Wire Gauge

Hi all,

Finally wiring together my beautiful looking electrical panel.
I started wiring carefree when I realized I oughta pay some attention to my wire sizes.

My electrical panel is pretty simple.

I have 120VAC input from a wall that is in series with an e_stop and power switch, and then a power distribution block. The 120 vac powers an hmi, 12 volt power supply, and 24 volt power supply.

the 24 volt power supply powers a micrologix 1000. there are no inputs, the hmi is connected via an allen bradley cable, and software memory is used to communicate between the touch screen and the controller.

the 12 volt power supply powers a fuse box that is used to vary the amperage of four different relays.

The AB controller has 4 outputs, each going to a different relay. The relays are grounded. The relays output to a light, because the light is subject to vary, and the power will vary, we have the fuse box to change amperage.

I realize this is not enough information for you all to tell me what wire gauges to use. But how do I figure out what gauges to use? I'm not to sure the range of amps the lights...is this what i need to know to answer my question?

I realize this is not enough information for you all to tell me what wire gauges to use. But how do I figure out what gauges to use? I'm not to sure the range of amps the lights...is this what i need to know to answer my question?

Click to expand...

Amperage the wire will carry and length of wire ( to long will cause a voltage drop) determine the gauge needed.

a fuse box that is used to vary the amperage of four different relays.

Click to expand...

Fuses do not change (vary) the amperage they limit the amperage by breaking if their limit is exceeded.

This is the chart our panel shop goes by:



From UL 2011

Mickey said:

Amperage the wire will carry and length of wire ( to long will cause a voltage drop) determine the gauge needed.

Click to expand...

Length wont be more than 5 feet....at most 10.

Each wire should only carry 3 amps i believe. fuse box (i now understand but didnt minutes ago) will not let amps exceed 3.

So would 18 gauge be appropriate?

Gomez_ said:

This is the chart our panel shop goes by:

From UL 2011

Click to expand...

so then yes 18 would be fine and possibly 20. Ill do 18 to be safe if that makes sense?

18 is pretty common for our control wiring.

thanks gomez and mickey.

Does it ever hurt to use a wire thicker than needed?

Is your AC line(cord) protected with a fuse(other than 15 or 20 amp breaker at building distribution box)?

Typically our panels (and what I have seen in other panels) control wiring is 16 THHN. A 10A CB on the line side is typical then branch off proper sized fuses or CB based on the devices in the circuit. Branch wiring remains 16.

Same is typically used in the field if running conduit. If we get into low-voltage proximity sensors and such we may use a multi-conductor cable w/smaller wire gauge from the IO to the device in the field.

I would choose to standardize control wiring and not change it unless of course your building something other than 120VAC/24VDC panel.

EDIT: Larger wire than needed can be a hassle if you have devices with smaller terminal blocks.

khewes said:

thanks gomez and mickey.

Does it ever hurt to use a wire thicker than needed?

Click to expand...

No...sometimes better to use 1 size over required size. This may save you in the long run.

There are certain gotcha's in wire sizes.
A lot of circuit breakers have a 60 degree rating on the lugs, sometimes 75, but never 90.
Which means if you size using a 90 degree wire (like MTW, THWN or THHN), you need to use the next size (or 2 sizes) up to cover the 60 degree rating requirement.
60 degree wire is like TW and 75 degree wire is THW or THWN.

Paully's5.0 said:

Typically our panels (and what I have seen in other panels) control wiring is 16 THHN. A 10A CB on the line side is typical then branch off proper sized fuses or CB based on the devices in the circuit. Branch wiring remains 16.

Same is typically used in the field if running conduit. If we get into low-voltage proximity sensors and such we may use a multi-conductor cable w/smaller wire gauge from the IO to the device in the field.

I would choose to standardize control wiring and not change it unless of course your building something other than 120VAC/24VDC panel.

EDIT: Larger wire than needed can be a hassle if you have devices with smaller terminal blocks.

Click to expand...

We do the same with our panels. I can't remember if its 16 AWG or 18AWG that we use standard. We don't use THHN, just MTW in our panels.

We do no use anything smaller than 14AWG on 480V wiring however.

Gomez_ said:

This is the chart our panel shop goes by:

From UL 2011

Click to expand...

NFPA 70 derates some of the conductors in your list (just happens to be the wires commonly used n control panels) as far as maximum overcurrent protection. See 240.4 (D).

18 = 7 Amps
16 = 10 Amps
14 = 15 Amps
12 = 25 Amps
10 = 30 Amps