Recommended track width/clearance

Community PCB Design
1

I am working on a project that will allow me to control a relay with an Atmega328. I have the code mostly completed and have started thinking about the PCB design and layout. I’m running into to a problem because I intend to have the relay controlling 110V AC at around 15 amps. This is a PCB mount relay sold by SFE (http://www.sparkfun.com/products/10924).

How large should I make my tracks that carry this AC voltage and amperate? I am wanting to build bigger if I can to plan for 20 amp max, but 15 continuous. Additionally I am wanting to use stranded hook-up wire and need a gauge recommendation.

Does anyone have recommendation for track size, spacing between the AC tracks, and distance from the low voltage tracks?

2

You’re going to need some thick traces! Maybe you can duplicat the track on the top and bottom of the board to make the trace size smaller. I’m not sure if that is entirely feasible for a pcb.

3

Standard 1oz copper PCB isn’t thick enough for 15A (trace thickness, not width). I’m not sure if even 2oz plating is enough. You’re better off using a relay with spade or screw lugs and leaving it off the PCB, or using a Triac. Somewhere I saw a web page that showed the calculations for max current for .5, 1 and 2oz copper pcbs.

You need 12GA wire for 15A, stranded or solid. 10GA for 20A.

Going to 20A presents another problem. The maximum permitted current for a “normal” house outlet is 12A, even though the outlet is rated for 15A. If you want 20A, you need a dedicated breaker and outlet.

Spacing between the AC tracks can be small, .1" or so, same with spacing between AC and DC tracks.

4

There are lots of PCB trace calculators out there on the web. You input Cu weight on PCB, length, current, voltage, acceptable temp rise, and it tells you the recommended trace width.

One is at

http://home.comcast.net/~pcb.george/trace.html

5

The answer to your questions are unfortunately not trivial. Especially in the case of PCBs as traces will dissipate heat into the PCB and it will be cooled by convection. You must also decide what is a “safe” temperature to operate at. Many other factors contribute to estimating the correct trace size for a specific current.

Regarding the spacing of your external traces the standards say (for 110V) that PCBs with no solder mask should be spaced 0.6mm apart, with solder mask 0.4mm. That is assuming your application altitude is less than 10,000 feet. Having said that more space is always better!

The question of how wide to make traces to handle a specific current load is not trivial. This will depend on the thickness of your copper on the PCB and the temperature rise you can accept.

Assumptions:

External Traces

PCB Substrate is greater than 0.8mm thick

Copper Thickness is less than 108um

Temperature rise from ambient @ STP

Here is the temperature rise for specific cross sectional area for 20 A:

1 oz copper (1.4 mils)

100 C 250 Sq. Mils

75 C 300 Sq. Mils

60 C 350 Sq. Mils

45 C 400 Sq. Mils

30 C 500 Sq. Mils

20 C ~700 Sq. Mils

So for a reasonable temperature rise (30 C) you can calculate the size of the trace. Add a 10% derating to account for copper thickness, conductor width estimates, and cross-sectional area.

1 oz copper (1.4 mils) - 360 mils + 10% = ~400 mils

2oz copper (2.8 mils) - 180 mils + 10% = ~ 200 mils.

Regarding the Stranded wire gauge. You will have to calculate the equivalent cross sectional area. The size of a stranded wire is defined in the following way: 18AWG 16/30. The overall diameter of the wire is AWG 12 (2.05mm) and it is made up of 41 strands of AWG 30. AWG 30 has a 10 mil diameter so the cross sectional area is 0.05mm. Multiply that times 41 and we have our equivalent cross sectional area 2.05 mm^2 . That is about the same size as 14 AWG (2.08 mm^2) solid wire. (I know it looks strange because of the relationship of the cross sectional area to diameter of 14 and 12 gauge).

Now that you have your equivalent wire size, you must once again consider the temperature rise. Checking the National Electric Code will tell you that for a wire with insulation rated to 90 C you can run 14 amps through the stranded 12 gauge wire (solid 18 gauge). The NFPA (they write the NEC) has determined what they deem safe. You can find all the info on current carrying capacity for solid copper wire on wikipedia.

http://en.wikipedia.org/wiki/American_wire_gauge

For long runs you will also have to consider the distance you will run your wire. 14 Gauge copper has a resistance of 2.525 mili Ohms per foot. The longer your run the wire the less power you can deliver.

Keep in mind this only holds for copper wire. Aluminium wire for instance would have a different diameter for a given AWG. AWG is the resistance per unit length in decibels. AWG = 10 x log10(R) + 10.

Hope that helps.

6

You may want to consider copper pours instead of traces for this. Locate the relays and mains circuitry in a tightly confined area of the board to minimize trace lengths. Make sure you provide fusing.

As for clearances, when dealing with mains, be very careful. Consider consulting standards such as IEC 60950 to determine minimum safe creepages and clearances between points of opposite polarity and between mains and the enclosure, the logic/control circuitry etc for ITE equipment. A good rule of thumb for mains circuits is to allow 8mm of creepage between primary and secondary circuits and 4mm between mains and the ground. For creepages between parts of different polarity in the mains circuit consider 2mm. You can mitigate some of these creepage distances by putting slots in the board that are more than 1mm wide…

7

Sorry for the late responses, just getting back into the swing of things after the new year.

There is definitely much more information regarding this topic than I had anticipated. This is definitely something I want to do right and think a pre-designed solution may be the direction I should go in the interest of safety. For the next time I have a wealth of new information to analyze and learn from.

Has anyone reviewed the SparkFun power control tutorial? http://www.sparkfun.com/tutorials/119

If the eagle files they provide are considered safe I think I will go that route and see about having BatchPCB make me up a couple.

8

For higher powered applications, it’s not uncommon to solder a physical wire in place of a PCB trace to handle currents that would otherwise lead to unrealistic trace widths.

-Bill