Relays for product with high inrush current


I am looking for a mechanical relay to replace a SSR we are using today, our product contains three different SMPS and have an inrush current that peaks just below 80A when measured with a hall sensor. The steady state current is a maximum of 8A.

So far I have found G4A-1A-PE DC5 that has 80A inrush listed in the datasheet and ALF1P05 with 100A inrush. I know there are also some relays advertised for EV chargers but they seem pretty expensive.

I have also looked at G4W-1114P-US-TV8-HP-DC24 and T92S7D12-24 with TV-8 and TV-10 ratings, but as I understand it even though the TV- rating does state an inrush current it is with 120VAC and our product is used with 240VAC, is there any derating i can apply for 240VAC?

And are there any alternative relays that has a decent stock that i should look at?

I have similar need. To that end, I am wondering how to interpret the specs of relays that are intended for motors. Those have a locked rotor current rating. I wonder if I can use that as an inrush spec if used with power supplies.

I’m finding it inconvenient to track down an authoritative definition of those ratings, but from what I gather it’s based based on empirical testing with an incandescent lamp load for a relatively low 25K cycle life.

Electronic loads such as SMPS generally present a fair bit of capacitance at their inputs . That can look a lot like a short circuit while the contacts are bouncing during closure, with current flow limited in large part by parasitic resistances and inductance in the wiring. It doesn’t take much imagination to see how melting a puddle on the contacts and then sticking them together could lead to rapid wear… It’s a different animal compared to a lamp load (surge due to lower resistance while filament heats, but not much reactance) and motor loads, which involve an inductance-limited startup surge, and some ugly arcing potential when interrupting a circuit due to that same inductance.

Since the energy that must be transferred to a capacitor to charge it scales with the square of the final voltage, it would seem reasonable as a starting point to expect current ratings to scale by the inverse of that, e.g. a contact good to 30K cycles for 80A @ 120VAC might do OK with 20A @ 240VAC. Actual results will almost certainly deviate somewhat and probably not to one’s favor.

Taking the G4A series as an example, there’s a few different reference points mentioned;

  • a “normal” motor on 250VAC with startup surge but turn-off at modest load and decent power factor (200K cycles, not bad)

  • an “inverter” load (something with a capacitor up front) sucking 200 amps on make and 20 under normal load ( down to 30K cycles at only 100VAC… Ouch)

  • an “overload” (locked rotor) condition at 250 VAC where turn-on & turn off occur with the same nasty 80A and 0.7 p.f. conditions. (only 1.5K cycles, more than 100x worse than the “normal” conditions)


There’s a lot to be said for a zero-cross SSRs in terms of switching inverter loads. The on-state losses aren’t as good as a mechanical relay, but with some (careful) planning it’s possible to use them in parallel to get advantages of both.

At day’s end, I don’t think there’s a good substitute for qualifying a product in context of one’s own application in this sort of situation. 30K cycles at 8 seconds per takes only about 3 days…

Also, don’t forget to open the vent.


Thank you for your thorough answer.
Yes ive seen the same in some datasheets ive been looking at like these ones.

Our product is used in hard to reach locations and are not service friendly at all but I have to option to service them at something like a 2 year interval that equals somewhere between 1500-7500 cycles.

I am kind of leaning towards just using a different SSR with zero cross as we are using today, even though we wanted to switch to mechanical relays for their availability. But it seems that the kind of relays we would need have about the same availability as the other SSRs I have found and the cost savings would be eaten up by the servicing cost.

And thank you for the tip about venting, I have been looking at so many datasheets but ive completly missed that some of them requires venting.

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Thanks Rick - Very useful.

I did some research on HVAC motors used in home furnaces, since there are many relays designed for these.
Many of these motors use capacitors. And the properties of a motor are different before the magnetic fields are established. This results in an inrush at startup.
Typically these use 5A to 10A running, with inrush of 6 to 10 times that.
However, the duration of the inrush might be shorter than for SMPS.

My project is one-off at home, so I’m going to skip lifecycle testing.

It’s used with a light switch, so cycles should be less than a furnace.
Since I don’t know the inrush duration (and relay’s only $22 ), I’ll try a 32A (380 at make) relay for a 5A SWPS (80A max inrush)

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Same problem want to go to a Relay as the SSR and the Photovoltaic drive creates problems

back ground:
Solar 48 volt system with 5 arrays ~14 KW System has fork-lift battery’s ~100 KWh
(2500 A) and LiFeP04 pack (200 amp)
So its been installed since 2012 added the Li Fe in 2014 and then used [3] 80 amp SSR
hooked directly to a solar charge controller SCC this worked for about maybe 2 years.

The main battery [LA] will charge up to ~60-64 volts depends on temperature, but that will over-charge the Lithium pack, it must not go above ~57 volts so and automatic switch AUX is used by the SCC it works perfect. Limit is the volts 12 and amps .200

The max breaker is 100 amps for the panel is also problem (limit on Lithium pack only)
battery’s have supercaps, the ones on the lithium pack have higher ESR ~0.01 Ohm and the inrush is high, mostly if it real sunny as the Lithium pack is not charged yet as the 100 amp breaker had/has a smaller cable #4 limit the amps to ~80. or breaker flips off.
Then the volt drop I see around 53V-57v when turned back on.
or at least 4 volt difference 4 / .01 = 400 amps cause of the ESR of supercaps…

Most likely not turning on at exact same time… maybe

seems I can not find a relay input 12 volts at less than 200 milliamps.
load 400 amps volts are low less than 10. the high load is very short time say 14 sec.

Hello machinationus,

Using the #4 cable as a series current limiting resistor is not recommended. My fear is that a sustained equipment fault could cause the wire to overheat and potentially cause a fire. Stated another way, the wire burns to protect the circuit breaker.

Have you considered using a step start process:

  • Step 1: we close a small relay allowing the initial capacitor charging current to pass though proper current limiting resistors.

  • Step 2: After a suitable time delay, e.g. 10 to 20 seconds, we close the primary contactor effectively bypassing the current limiting resistors.

  • Step 3: Close the output contactor thereby connection the super capacitors to the load.

The advantage of this approach is that we can use contactors designed to withstand the 100 A current limit as opposed to the full surge of the super capacitors.

Best wishes,


sorry maybe I confused you
The supercaps are on each cell, or part of battery.
I really don’t like using a resistor as it will waste a lot of power.

would this work I not sure about the hook-up
thank you

hello @machinationus

The Altran AEV250-G is a capable contactor. It can easily handle the 100 A current as described in your system.

Recommend consulting with a licensed solar installer for assistance in configuring the system. In theory, an application of 48 ~ 72VDC to the coil will close the contactor. In practice, it may be prudent to add additional monitoring the control circuitry.



I got the latest data sheet but will have to contact them as no hook-up shown
they talk about a AUX and some info about the coil amps that is confusing…
Thank you

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Hello @machinationus

The auxiliary contact is typically used by external control and monitoring circuitry. This is not applicable for the AEV250-G as it does not include the optional “A” or “B” suffix in the part number.

And yes, the datasheet can be difficult to interpret as it includes dynamic operation of the contactor. For example, the chart on page 4 shows the time-limited (overcurrent) capacity when the contactor is used to drive a capacitive load. As you pointed out in your initial note, time is an important aspect to prevent nuisance tripping of circuit breakers and for long equipment life.

Please let us know how we can assist.