Contactor Rating for Power Switching

Thread Starter

meowsoft

Joined Feb 27, 2021
494
How can I choose contactor size for power switching safely ?
It is safe to use 95a (AC-3)/25 KW rated contactor for switching 100a 220v circuit in a building mainly AC-1 load ?, between grid source and backup generator
 

Reloadron

Joined Jan 15, 2015
6,136
100 Amp 220 VAC service is 22 KW.

I looked up the CHNT NC1-9504 which is a contactor.
between grid source and backup generator
What you want is a transfer switch designed around transferring power between a mains power source and a backup generator power source rated for the voltage and current you plan to switch. Below is a 220 VAC 100 AMP transfer switch.

Transfer Switch.png

The solenoid coils on this one are 12 VDC. The blue wires beside each solenoid connect to two micro switches. The main actual contacts are configured with a mechanical linkage to make sure only one or the other can engage at any given time. This is always a major consideration when backup generator power is concerned.

As to just a contactor? Yes, I agree with Max, you might get away with it. Normally you want a contactor rated 20% over the maximum load you anticipate.

Ron
 

MaxHeadRoom

Joined Jul 18, 2013
23,793
This is why I mentioned that he might just get away with it on the assumption that although the circuit for the building is rated for 100a, how often does the load reach this, if at all.
But it is still a gamble.
 

Thread Starter

meowsoft

Joined Feb 27, 2021
494
If it is a AC1 load and the 100a is rated load, not necessarily the actual, or at least fluctuating, you might get away with it.
Some sites say it's 115a for AC-1 and 95a for AC-3

I can't find official documents, but some site at British market says that's rated at 115a for AC-1
 

Thread Starter

meowsoft

Joined Feb 27, 2021
494
This is why I mentioned that he might just get away with it on the assumption that although the circuit for the building is rated for 100a, how often does the load reach this, if at all.
But it is still a gamble.
If I use two poles for phase and two poles for neutral, is that safe scenario ?
This contactor is rated for 95a per poles and 45kW for all poles, is that mean I can use two poles in parallel for phase and two poles in parallel for neutral ?, so... load per poles is only about 50a, far from 95a
 

Thread Starter

meowsoft

Joined Feb 27, 2021
494
It should, especially if you could guarantee dry contact switching.
Ok... do you think it's should safe with dry contact ?, also I use two contactor with mechanical interlock and controlled by PLC, and contactor coil plus PLC is powered 24v DC from battery

Wiring :
Contactor - 1
Pole 1, 2 (RST/Top) : Phase - Utility
Pole 3, 4 (RST/Top) : Neutral - Utility
Pole 1, 2 (UVW/Bottom) : Phase - Outgoing
Pole 3, 4 (UVW/Bottom) : Neutral - Outgoing

Contactor - 2
Pole 1, 2 (RST/Top) : Phase - Generator Set
Pole 3, 4 (RST/Top) : Neutral - Generator Set
Pole 1, 2 (UVW/Bottom) : Phase - Outgoing
Pole 3, 4 (UVW/Bottom) : Neutral - Outgoing

If each pole maximum is 95a and 4 poles maximum is 45kW, is that wiring safe for 1x100a load ?
 

Thread Starter

meowsoft

Joined Feb 27, 2021
494
I was called schneider technical services, they tell me, for wiring scheme that I was described, there is difference in time for closing and opening time for difference poles, even in one contactor, if differences per poles is not exceeds 20 milliseconds is OK, if exceeds is can burnt that contactor contacts
 

Thread Starter

meowsoft

Joined Feb 27, 2021
494
But If I my case, I use smart office system, that each circuit branch open and close is controlled by PLC and relay, I can open some non critical circuit branch when switching power, and wait for some seconds, that close that circuits again, so... there not exceeds 95a at switching time, I think 5 seconds delay is enough
 

Reloadron

Joined Jan 15, 2015
6,136
I was called schneider technical services, they tell me, for wiring scheme that I was described, there is difference in time for closing and opening time for difference poles, even in one contactor, if differences per poles is not exceeds 20 milliseconds is OK, if exceeds is can burnt that contactor contacts
That would be true. Though I never looked at it as to switching time and normally relied on a single set of contacts. Also, I never switched the neutral of ground lines, only the hot lines. The transfer switch I posted earlier is switching 240 VAC (phase to phase) or what we call 240 VAC split phase.

Ron
 
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