Size of wire needed. Please help.

[ricksan76]

I have a customer that has a 200 amp service at the rear of his home. He is going to be doing a addition. Because of that he wants to move the meter to the side of the house. So, on the side of the house there will now be just a meter, with a 200 amp breaker which will be feeding the existing 200amp panel that feeds the entire house. Now for the question he want to run the wires on top of the roof in EMT. Our ambient temp is 111 F here in Palm Springs CA. What size wire would I be using, since it is on the roof? If you could please show the calculation, this would be greatly appreciated. Service will be 120/240 single phase.

 

[ricksan76]

Just a little more information. The distance from panel to panel is on 75'

 

[cpinetree]

Not a diect answer, but try this

Not a diect answer, but try this

See if this will help: http://www.electrician2.com/calculators/wireocpd_ver_1.html

You will need to supply additional information, loads, cu or al, type of wire insulation etc.

The above calculator can figure it, if you supply the numbers.

Hope it helps

 

[kwired]

Interesting question.

In most cases you would have to derate the conductor for ambient temperature and could end up with a pretty large conductor.

Since this is feeding (an entire) dwelling unit though, you are permitted to use 310.15(B)(7) to select the conductor size, but conductors selected from that section is not subjected to any ampacity adjustments and could be a 2/0 copper or 4/0 aluminum conductor for 200 amps.

 

[kingpb]

Interesting question.

In most cases you would have to derate the conductor for ambient temperature and could end up with a pretty large conductor.

Since this is feeding (an entire) dwelling unit though, you are permitted to use 310.15(B)(7) to select the conductor size, but conductors selected from that section is not subjected to any ampacity adjustments and could be a 2/0 copper or 4/0 aluminum conductor for 200 amps.

All that you stated is 100% correct, however; I would have to invoke 90.1(B) and say that consideration for temperature should be made for convenience and adequacy for good service; i.e. happy customer, reputation, and no call backs.

I believe by derating all you need to do is jump to a #3/0 AWG instead of a #2/0 AWG. This is based on the #2/0 90 deg C column, which is consistent with the amperage of Table 310.15(B)(7), i.e. 200A. Then using the temperature derating of 0.87 (111 deg F - 44 deg C) which means you need #3/0.

With all that said, I would probably use a #4/0 AWG and sleep well at night.

 

[resistance]

I agree with others for the most part, but I'm not sure why the OP want try doing the calculation (at least make an attempt). If you get stuck, or feel your numbers are wrong, then I?m (and I?m sure others) here to help. Hope i?m not breaking the rules!?

 

[Dennis Alwon]

If the OP is on the 2014 that changes things as now it is a percentage method. So ambient temp and any other derating must be considered.

310.15 (B)(7) 120/240-Volt, Single-Phase Dwelling Services and
Feeders. For one-family dwellings and the individual
dwelling units of two-family and multifamily dwellings,
service and feeder conductors supplied by a single-phase,
120/240-volt system shall be permitted be sized in accordance
with 310.15(B)(7)(1) through (4).
(1) For a service rated 100 through 400 A, the service
conductors supplying the entire load associated with a
one-family dwelling, or the service conductors supplying
the entire load associated with an individual dwelling
unit in a two-family or multifamily dwelling, shall
be permitted to have an ampacity not less than 83 percent
of the service rating.
(2) For a feeder rated 100 through 400 A, the feeder conductors
supplying the entire load associated with a onefamily
dwelling, or the feeder conductors supplying the
entire load associated with an individual dwelling, unit
in a two-family or multifamily dwelling, shall be permitted
to have an ampacity not less than 83 percent of
the feeder rating.
(3) In no case shall a feeder for an individual dwelling unit
be required to have an ampacity greater than that specified
in 310.15(B)(7)(1) or (2).
(4) Grounded conductors shall be permitted to be sized
smaller than the ungrounded conductors, provided that
the requirements of 220.61 and 230.42 for service conductors
or the requirements of 215.2 and 220.61 for
feeder conductors are met.

 

[kingpb]

Dennis, I see the 83 percent, but based on what?

 

[resistance]

Dennis, I see the 83 percent, but based on what?

83 percent of
the feeder rating.

 

[ricksan76]

Thank you for all of your responses. This is for 2014 NEC. Greatly appreciated. I believed I could use 310.15B7, but was told by another EC that I could not. That got me confused, and I wanted to make sure I was on the right page.

 

[kwired]

Dennis, I see the 83 percent, but based on what?

I see it as meaning the conductor must have an ampacity of at least 83 percent of the overcurrent device, meaning ampacity adjustments are included in determining that ampacity just like for other applications. A 100 amp feeder would need an adjusted conductor ampacity of at least 83 amps.

If that was not the intent then why the change? Possible the wording still isn't quite right but I think the intent probably was to figure in ampacity adjustment where necessary in this kind of situation. The way it was before - and for a very long time gives us exactly what I mentioned earlier. Size the conductor with 310.15(B)(7) we get a relatively small conductor, but don't use (B)(7) for the same load in a situation where adjustments are needed and you could have a fairly significant increase in conductor size in some instances.

 

[kingpb]

If that was the intent, I think they missed the mark with the wording. As particular the NEC is about wording, it just makes you wonder. More for the AHJ to interpret I guess.

 

[Sahib]

The ambient temperature inside the EMT in the condition of the OP is higher than the outside ambient temperature during peak summer and it depends on how the conduit is installed:directly on the roof or elevated, color of the roof:dark or light. So 111 degree F ambient derating is not correct.

 

[Sahib]

In the worst case scenario add 60 degree F to 111 degree F and take 171degree F as the ambient temperature for derating purpose.

 

[kwired]

In the worst case scenario add 60 degree F to 111 degree F and take 171degree F as the ambient temperature for derating purpose.

That is fine for usual ampacity selection assuming the ambient temp of 111 has some validity, problem is if sizing conductors according to 310.15(B)(7) which allows smaller conductors if conditions are met for dwelling unit feeds did not take ambient temp into consideration. But has been brought up that 2014 NEC does address that issue now and has made changes to that section. That still kind of means nothing though if you are not on the 2014 NEC yet.

 

[Sahib]

That is fine for usual ampacity selection assuming the ambient temp of 111 has some validity, problem is if sizing conductors according to 310.15(B)(7) which allows smaller conductors if conditions are met for dwelling unit feeds did not take ambient temp into consideration. But has been brought up that 2014 NEC does address that issue now and has made changes to that section. That still kind of means nothing though if you are not on the 2014 NEC yet.

So ultimately the house owner is going to be the loser in the very likely case of having his roof top in conduit conductors insulation damaged by peak summer temperature.

 

[kingpb]

So ultimately the house owner is going to be the loser in the very likely case of having his roof top in conduit conductors insulation damaged by peak summer temperature.

Only if the installer follows minimum requirements; which are not at all consistent with good design.

 

[kwired]

So ultimately the house owner is going to be the loser in the very likely case of having his roof top in conduit conductors insulation damaged by peak summer temperature.

Only if the installer follows minimum requirements; which are not at all consistent with good design.

What kingpb said, plus we can also consider that many supply conductors for dwellings are sized well above the amount of load that is actually seen in many cases. Time and heat will still possibly catch up with these conductors but consider that many 200 amp dwelling supplies often don't get loaded to more than maybe 70 amps, and if they do it often is not for very long.

 

[Sahib]

What about reducing the size of the service OCPD for such specific cases taking into account the adjustment factor for roof top conductors. Of course minimum service size is 100A for residence, a bottleneck.

 

[kwired]

What about reducing the size of the service OCPD for such specific cases taking into account the adjustment factor for roof top conductors. Of course minimum service size is 100A for residence, a bottleneck.

That is just a different way of saying "derate the conductors" Problem is they still need to have enough ampacity to carry the calculated load. But there are many dwellings that are supplied with 200 amp supply that probably could get by on less. In general 150 amp supply cost about the same as 200, so many just go with 200 to leave a little extra capacity just in case or for sometime down the road.