PCB trace width question

I have a relay on my board, rated for 10 amps AC. I want to turn a lighting contactor on and off. i has a 120V coil. the AC coil has a 350VA inrush current and then 50VA holding.

the trace width on the board is 3mm (2oz) and should be good for about 4.25 amp.

my question is if I should put traces on top and bottom of the board.

I don't see why you would need traces on top and bottom. I didn't verify your width calc, but if it's correct, you have room for 4.25 amps and your peak draw is just shy of 3 amps, and that is not even a continuous draw.

Are you asking bc you want the extra amperage or because you want isolation. For isolation with 120VAC, you only need 0.6mm (0.024 in) separation to avoid creepage, and even less if the board is coated.

the application is to energize a coil for a lighting contactor. it is an AC coil.

currently, I have 2mm separation between traces and also between the next closes 5vDC pad.

How much current is needed? 2mm wide trace, about 78 mil.

Can put your specifics in a calculator and see how current the trace will be capable of. http://www.multek.se/engelska/engineering/pcb-structures-2/trace-width-calculator

350VA inrush on a 120VAC line.

350 / 108 (assume 10% reduction) = 3.2 amps

trace length is just under 1 inch.

assuming a 10% higher draw, would put me at about 3.5 amp.

different calculators show different results, the one offers that my 3mm wide, 2oz copper trace would handle about 4.4 amps.bout 20% over my estimated needs.

as we know, most things don't fail within the expected zones, but when a series of events pushes them outside of the expected parameters.

couple years ago, I had to put in some 200 amp 3 phase transformers to handle a continuous brown out condition in Brooklyn for HVAC equipment. too far down the line from con edison generating facility. the trick was to handle the low side so as to not burn up the motors and to handle the high side in the event power did come up to full voltage. IIRC, we dipped into the low 180's on the 208 line and saw 205 as the peak. when you do your homework, and actually solve the problem, it fades from memory and never comes back to bite you.

I am not a big fan of putting 120 volts AC on a pcb with an Arduino.

My favourite for beefing up a trace is to just puddle solder along the whole trace. Specify the soldermask layer to leave this trace un-masked.

For bigger current, lay a copper wire along the trace and solder that down all the way along.

MorganS:
My favourite for beefing up a trace is to just puddle solder along the whole trace. Specify the soldermask layer to leave this trace un-masked.

For bigger current, lay a copper wire along the trace and solder that down all the way along.

I was thinking about doing this, or even adding more copper. make it 4oz.
I think for the first run I will go with the un-masked and see how much I can push it till it fails.
The funny part is that we will use this part of the board more in the first two days of commissioning, that means 5 or 6 times. than in the next 5 years.

I forgot to mention that if you are using a 10 amp breaker, you need to design the trace for 10 amps.

You might want to drop to a 5 amp breaker and go with the 4 ounce copper, or make it wider if you can. I've done traces for 10 amps on 2 ounce copper. I think they were .25".

Also, I've never done an uC on the same board as 120v. I've always done 2 separate boards. Mainly bc I didn't want to have to troubleshoot some stray signal.

I went with a 5V relay board whose contacts provide power to the contactor's coil. - Scotty

scottyjr: I went with a 5V relay board whose contacts provide power to the contactor's coil. - Scotty

that is what I am doing now. I am trying to re-design for an on-board relay to reduce size, but as pekasus said, circuit protection should be the lowest value, everything, all wires, connectors, traces, etc should be rated higher that the circuit. more to ponder

Just figure out what gauge wire will carry the load, and solder it down to the trace, being sure that you solder right at the ends so that the trace isn't carrying the load for any distance.

jrdoner: Just figure out what gauge wire will carry the load, and solder it down to the trace, being sure that you solder right at the ends so that the trace isn't carrying the load for any distance.

I think that I could put in an extra pad at each end of the trace. as a frame of reference, here are the two traces. then feed the wire and solder it in place.

Thanks for the picture. That is helpful. It looks like a two sided board. Are the traces requiring reinforcement on the bottom, non-component side? It is easier if they are.