15KW Heat

[wmeek]

So When sizing Conductors for 15kw heat with a 3.9A Blower motor.
You Have 15000/240=62.5A + 3.9A = 66.4A x 125%= 83A

Conductor is 83A = #4 Thhn
OCP is 83A so Code says if not a standard size fuse you need to go up to next standard size.(240.4B) 240.6A)
With all that said is it all right to run a #4THHN and install 90A Fuses.
Also I am running one feeder and using the tap rule to tap a #10 thhn for 5kw heat strip on 30A breaker in unit and #6 thhn for 10KW heat strip on 60A breaker in unit. I do not have room to install a panel and use branch circuits to unit and not comforable with the tap rules
Is this all code compliant

 

[Carultch]

So When sizing Conductors for 15kw heat with a 3.9A Blower motor.
You Have 15000/240=62.5A + 3.9A = 66.4A x 125%= 83A

Conductor is 83A = #4 Thhn
OCP is 83A so Code says if not a standard size fuse you need to go up to next standard size.(240.4B) 240.6A)
With all that said is it all right to run a #4THHN and install 90A Fuses.
Also I am running one feeder and using the tap rule to tap a #10 thhn for 5kw heat strip on 30A breaker in unit and #6 thhn for 10KW heat strip on 60A breaker in unit. I do not have room to install a panel and use branch circuits to unit and not comforable with the tap rules
Is this all code compliant

If you are using a tap, the 240.4(B) rule doesn't apply. In general, all taps require at least as much ampacity of conductor as you have amps of OCPD on the end of the tap. Please see 240.21(B), and then select the applicable rule based on length or inside/outside.
If you have a typical feeder, it does.

In the case of the 83A circuit on #4 Cu wire, make sure that termination equipment on both sides is listed and labeled for 75C conductors. Most of the time this is true, but you do have the burden of proof to confirm this for 100A and less.

 

[augie47]

The air handler should have a nameplate showing you the minimum circuit amps (MCA) and maximum over current protection (MOCP) for one and/or two circuits which should be your guide.

 

[Mgraw]

Yes like augie47 said, use the name plate numbers. There is a good possibility that 15kw heater is really 14.4kw.

 

[wmeek]

The nameplate reads 83A Minimum and OCP max is 90A

 

[wmeek]

I am going to run #3 and fuse at 90A

 

[Dennis Alwon]

I am going to run #3 and fuse at 90A

#4 at 75C is sufficient because it is good for 85 amps. You can still fuse it at 90 amps

 

[wmeek]

I am reusing existing 100A Sq D disconnects and I am not sure of the terminal ratings being rated at 75degree. So im figuring on being on the safe side to use #3 Thhn

 

[augie47]

No problem being safe with the #3 but 424.22 is going to require your maximum overcurrent protection for a resistance heating element not exceed 60 amps so at some point, as you mentioned, you will need a 60 amp max breaker. Your unit will likely have a factory kit (fuse or breaker) available to split the load. If not you will need to do so with separate devices which should allow you to use the 75° rating.

 

[wmeek]

The unit has 30A an 60A Breakers for ocp of heaters elements. I am using the 3ft tap rule and running #10 an #6 from The #3 in a jbox on side of unit.

 

[Dennis Alwon]

The unit has 30A an 60A Breakers for ocp of heaters elements. I am using the 3ft tap rule and running #10 an #6 from The #3 in a jbox on side of unit.

I would set a disconnect and bring in the 2 circuits. Never thought about using the ta rule. Some units require 2 circuits but others will only take one circuit.

 

[augie47]

You might check the manufacturers info closely. A lot of those have a common tie bar installed or available so you could being your feed right to the breakers.

 

[kwired]

You might check the manufacturers info closely. A lot of those have a common tie bar installed or available so you could being your feed right to the breakers.

If they don't have a tie bar, or sometimes if it will take too long to get one, I have when replacing an old unit fed from single supply circuit just used polaris connectors or similar and made taps right inside the unit.