Current rating based on conditions of use

[mbrooke]

If the installation will be known to have temperatures below 30*C, is it permitted to use 310.15 (B)(2)(a) to increase the current rating of the conductors?

 

[petersonra]

If the installation will be known to have temperatures below 30*C, is it permitted to use 310.15 (B)(2)(a) to increase the current rating of the conductors?

I think so. as long as it is always that temperature. keep in mind there are maximum ratings of OCPD allowed for wires #10 and smaller, regardless of the calculated ampacity.

 

[Carultch]

If the installation will be known to have temperatures below 30*C, is it permitted to use 310.15 (B)(2)(a) to increase the current rating of the conductors?

It is permissible, but in my opinion, I do not recommend it. Most geographical locations have a hot temperature that is at least 30C.

A place you are most likely to see this, is a room that is kept cold by artificial means. Such as a walk-in cold room for food storage. In the event that the refrigeration system fails, this means that the ambient is now no longer guaranteed to be below 30C, and you no longer can take credit for the increased ampacity.

Also keep in mind 110.14(C). You cannot "have your cake and eat it too" with this rule. This rule doesn't require you to correct terminal ampacity for ambient temperature increases. Therefore, you cannot take credit with terminal ampacity for ambient temperature decreases.

 

[mbrooke]

It is permissible, but in my opinion, I do not recommend it. Most geographical locations have a hot temperature that is at least 30C.

A place you are most likely to see this, is a room that is kept cold by artificial means. Such as a walk-in cold room for food storage. In the event that the refrigeration system fails, this means that the ambient is now no longer guaranteed to be below 30C, and you no longer can take credit for the increased ampacity.

Also keep in mind 110.14(C). You cannot "have your cake and eat it too" with this rule. This rule doesn't require you to correct terminal ampacity for ambient temperature increases. Therefore, you cannot take credit with terminal ampacity for ambient temperature decreases.

Bummer.

I was thinking a roof with electric heat coils. With temperatures below 50*C it would be permitted to load the conductors higher at the portion, ditto if the OCPD was on the exterior.

 

[mbrooke]

I think so. as long as it is always that temperature. keep in mind there are maximum ratings of OCPD allowed for wires #10 and smaller, regardless of the calculated ampacity.

True, though motors would be excluded.

 

[mbrooke]

It is permissible, but in my opinion, I do not recommend it. Most geographical locations have a hot temperature that is at least 30C.

A place you are most likely to see this, is a room that is kept cold by artificial means. Such as a walk-in cold room for food storage. In the event that the refrigeration system fails, this means that the ambient is now no longer guaranteed to be below 30C, and you no longer can take credit for the increased ampacity.

Also keep in mind 110.14(C). You cannot "have your cake and eat it too" with this rule. This rule doesn't require you to correct terminal ampacity for ambient temperature increases. Therefore, you cannot take credit with terminal ampacity for ambient temperature decreases.

Does the last sentence mean I am ok?

(C) Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity
adjustment, correction, or both.

 

[kwired]

Does the last sentence mean I am ok?

It means you can adjust ampacity of the conductor based on ambient temperature or number of conductors in the raceway. It does not mean you can change the termination temp rating requirements. So a 75 deg terminal still must have a conductor with 75 degree ampacity table selection though the rest of the conductor can be adjusted from 90 degree column should portions of the conductor be in high ambient situations.

 

[Smart $]

Bummer.

I was thinking a roof with electric heat coils. With temperatures below 50*C it would be permitted to load the conductors higher at the portion, ditto if the OCPD was on the exterior.

Hmmm... that brings up an interesting scenario. The heating circuit would not be active unless inside calls for heat, meaning the ambient temperature outside would likely be not more than cool, if not outright chilly. The temperature adder for raceways exposed to sunlight would only be added to this "active" temperature.

PS: It may help if you give us the specifics rather than dancing around the issue with questions about concepts... :happyyes:

 

[mbrooke]

It means you can adjust ampacity of the conductor based on ambient temperature or number of conductors in the raceway. It does not mean you can change the termination temp rating requirements. So a 75 deg terminal still must have a conductor with 75 degree ampacity table selection though the rest of the conductor can be adjusted from 90 degree column should portions of the conductor be in high ambient situations.

Wait, Im confused. Or maybe not...


It means that the final ampacity can not exceed the 75*C column in 310.15? So anything below 30*C does not apply since adjustment factors can increase the 75*C ampacity?

 

[kwired]

Hmmm... that brings up an interesting scenario. The heating circuit would not be active unless inside calls for heat, meaning the ambient temperature outside would likely be not more than cool, if not outright chilly. The temperature adder for raceways exposed to sunlight would only be added to this "active" temperature.

PS: It may help if you give us the specifics rather than dancing around the issue with questions about concepts... :happyyes:

Kind of what I was thinking - only time this heater circuit is in use is when the ambient temp is low. But more details are needed also. Not all of the circuit may be in a low ambient and 240.4(D) still applies to small conductors.

 

[mbrooke]

Kind of what I was thinking - only time this heater circuit is in use is when the ambient temp is low. But more details are needed also. Not all of the circuit may be in a low ambient and 240.4(D) still applies to small conductors.

Wire sizes are over 10 gauge. The OCPDs are outside where conduit leaves the enclosure, up the wall onto the roof into disconnects and then whips into electric coil packs. The blower and AC would not apply, however the concept would hold for the heaters imo.

 

[Smart $]

Wire sizes are over 10 gauge. The OCPDs are outside where conduit leaves the enclosure, up the wall onto the roof into disconnects and then whips into electric coil packs. The blower and AC would not apply, however the concept would hold for the heaters imo.

Then as stated... but still need specifics to go beyond confirming the concept is sound.

 

[kwired]

What kind of current are we talking about for these heat cables? That is some serious heat cable if it needs more then 10 AWG. Most I know of probably are not designed for more then a 20 amp circuit either.

 

[RichB]

I have always used this site http://www.worldclimate.com/ to get an average ambient for the area I am working in---then use any correction factors, such as for on a rooftop--I am still trying to convince our designers that we can "uprate" our conductors:angel::lol:

 

[mbrooke]

What kind of current are we talking about for these heat cables? That is some serious heat cable if it needs more then 10 AWG. Most I know of probably are not designed for more then a 20 amp circuit either.

They are in the 20 kw+ range.

 

[kwired]

They are in the 20 kw+ range.

I only have 15 kW of backup heat on my heat pump and here you are trying to heat the outdoors in the winter months

 

[mbrooke]

I only have 15 kW of backup heat on my heat pump and here you are trying to heat the outdoors in the winter months

Not a home, commercial property. But the concept would still be the same imo.


So say I had a 3 phase 208 volt circuit for a bank of heat strips totaling 25kw. 70amps per phase, 75*C terminals. 70x125%= 87.5 or 90amps, giving me #3 cu. If I apply a correction factor of 50*F #4 cu I get 102amps. The wire is good, but technically I am on the 90*C column for the terminals so its no to code? :?

 

[Carultch]

Bummer.

I was thinking a roof with electric heat coils. With temperatures below 50*C it would be permitted to load the conductors higher at the portion, ditto if the OCPD was on the exterior.

(You mean 50F. If outside temperatures were 50C, it would be hot as Hades.)

That is a good point. When the natural temperatures are below 30C, by the purpose of the circuit in use, I would agree that you could take credit for the temperature correction that increases ampacity.

It is when you rely upon artificial means to get it cold, for which I would disagree.

 

[mbrooke]

You mean 50F. If outside temperatures were 50C, it would be hot as Hades.

Ooops :lol::ashamed1:

I meant 50*F, or 10*C.

 

[iwire]

So voltage drop and the inefficiencies it brings are of no concern?

Is the goal saving money or copper?