Lockdown restrictions will also be enforced in the Cardiff and Swansea County areas and L’Nellie.
They will be implemented in Leleneli, Carmathenshire, on Saturday at 18:00 BST and in the two largest cities in Wales from 18:00 on Sunday.
That means let 1.5 million people, about half the population, be under lockdown.
But the first area to lockdown has been a significant drop in cases of carpelli, Health Minister Vahan Gehing said.
The sanctions will be similar to those already affecting people living in McLaughlin Tidefill, Bridgand, Blaenau Gwent, Newport, Ronda Nada Sinan Taif and Kerfili, which are already in lockdown.
Mr Gehing said the people of Cardiff and Swansea “should not take a big hit this weekend”.
He also said that the cases in Carmathenshire are linked to the “overwhelming” Lalenley, there are eight out of 10 cases.
“We have a special challenge at Leleneli that is the good identity of this city, a significant spread that is out of control,” he said at a Welsh government press conference.
“We have talked to local authorities, we have talked to the local health board and we understand the need to take action and act as soon as possible.
“If the same prevalence rates were in Cardiff and Swansea, the infection rates would be much higher.”
The rules in Cardiff and Swansea should come into force a day later, which “allows people to plan”.
In the last seven days, 85 out of 109 cases in Carmathenshire have occurred in Llanley where the infection rate is 151.6 per 100,000 people, compared to 57.7 in the vast county.
Lelenelli Lockdown is the first to not cover the entire local authority area. Mr Cohing said the postcode finder would be made available online to clarify which areas are under lockdown.
Llanelli in most cases engage with socialist people without social distance.
Bury Burt, also in Carmathenshire, will not be included in the restricted area.
What about university students?
As soon as the students returned, Mr Gehing said the impact on university cities would be monitored.
He said: “One thing we need to do is understand how the pattern is changing in those university towns and cities once the students get there.
“With the huge movement of people in the UK at the moment, we need to understand what it does in terms of local pervasiveness so I don’t think it’s time to make a definite choice about what will happen now. It’s your pause, but it’s worth considering. The obvious question is. “
What can i do
Under the rules, eight counties and one city are not able to enter or leave without any “reasonable excuse”.
People are allowed to travel outside the area for limited reasons.
In this they can go to school if they cannot work from home, provide care and go to work if they buy food or medical supplies.
What about unregistered areas?
Ministers and officials said they would look at the data over the weekend before deciding whether to extend the ban on Glamorgan and Torfen’s Well, Nethport talbots.
There will also be a meeting next week with council leaders in North Wales, where Mr Gehing said the picture was “more mixed”.
Only part of Carmathenshire will be locked from Saturday.
Leader Emilin Doll said: “It is worrying to see how quickly the number of positive cases in the Lalenli area has increased, and steps have been taken to break the chain of infection and invasion in this area to prevent an epidemic.
“We should all do the right thing, follow the advice and protect each other. In parts of Leleneli, we call on people and businesses to make even more sacrifices – we fully appreciate the impact, but there is no other way.” Spread should be stopped. “
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