Derating wire for Heat Trace.

[muckmail]

Wire Derating Question for 8 circuits.


I have 8 circuits going to heat trace.
I have three 30A cb for 3 circuits and five 20A cb for 5 circuits.
They are 120V circuits.

This is a total of 16 current carrying conductors
From my calculations I think I need Use #8 wire for the 30A
circuit breaker circuits and #10 wire for the 20 amp circuit
breaker circuits.

From Table Derating Table_310.15(B)(3)(a) I get a derating
factor of 50% for 16 current carrying conductors.

From table 310.15(B)(16) I get 30A, 90deg THHN for # 12 wire.
From table 310.15(B)(16) I get 40A, 90deg THHN for # 10 wire.

Ok,

For Number #12 wire to 20A CB I take (.8 * 20) to get 16A
From the Tables above (.5 * 30) is 15A
Since 16A > 15A so # 10 (one size bigger) is required.


For Number #10 wire to 30A CB I take (.8 * 30) to get 24A
From the Tables above (.5 * 40) is 20A
Since 24A > 20A so # 8 (one size bigger) is required.

Is this correct?
If not where is my error.
Thank you,

 

[kwired]

Wire Derating Question for 8 circuits.


I have 8 circuits going to heat trace.
I have three 30A cb for 3 circuits and five 20A cb for 5 circuits.
They are 120V circuits.

This is a total of 16 current carrying conductors
From my calculations I think I need Use #8 wire for the 30A
circuit breaker circuits and #10 wire for the 20 amp circuit
breaker circuits.

From Table Derating Table_310.15(B)(3)(a) I get a derating
factor of 50% for 16 current carrying conductors.

From table 310.15(B)(16) I get 30A, 90deg THHN for # 12 wire.
From table 310.15(B)(16) I get 40A, 90deg THHN for # 10 wire.

Ok,

For Number #12 wire to 20A CB I take (.8 * 20) to get 16A
From the Tables above (.5 * 30) is 15A
Since 16A > 15A so # 10 (one size bigger) is required.


For Number #10 wire to 30A CB I take (.8 * 30) to get 24A
From the Tables above (.5 * 40) is 20A
Since 24A > 20A so # 8 (one size bigger) is required.

Is this correct?
If not where is my error.
Thank you,

There is more than one way to calculate this but you still have the right result.

I will add some comments about the installation. If you were to use multiwire branch circuits you could cut the number of current carrying conductors to 8 (or 9 if three phase MWBC's) then your deration values are only 70% and you can still use 10 and 12 AWG conductors, but will need handle ties on the breakers, or multipole breakers to use a MWBC.

 

[hillbilly1]

Be sure to note the maximum OC protection the manufacture's installation instructions state, a lot of them have a 20 amp maximum requirement. CYA on anything to do with heat trace, it does not have a very good track record.

 

[kwired]

I was not thinking about the fact that ground fault protection for equipment is likely going to be required which may complicate the use of MWBC's

 

[muckmail]

Derating wire.What about voltage drops and wire sizing?

Derating wire.What about voltage drops and wire sizing?

What about voltage drops and wire sizing?
I have serveral hundred feet on some of my circuits
so how do I find wire size for voltage drop?
Thank you,

 

[Sierrasparky]

you could use two pole EPD ground fault devices

 

[don_resqcapt19]

What about voltage drops and wire sizing?
I have serveral hundred feet on some of my circuits
so how do I find wire size for voltage drop?
Thank you,

As far as voltage drop, I use an online calculator. You calculate the voltage drop on the actual load and not on the breaker size. If you have a lot of heat trace, that is the load of the trace is close to the breaker rating, you also have to look at the "cold start" current to be sure the breaker will hold on a "cold start".

 

[kwired]

Having to increase conductors for voltage drop reasons does help with deration issues as the conductor is larger than usual to begin with. You do not have to increase size again for heat developing issues vs voltage drop issues.