Electric furnace heat strip calculation shouldn't be confusing!?!?

[bamit]

can someone explain to me how a electric furnace with a 10kw heat strip at 240v has a minimum ampacity of 58.5 ? The furnace fan motor draws 2.5 amps.
Here's my math
10,000 / 240 = 41.6
41.6 x 125% = 52
52 + 2.5 = 54.5 amps

The 6/2 Romex is only good for 55amps

I've used 6/2 Romex for decades! Maybe I need to change that.
Attached is a data

 

[Smart $]

Are you sure the minimum ampacity when connected 240V isn't the 53.8A in the chart? The reason I ask is because 240V is listed to the left under the kW columns.

 

[GoldDigger]

The top of that section of the table shows "208/230".
That indicates to me that the 53.8 in 53.8/58.5 is the MCA for a 208V connection.
And if the heater is designed for full wattage output at 230V, that explains in part why the current calculation based on full output at 240V does not match.

 

[bamit]

The top of that section of the table shows "208/230".
That indicates to me that the 53.8 in 53.8/58.5 is the MCA for a 208V connection.
And if the heater is designed for full wattage output at 230V, that explains in part why the current calculation based on full output at 240V does not match.

Thanks for y'all's help. So do you think the inspector should use the name plate rating using the 230v calculation, or should they recalculate using the voltage provided, 240volt ?

 

[infinity]

Why would half of the table use 240 volts and the other half 230 volts?

 

[Smart $]

Thanks for y'all's help. So do you think the inspector should use the name plate rating using the 230v calculation, or should they recalculate using the voltage provided, 240volt ?

A inspector must (should) inspect per Code, which requires the use of the nameplate MCA value.

So the question here does the nameplate data match what is in the table?

 

[infinity]

A inspector must (should) inspect per Code, which requires the use of the nameplate MCA value.

So the question here does the nameplate data match what is in the table?

Good question, where is the nameplate data?

 

[bamit]

Good question, where is the nameplate data?

The name plate states the same thing as the data sheet, calculated at 230v. Looks like I need to change the heat strip down to 8 kW. It's a shame to do that when technicality it's only drawing 52 amps.

Thanks again

 

[Smart $]

The name plate states the same thing as the data sheet, calculated at 230v. Looks like I need to change the heat strip down to 8 kW. It's a shame to do that when technicality it's only drawing 52 amps.

FWIW, changing the heat strip doesn't change the nameplate.

Modified equipment must be approved by the AHJ... and many are unwilling to do that with listed equipment.

 

[bamit]

FWIW, changing the heat strip doesn't change the nameplate.

Modified equipment must be approved by the AHJ... and many are unwilling to do that with listed equipment.

Yes, that option has been approved, these heat packages are a option to the unit,with the changed nameplate, according to the hvac owners it well within the manual j requirements. (2 additional furnaces here) just feel the way it is would be to code & safe.

 

[Smart $]

Yes, that option has been approved, these heat packages are a option to the unit,with the changed nameplate, according to the hvac owners it well within the manual j requirements. (2 additional furnaces here) just feel the way it is would be to code & safe.

Quite simply, it is not to Code. You can debate safe or not until you're blue in the face and it won't change what is required for Code compliance... or AHJ approval of modifications.

Your only alternative is to get involved in the Code change process. If others agree with you, you may see a change in a future edition... but will not change anything for a project currently at hand.

 

[bamit]

Quite simply, it is not to Code. You can debate safe or not until you're blue in the face and it won't change what is required for Code compliance... or AHJ approval of modifications.

Your only alternative is to get involved in the Code change process. If others agree with you, you may see a change in a future edition... but will not change anything for a project currently at hand.

Okay, thanks, just wasn't sure if a nameplate rating was the inspectors only code alternative. Not a big deal, wish I had time to put my 2 cents worth out there to the code committee.
Thanks again

 

[kwired]

can someone explain to me how a electric furnace with a 10kw heat strip at 240v has a minimum ampacity of 58.5 ? The furnace fan motor draws 2.5 amps.
Here's my math
10,000 / 240 = 41.6
41.6 x 125% = 52
52 + 2.5 = 54.5 amps

The 6/2 Romex is only good for 55amps

I've used 6/2 Romex for decades! Maybe I need to change that.
Attached is a data

what don't make sense to me and has been mentioned by others is why the table lists 240 and 208 volts for the kW ratings columns, but then in the amp ratings columns they change to 208 and 230 volts. If they are going to rate the thing for 240 volts I have no idea why they don't keep 240 volts consistent throughout the data charts - they did keep 208 volts consistent:?


That said most "10 kW" units I ever run into are just called 10 kW as that i rounded off to nearest kW unit. Most usually are actually rated something like 9.6 kW @ 240 volts - which will give you 40 amps just like this one has listed - my guess is the manufacturer did a poor job of labeling this unit.

 

[wwhitney]

The furnace fan motor draws 2.5 amps.

Are you sure that's the only non-heater load? Because the numbers in the highlighted line of the chart imply the non-heater load is 8.5 amps at both 208V and 230V. (Min Ampacity - 125% of Heater Amps).

Cheers, Wayne

 

[kwired]

Are you sure that's the only non-heater load? Because the numbers in the highlighted line of the chart imply the non-heater load is 8.5 amps at both 208V and 230V. (Min Ampacity - 125% of Heater Amps).

Cheers, Wayne

There is consistency there - though the blower motor likely does draw more amps @ 208 then it does at 230 or 240, still think this manufacturer don't know how to mark equipment.