Control panel wiring - terminals/daisy chaining

[GlenGineer]

I've seen sooo many panels that had grounded wires daisy chained. I've also seen a few that had line side motor wires (going in to contactors) daisy chained throughout panels. It drives me crazy particularly when suddenly a wire gets disconnected and your control circuits stop working.

I think it is probably done to save a few dollars during the panel build. Us amateurs sometimes make the decision to do this when we size the panel wrong.

I've never seen terminals represented on the electrical ladder. (Well except divisions between panel wiring and field wiring) On panel drawings and sub-panel drawings the terminals are normally shown.

 

[CapinWinky]

I have no problem with daisy chaining similar devices that are in an obvious group, especially if the devices have provisions for daisy chaining. An example would be 24V control power to drives that have 4 pin, internally-jumpered plugs for 24V (so you can go in on 2 terminals and out on the other two). It is not okay to come off the last drive an go to some other type of device, or even another drive located away from the group.

As far as wires and terminals, we label each supply of 24V as 24V1, 24V2, etc. and label the terminals the same (by supply, I mean unswitched, switched, from a different PS, etc; each wire does not increment, there are several 24V1 for example). Something we don't do that I really wish we would is somehow mark which of those wires is the INPUT to the terminal blocks (different wire color, or labeled Input-24V1 or some such). We do call out daisy chaining with 24V1, 24V1A, 24V1B, etc. to make it clear when we are allowing it to be done.

I think the most confusing bit in our drawings is if we have safety devices in series, we always bring each one back to a terminal so we can easily add or jumper them, but we show them in the drawings as <WireNumber>, <WireNumberA>, <WireNumberB> just like we would for daisy chaining. I've argued we should clearly show the terminals on the drawings, but we still don't.

 

[nonuke]

Electrical panel Busbar for the +/- 24VDC etc.

http://catalog.weidmueller.com/proc...([prod13321030937074])&page=TechnicalInfoNode

 

[Bit_Bucket_07]

Is this always represented in your schematics? I.e every terminal shown?

I don't always provide the schematics when I build a panel, but I never daisy chain power circuits.

1. Every device within a control panel should be capable of having it's power wiring disconnected without affecting other devices.

2. Daisy chaining power circuits results in derating the wire, due to multiple loads being attached to a single source wire.

 

[ajph]

2. Daisy chaining power circuits results in derating the wire, due to multiple loads being attached to a single source wire.

This is an excellent point! Is this true for jumper-bars on terminal blocks too?

 

[ajph]

For example...

If I have a jumpered strip supplied by 6.0mm^2 wire - do all of the connections from this bus need to be 6.0mm^2 or does it depend on the load of the device connected to the bus?

 

[GlenGineer]

There was a point in time when I lived and breathed our electrical code book but the last two years I've barely had the need to look at it.

2. Daisy chaining power circuits results in derating the wire, due to multiple loads being attached to a single source wire.

I spent 2 minutes looking in the code book today and I'm still not sure what you are referring to. Daisy chaining would cause additional length which could cause a voltage drop (but you will not see much until you go at least 100 ft) If you have a long enough run you might need to increase the size to get the voltage you want.

EDIT we are talking low voltage DC so what will not effect me for 100 ft at 110VAC would probably effect you in 10 ft - I think I understand.

The addition amperage draw could cause an increase in the ampacity. When you compare it to a home run wire the daisy chained wire might need to be bigger.

Unless there is something I'm forgetting I would think the answer to AJPH's question is that between the terminal strip and the control device the ampacity would be at the minimum as would the length. This would allow you to use a smaller wire. Between the terminal strip and the DC source you have a higher ampacity. The distance from the DC source to the terminal would be the basis of the first voltage drop calculation. So let us say the your power supply is actually 24V with the voltage drop it might be 23.75 (for instance). The next part of the calculation would be between the terminal strip and each of the control devices. Or you could take a short cut and just calculate the longest one. If it comes up 22.75 and your devices can tolerate that low voltage then you are golden. Otherwise you would need to bump something up to the next size and recalculate.

Glen

 

[sparkie]

I have seen this done quite a bit safely and effectively.

For outputs:

AC: Run the hot through a circuit breaker and to your first output terminal hot. Then jumper to your next hot input or use terminal blocks that have busses. Please note that it is very important that you use fused terminal blocks with a fuse for each individual output. Lighted/fused terminal blocks are even better because you can immediately see which fuse is blown. For DC you can do the same, but put a master fuse between your power supply + and first + out terminal block.

For inputs:
You run through a circuit breaker and fuse each individual hot out to field devices and the common will be on the input.

We also tend to use AC on longer runs for our field devices so voltage drop isn't such a huge issue and only use 24V on local panels/systems without long runs.