of blocks needed. I know that the block used would have to be rated for multiple wires and current that could be seen but what is considered the common practice? Thanks Z

[TimWilborne 108]

Personally I like to only have 1 wire per terminal block, but I am aware of several plant electrical specifications that allow for two wires per terminal block. I get the feeling that this was intended to provide a way to jumpering power from one block to another.

[paulengr 44]

Most terminal blocks are certified with UL for only one wire. Putting two wires in a terminal is a Code violation unless it is specifically rated for it, in the U.S., because you are not using the device in the way it is Listed, making it an unlisted (and illegal) installation. The way you make it work is by buying terminal blocks that have ports running down the center of the terminal block where you place a jumper bus bar. If you want to minimize installation time and enforce the "one wire per terminal" rule, buy insulation displacement terminals. There's no wire stripping and you flat out can't overload the terminal block because it won't work. They are now almost as cheap as regular terminal blocks and the wiring time is much shorter since all you have to do is cut the wire and insert it. It makes vibration-proof, sealed (no oxidation) connections. I just bought the last few boxes of IDC terminal blocks from Omega. They seemed to have the best price from a major vendor. I have bought them from Mouser or Newark before (Phoenix Contact or Weidmuller) but I was very disappointed. Seems they didn't stock them and the special order took months to arrive (way beyond the stated delivery date). The blocks from Omega are actually coming from Telemechanique and are very well built. You can't get high power or fused or anything but feed-through terminals this way for the most part. If you're wiring up power circuits, you can get 3 phase distribution terminals which have say 1 or 2 large terminals on one side and as many as a dozen smaller ports on the other side. The major disadvantage to them is that at least theoretically unless you make the wiring on both sides identical in size, it's a Code violation because the outgoing wires are not short circuit protected. It's the same argument you run into with MCC's and other modular power distribution systems where the horizontal runs are usually huge but the vertical runs are rated for perhaps half that much. In theory, the vertical runs are totally unprotected if you get a short on the bus. Since power failures are very rare in this area, most people choose to assume that almost all failures are going to be dead shorts which will quickly be removed by the upstream protection, and that the risk is minimal.

[Lordzorak 0]

Let me ask this then. If you prefer to have each wire to its own block, do you typically use a 6mm or even a 5mm block (when dealing with nothing larger than size 14 wire)? This would seem to be the route to go as the larger blocks would only be needed for single larger wire size or a combination of two smaller wire sizes. Further, when counting the number of blocks for a series of blocks with the same function, do you count wire on only one side of the block. For example, if I have a string of blocks acting as neutrals that are center jumpered and I know that I will have 20 wires coming to them, do you figure 20 blocks or figure 10 blocks (plus one for the circuit neutral). It would seem a waste of real estate to not use each side of a block when possible, but does the asthetics of the panel ever come into play in that you have all the same wire coming down the same side of the tray and all land in succession? Again, thanks for your responses and willingness to share. Z Edited 18 Jul 2008 by Lordzorak

[panic mode 246]

that would depend on from which side wires come to the strip. so for one circuit terminal (2 termination points) you may need 20 terminals (which is unlikely). note that there are also single circuit terminal blocks with more than two termination points (3 or 4 for example). also on i hate to see every single termination point used on jumpered part of the strip, should have some spares too...

[paulengr 44]

Aesthetics does matter. If you have to have wiring running back over the top of the terminal blocks, then the whole point of terminal blocks (organization and neatness) is gone. You may as well just wire everything up with wire nuts and overstuffed device terminals. If you want to win an ugliness competition some time, try the "dead bugs" or wire wrap methods of circuit board fabrication. Because of the order I lay things out in, I can frequently use the "top" of one terminal block to feed components in the row above, and the "bottom" to feed components in the row below. Frequent places where you will find this is aesthetic and works well are also "power distribution" points. You will have so many disparate destinations that it tends to work out without getting ugly or having a bunch of "loops" going through the vertical channels at the end of a row. I find where aesthetics and practicality meet head-on is wires running "vertically". This is where the wire really needs to go from one horizontal row of components to another. The panduit strip at the end tends to get very, very full, unless you start adding additional vertical channels (and chewing up a lot of space). I tend to have only 2-4 I/O cards per panel anyways (very distributed I/O) so most of the time, I can fit comfortably in a 24x24 panel. I'm "cheating" somewhat because I'm running Autocad Electrical. It's not the best CAD system for electrical work but it does OK once you get past all the odd behaviors. First I draw the schematics. I try to rely very heavily on the catalog because without that information, the system is very limited. Then I populate the schematics with terminals where I know I'm going to need them. As I'm doing this, I mark the terminals into groups ("terminal strips" in Autocad Electrical terminology). Finally I lay out the panel, including the terminal strips that are automatically populated for me. Then after printing out both, the electricians can pretty easily wire everything up right off the prints. I just lay out all the terminal blocks in a row (or multiple rows). When we actually assemble them, we put a spacer (actually an end cap) between each "strip" shown on the drawing. The wire labels on the "top" terminals at the bottom (the "interface area") also double as labels for the terminal blocks themselves. All wires are labelled at both ends, except power. This is the color code we're using: blue -- common brown -- +24VDC red -- DC signal wire green -- ground white -- AC neutral black -- AC power yellow -- AC signal wire We don't have to label the power wires because those are self-identifying. All fuses are lighted with "fuse blown" indicators and they are also terminal blocks (with 5mm fuses). If I can help it, I try to incorporate the fuses directly into the terminal strips so there isn't a "double row". Layout-wise, I put the PLC I/O blocks in the top rows (we're a manufacturing plant and we use distributed I/O extensively). I frequently put the I/O power supply in the top left corner next to the I/O blocks. Next comes a filter/surge protector, any remaining power supplies (for hydraulic valves for instance), relays, or other related devices. Finally the middle/bottom rows are populated with the external interface blocks. Other than safety relays and the occasional interface relay or some sort of transmitter/amplifier, we don't generally have much inside the panels anyways except I/O and terminals. I try to locate starters are drives outside the I/O panels. These tend to be very spartan because we don't have a lot of motion control stuff to speak of. If I have a high enough density of components to justify it, I buy MCC. Otherwise, I just buy fused combination starter/disconnects with electronic overloads, or fused safety disconnects with extended cabinets (same as a starter cabinet minus the starter & overload), and put the drive inside the cabinet. I'd buy drives and starters with integrated cabinetry but these tend to be either very expensive or very limited. In terms of I/O wiring, for analog, I try to stick to 4-20mA loops. I try to use M12 connectors and patch cords wherever possible. I try to run in conduit until I get close to the sensor. The connections to the cord are made with wire nuts. The wire nuts are inside an LB and the cord passes through a cord connector screwed into the end of the LB. That way it is fairly easy to do diagnostics as well as replace the cord when necessary. For digital, I'm slowly adapting passive I/O blocks and 24VDC. These make cable replacement and a lot of troubleshooting very simple. I'd switch over to "active" I/O blocks ("on-machine I/O) but so far, they seem to be very cost prohibitive and they are mostly only available in ASI bus, Profibus, or DeviceNet. I'm not particularly fond of any of those network standards so at this point I'm still waiting. Edited 19 Jul 2008 by paulengr