Heater Branch Circuits

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Pitt123

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We are installing 5kW heaters in some storage trailers and I had a couple of questions regarding the branch circuit installation. Each heater circuit (3) total will be fed from a 120/240V panel. Heaters are rated 208-240V.

At 240V a 5kW heater draws 20.8A. Do we need to take 125% of this value since heater will run continuously. If so this gives us 26A. Does this mean that we need to use a #10AWG (ignoring voltage drop). If so we would use a 30A OCPD?

Can we round the heater load calc down to 20A muliply this by 125% to come up with 25A and then use a #12AWG? If so I dont think a #12 is permitted to be on an OCPD greater than 20A so this then forces us to use #10?

Do we need some sort of disconnect in the storage trailer (conex) or can we wire the branch circuit straight into the heater.

Circuit will be underground in PVC for small 10-20ft length. Do we need to derate for this underground routing? What factor would be applicable?
 
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brother

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Pitt123 said:
We are installing 5kW heaters in some storage trailers and I had a couple of questions regarding the branch circuit installation. Each heater circuit (3) total will be fed from a 120/240V panel. Heaters are rated 208-240V.

At 240V a 5kW heater draws 20.8A. Do we need to take 125% of this value since heater will run continuously. If so this gives us 26A. Does this mean that we need to use a #10AWG (ignoring voltage drop). If so we would use a 30A OCPD?

Can we round the heater load calc down to 20A muliply this by 125% to come up with 25A and then use a #12AWG? If so I dont think a #12 is permitted to be on an OCPD greater than 20A so this then forces us to use #10?

Do we need some sort of disconnect in the storage trailer (conex) or can we wire the branch circuit straight into the heater.

Circuit will be underground in PVC for small 10-20ft length. Do we need to derate for this underground routing? What factor would be applicable?
Click to expand...

All branch circuits for fixed heating equipment has to 125% of the rated load NEC 424.3(B), and yes you would use the #10 copper with a 30 amp breaker. Give NEC 2008 424 a good read. 424.19 Deals with the disconnecting means.
 
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Pitt123

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Since I am running all (3) heaters in the same conduit (6 current carrying conductors) then I need to derate the ampacity of the #10 by 80%. This gives me an ampacity of 28A for these conductors however using the next size up OCPD I can still use a 30A OCPD is this correct?

Had there been any more conductors in the conduit then for my calculated load of 26.04A I would have probably had to use a #8 because the derating on the #10 would be too small for the load (.70 * 35 = 24.5A). Should I use a #8 anyway considering the derated capacity of #10 is 28A which is close to my 26.04 calculate load?

When derating conductors and increasing cable size as a result of derating do you ever increae the breaker size? Or do you leave the breaker size small enough to cover the final current carrying capacity of the cable?

Should I derate any for the small underground run of PVC conduit?
 
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Pitt123

Senior Member
brother said:
Give NEC 2008 424 a good read. 424.19 Deals with the disconnecting means.
Click to expand...

I'm confused with some of the terminology in this section. What are they referring to as supplementary overcurrent protection? Is this an OCPD on the heater itself or is this the OCPD on the branch circuit in the panel?

Since these heaters are in a conex would it fall under 424.19(B)(4) and thus the panel OCPD could serve as disconnecting means?
 
kwired

kwired

Electron manager
Location
NE Nebraska
Pitt123 said:
Should I derate any for the small underground run of PVC conduit?
Click to expand...

That is exactly what you are doing here:

Since I am running all (3) heaters in the same conduit (6 current carrying conductors) then I need to derate the ampacity of the #10 by 80%. This gives me an ampacity of 28A for these conductors however using the next size up OCPD I can still use a 30A OCPD is this correct?
Click to expand...

If is is less than a 2 foot nipple then deration is not required.

When derating conductors and increasing cable size as a result of derating do you ever increae the breaker size? Or do you leave the breaker size small enough to cover the final current carrying capacity of the cable?
Click to expand...

Final conductor ampacity is what it is to be protected for.


240.4 Protection of Conductors.
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
Click to expand...

310.15 includes ampacity adjustments.
 
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