Changing Wire Size on terminals

[DM2-Inc]

Does the NEC limit the maximum difference in wire size, of the same circuit, at a screw terminal? I was told many years ago that it was 2 wire sizes, but I've never found it in the NEC.

We have a situation where the power supply (24 VDC, 75 Amps) is designed for connecting a 4 AWG wire. Some of the devices this power supply is powering are 1,000 to 2,000 feet of cable away. To minimize the voltage drop, I want to connect a 4 AWG wire to the power supply and run about 2 feet to a power distribution block that will allow me to increase the cable size to 350 kcmil at which point It needs to be run about 50ft to a marshaling cabinet. At the marshaling cabinet will go to another power distribution block and get distributed to a series of fuses that provide the power to the field devices. The wire out of the fuse will range from 4 AWG to 8 AWG. The load in the field varies from 1 Amp to 7 Amps. My voltage drop calculations start at 20.1 VDC (85% of the power supply rating...a code requirement). Because I need to have a voltage at the device no less than 18 VDC, I need the huge cable.

I'm also not sure that short circuit calculations wouldn't govern this either.

 

[Buck Parrish]

Does the NEC limit the maximum difference in wire size, of the same circuit, at a screw terminal? I was told many years ago that it was 2 wire sizes, but I've never found it in the NEC.

We have a situation where the power supply (24 VDC, 75 Amps) is designed for connecting a 4 AWG wire. Some of the devices this power supply is powering are 1,000 to 2,000 feet of cable away. To minimize the voltage drop, I want to connect a 4 AWG wire to the power supply and run about 2 feet to a power distribution block that will allow me to increase the cable size to 350 kcmil at which point It needs to be run about 50ft to a marshaling cabinet. At the marshaling cabinet will go to another power distribution block and get distributed to a series of fuses that provide the power to the field devices. The wire out of the fuse will range from 4 AWG to 8 AWG. The load in the field varies from 1 Amp to 7 Amps. My voltage drop calculations start at 20.1 VDC (85% of the power supply rating...a code requirement). Because I need to have a voltage at the device no less than 18 VDC, I need the huge cable.

I'm also not sure that short circuit calculations wouldn't govern this either.

It's all about the UL listing of the screw terminal.
It is common to splice to a smaller wire for voltage drop configurations. (Larger Cable)

Here is the calculator http://www.csgnetwork.com/voltagedropcalc.html

 

[resistance]

At the marshaling cabinet

Sorry, but I'm looking to learn something new today, so I have to ask. What's a marchaling cabinet? Please post a pic!!

In about 6 months-If my business stays afloat-we will be moving into the industrial commercial market, so I'm curious. If all goes well, I will be hiring a highly qualified J-man to handle this portion of the work load. Currently I'm semi-optimistic!

 

[DM2-Inc]

What's a marchaling cabinet?

Sorry...should have been "Marshaling..."

 

[edavis2293]

Marshalling cabinet

Marshalling cabinet

Industrials, refineries, chemical plants. etc, use marshalling cabinets. Basically, a marshalling cabinet is no more than a junction box with several rows of terminal strips to gather all incoming signals from the field for connection by cable to a PLC or DCS. A marshalling cabinet doesn't have to be a box either. I have seen and installed entire walls of a control room with terminals for the same purpose. There are several variations but marshalling is a synonym for junction.