How do you accuratly apply correction factors at the bottem of 310-16? I have 3-500mcm type TW copper with 60degree insulation in conduit rated for 320amps. The wire goes out the bottem of the MDP under the store concrete floor up a coluum in the middle of the store to a penthouse above the store ceiling. The new air handling unit has a MCA of 338. If i have a ambient temp. of 77degrees my wire is now good for 345amps. What is the temp. under store concrete? The store will have A/C set at 74degrees, can i make this legal without pulling new wire? john
Ambient correction
- Thread starter jociha
- Start date
- Status
- Location
- Massachusetts
Re: Ambient correction
How many people derate the wire for 87 C ambient?
How many people derate the wire for 87 C ambient?
pierre
Senior Member
- Location
- Westchester County, New York
Re: Ambient correction
Ambient temperature correction is not necessary until the ambient temps are higher than 86 degrees F or 30 degrees C. Your example is well below and should not be a concern.
Pierre
Ambient temperature correction is not necessary until the ambient temps are higher than 86 degrees F or 30 degrees C. Your example is well below and should not be a concern.
Pierre
- Location
- Massachusetts
Re: Ambient correction
Pierre he wants to rate the wire higher as the temp is less than 77 F
Pierre he wants to rate the wire higher as the temp is less than 77 F
pierre
Senior Member
- Location
- Westchester County, New York
Re: Ambient correction
Thanks Bob, it is not too often that we might go the other direction for ambient correction.
Pierre
Thanks Bob, it is not too often that we might go the other direction for ambient correction.
Pierre
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Ambient correction
The physics of the situation would tell you that it is OK. But the code official might make it difficult for you to take advantage of that fact. You might be called upon to produce an engineering report that includes meteorological data and a calculation method called the "Arrhenius Equation." I think you need to pull new cable or find another solution.
Here is a summary of the physics: High temperatures degrade a cable?s insulation system. But it?s not linear. A rule of thumb is that adding ten degrees to the year-round average ambient temperature will cut the expected life of the cable?s insulation system in half. But that also means that any time that the cable spends under the 86F temperature (i.e., on which 310.16 is based) will put some ?cable life? into a ?life bank,? in that the insulation system degrades more slowly, and will last longer than expected. So if you could produce a reliable history of temperature versus time on a day by day basis for several years, it could be possible to calculate the overall rate of degradation. If this calculation proves that the cable would degrade more slowly than it would at a flat temperature of 77F, then perhaps you could use the 1.08 factor. But I think such a calculation would cost more than pulling new cable.
The physics of the situation would tell you that it is OK. But the code official might make it difficult for you to take advantage of that fact. You might be called upon to produce an engineering report that includes meteorological data and a calculation method called the "Arrhenius Equation." I think you need to pull new cable or find another solution.
Here is a summary of the physics: High temperatures degrade a cable?s insulation system. But it?s not linear. A rule of thumb is that adding ten degrees to the year-round average ambient temperature will cut the expected life of the cable?s insulation system in half. But that also means that any time that the cable spends under the 86F temperature (i.e., on which 310.16 is based) will put some ?cable life? into a ?life bank,? in that the insulation system degrades more slowly, and will last longer than expected. So if you could produce a reliable history of temperature versus time on a day by day basis for several years, it could be possible to calculate the overall rate of degradation. If this calculation proves that the cable would degrade more slowly than it would at a flat temperature of 77F, then perhaps you could use the 1.08 factor. But I think such a calculation would cost more than pulling new cable.
- Status