Current Capacity of short AWG 4 wire

[Cobul]

An AWG 4 wire has rating of 85A for 75C temp.

If there was only 1 foot of AWG 4 connecting the bus bar and the electric meter. How much current can the 1 foot take?

How is the 85A for 75C temp measured or computed?

 

[davenn]

Hi there
Welcome to PF

If there was only 1 foot of AWG 4 connecting the bus bar and the electric meter. How much current can the 1 foot take?

Why would you think it is any different ?
it's the diameter NOT the length that is important

 

[davenn]

Google is your friendIEE Current Ratings Regulations Table 4E1A (cse-distributors.co.uk)

Wire Capacity Chart (rowand.net)

American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength (powerstream.com)

 

[Cobul]

Why would you think it is any different ?
it's the diameter NOT the length that is important

A busbar has metal enclosure box. And right below it is the electric meter with another larger metal conduit. So it can dissipate heat better than the smaller feeder lines going inside the home.

So I was wondering if the 85A at 75Celsius is measured in the feeder lines going inside the house and not inside big enclosure.

 

[DaveE]

This all depends on the temperature of the buss bar and meter connections. Most ampacity tables assume an infinite length. In other cases it is a thermal problem that must be calculated to account for the particular circumstances (or measured).

edit: My intuition says 1' is long enough that the center will get almost as hot as a long length. In any case why do you care, it's only 1 foot, just use the big wire.

 

[DaveE]

So I was wondering if the 85A at 75Celsius is measured in the feeder lines going inside the house and not inside big enclosure.

The temperature limits are based on the temperature of the wire insulation. The place you should measure the temperature is the hottest place that has insulation. The ampacity tables are a convenient short cut, the real test is to not ever (anywhere) exceed the insulation temperature rating in worst case conditions. But in practice, people just use the tables. The big distinction is in cases where the tables aren't applicable, then you have to test.

 

[davenn]

So I was wondering if the 85A at 75Celsius is measured in the feeder lines going inside the house and not inside big enclosure.

it's a wire rating determined in the lab/factory

did you not read my links ?

 

[Cobul]

Say. In the United States (or other countries), how can you put feeder lines for additional meter if the house has already been built? Was it required the feeder lines need to pass through underground before it crept up hidden in concrete walls?

 

[hutchphd]

My feeder line come off a phone pole. The power company will be interested.

 

[DaveE]

Say. In the United States (or other countries), how can you put feeder lines for additional meter if the house has already been built? Was it required the feeder lines need to pass through underground before it crept up hidden in concrete walls?

In Silicon Valley, you hire an electrician who works with the city building department, and perhaps the local utility (if you can't use a subpanel). This is the quintessential example of NOT a DIY project, no matter how much you know.

Our electrical service also comes from telephone poles.

 

[Cobul]

In Silicon Valley, you hire an electrician who works with the city building department, and perhaps the local utility (if you can't use a subpanel). This is the quintessential example of NOT a DIY project, no matter how much you know.

Our electrical service also comes from telephone poles.

I don't connect anything myself. First we have to secure electric permit, then hire an electrician to install it. But then the existing concrete building has the existing feeders already inside the concrete. And it is difficult to replace the wire going to the third floor. So I wonder if the feeder line to third floor can be put outside building. I want AWG 2 instead of AWG 4 to third floor. Once feasible, then I have to hire electrical engineer to draw me a plan. If you have typical house with feeder lines going to service drop at main entrance. It's no problem, but I'm describing a concrete building office, not a house.

 

[DaveE]

Ask your building department, or a local electrician. They might object to two feeders to one building. That's unusual and they may require a single service disconnect. Except for architectural concerns (i.e. ugliness), which is a thing here, I don't think they care where you run your conduit if it's the right stuff installed the right way.

 

[Babadag]

One foot length wire is cooled through end contacts, indeed, but it is difficult to take it in calculation.
3*4 awg copper conductors, 75oC insulation, rated ampacity of 85A it is according to NEC table 310.15(B)(16) for:
Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (directly buried).
If you have a single wire in open air Table 310.15(B)(17) is suitable then 125 A is the ampacity for 75oC insulation and 30oC air.
In the case of 3 loaded wires and 40oC air according to Table 310.15(B)(20) ampacity is 101 A. Both current and air temperature have to be measured.

 

[anorlunda]

Say. In the United States (or other countries), how can you put feeder lines for additional meter if the house has already been built?

That is not the kind of question where you can get good answers on the Internet. The answer depends too much on local codes, laws, and practices. Once you hire the qualified engineer, he should be able to tell you what is possible.

 

[berkeman]

Say. In the United States (or other countries), how can you put feeder lines for additional meter if the house has already been built? Was it required the feeder lines need to pass through underground before it crept up hidden in concrete walls?

I don't connect anything myself. First we have to secure electric permit, then hire an electrician to install it. But then the existing concrete building has the existing feeders already inside the concrete. And it is difficult to replace the wire going to the third floor. So I wonder if the feeder line to third floor can be put outside building. I want AWG 2 instead of AWG 4 to third floor. Once feasible, then I have to hire electrical engineer to draw me a plan. If you have typical house with feeder lines going to service drop at main entrance. It's no problem, but I'm describing a concrete building office, not a house.

Ask your building department, or a local electrician. They might object to two feeders to one building. That's unusual and they may require a single service disconnect. Except for architectural concerns (i.e. ugliness), which is a thing here, I don't think they care where you run your conduit if it's the right stuff installed the right way.

That is not the kind of question where you can get good answers on the Internet. The answer depends too much on local codes, laws, and practices. Once you hire the qualified engineer, he should be able to tell you what is possible.

@Cobul -- It is good that you are working with your building department and a local electrician on this project. We are not able to help you any more with this. Please keep discussing this with your local officials. Thread is closed.