loads of RTUs with ELec Heat

[raberding]

I am struggling(again) with how to include the load of an RTU with electric heat in a service load calc. We have had a few discussions in our office and have not yet come to a definitive solution.
I say that we should use MCA (it's simple) and others say that results in a higher than actual load, because the heat and cool do not operate simultaneously. But MCA is the only reliable info, I think....
Does anyone use any values other than MCA?
fyi, we deal with "ordinary" 5T-10T RTUs...maybe 15kw heat, usually 208/3
thanks

 

[Ingenieur]

You may use the mfgs mca/1.25 or add up the actual currents
some units may dehumidify ie, cooling and htg on simultaneously

 

[steve66]

I use the full MCA for each unit since electric heat is usually considered a continuous load.

 

[augie47]

I use the full MCA for each unit since electric heat is usually considered a continuous load.

'From what I was taught for service load calculations you do not figure the load at 1.25 and the Examples in Annex D seem to support that,

 

[raberding]

MCA and RTU heat

MCA and RTU heat

yeah, it's my understanding that the MCA, as calculated and published by the mfg, includes all of the loads + 25% of the largest motor. I have always presumed that it included 125% of the heater load.
And if the unit is heating and cooling at the same time (which is very likely in a restaurant in Florida), ALL of the loads just might be simultaneous.

I think I'll keep using MCA.
Just looking for some peer corroboration.

thanks!

 

[steve66]

'From what I was taught for service load calculations you do not figure the load at 1.25 and the Examples in Annex D seem to support that,

To be honest, I'm not 100% sure one way or another. But the examples in the annex seem to be heavily weighted toward residential, and there are definitely some demand factors in 220 that apply to specific occupancies. However, I don't see any general exception to the rule for continuous loads. Did you have a specific example that you are thinking of?

I'm going by 424.3(B) which says fixed electric space-heating equipment and motors shall be considered continuous loads, combined with 220.40 : "the calculated load of a feeder or service shall not be less than the sum of the loads on the branch circuits....."

The commentary in the handbook or 424.3(B) refers to sizing heaters and their motors for 125% of the total load, and it also mentions that the requirement also impacts services and feeders. Again, that's the commentary, so some people give it more weight than others.

 

[augie47]

Oddly enough, in the classes I've attended, they teach 125% of the largest motor on service calculations but don't take into account the "continuous" on heat (or water heaters). It does not appear that the "continuous" factor is taken int account on Mike's Load Calculator in "Free Stuff".
Perhaps we have a resident 'expert" that can chime in.

 

[raberding]

440.4(B) is leading me to believe that MCA is properly loaded.

 

[Ingenieur]

from my experience
if you look at the actual unit data and add up all the FLA currents (compressor, condenser fans, ahu fan, etc.) the mca is typically 1.25 times this value
so the actual load is mca/1.25 which is what I typically used

 

[kwired]

from my experience
if you look at the actual unit data and add up all the FLA currents (compressor, condenser fans, ahu fan, etc.) the mca is typically 1.25 times this value
so the actual load is mca/1.25 which is what I typically used

It is usually 1.25 times the largest motor rated current plus the rated current of other loads.

If you have higher resistance heat load then motor loads BUT it does not run at same time as compressors - then MCA will be based on the resistance heat load plus any other load that can run simultaneously. Most residential grade units however the resistance heat is an add on component and separate circuit from any compressor loads. I guess OP is asking about RTU's though.

 

[dkidd]

from my experience
if you look at the actual unit data and add up all the FLA currents (compressor, condenser fans, ahu fan, etc.) the mca is typically 1.25 times this value
so the actual load is mca/1.25 which is what I typically used

The MCA is the sum of all currents plus 25% of the largest.

 

[raberding]

RTU MCA

RTU MCA

yes.
I am referring to Commercial Rooftop Units.
Trane, York, Carrier, Lennox, etc
Between 5 and 10 tons.
With somewhere between 8 and 15 kw of electric heat.

 

[Ingenieur]

The MCA is the sum of all currents plus 25% of the largest.

you are correct sir
I looked at a unit
comp x 1.25 + cond fan + evap fan FLA's
you can teach an old dog new tricks lol
good thing it doesn't make much of a difference

 

[steve66]

you are correct sir
I looked at a unit
comp x 1.25 + cond fan + evap fan FLA's
you can teach an old dog new tricks lol
good thing it doesn't make much of a difference

Right, but if electric heat is included, it is usually the largest load.

From my point of view, we still need to multiply heating loads by 1.25 for branch circuit and service calculations. It's somewhat of a coincidence that just using the MCA is a pretty easy way to do this.

Actually, for a unit with electric heat, I think the MCA is 1.25 times the heat plus the largest motor that would run at the same time. Otherwise, even using the MCA for a service calc. might not really cover the load.

 

[Chamuit]

You take the MCA from the nameplate or cut-sheet on the unit. See the first post in this thread. It shows the accessory heater model and the resulting MCA for the particular model installed...

You don't heat and cool at the same time with the same unit.

 

[raberding]

RTU loads

RTU loads

i'm liking 440.4(B).
seems to say that MCA is all-in.

 

[topgone]

i'm liking 440.4(B).
seems to say that MCA is all-in.

Here:

• For direct expansion units, the MCA is calculated by adding 125% of the rating of the largest motor (normally the compressor) plus the rating of all of the other loads in the equipment that exceed one ampere. If the unit has electric heaters, 125% of the rating of the heaters is added into the calculation.
• For chilled water units, the MCA is calculated as 125% of the sum of the ratings of all of the loads in the equipment over one ampere.

 

[kwired]

MCA is for branch circuit calculations. If you use if for service or feeder calculations, you might be fine for a single unit, multiple units start accumulating a little extra in that 125% of largest load - which doesn't hurt anything from safety perspective, but your service/feeder load calculation will be higher then what it could be and still be NEC compliant.

If you had ten identical units you have that extra 25% of largest load on all units in your feeder/service calculation if you just go with MCA marked on the units., when you only need to include that extra 25% just one time in your feeder/service calculation.

If you had 25 KVA heaters as the highest load in ten identical units - the extra 25% is 6.25 KVA each unit. If you have 10 units and just use MCA you will have 56.25 more KVA in your feeder/service calculation then you need to have.

 

[steve66]

MCA is for branch circuit calculations. If you use if for service or feeder calculations, you might be fine for a single unit, multiple units start accumulating a little extra in that 125% of largest load - which doesn't hurt anything from safety perspective, but your service/feeder load calculation will be higher then what it could be and still be NEC compliant.

If you had ten identical units you have that extra 25% of largest load on all units in your feeder/service calculation if you just go with MCA marked on the units., when you only need to include that extra 25% just one time in your feeder/service calculation.

If you had 25 KVA heaters as the highest load in ten identical units - the extra 25% is 6.25 KVA each unit. If you have 10 units and just use MCA you will have 56.25 more KVA in your feeder/service calculation then you need to have.

As I said before, I'm not 100% sure, but I don't see anything in the code that supports what you are saying. I think the extra 25% on all units is required.

 

[Besoeker]

I am struggling(again) with how to include the load of an RTU with electric heat in a service load calc. We have had a few discussions in our office and have not yet come to a definitive solution.
I say that we should use MCA (it's simple) and others say that results in a higher than actual load, because the heat and cool do not operate simultaneously. But MCA is the only reliable info, I think....
Does anyone use any values other than MCA?
fyi, we deal with "ordinary" 5T-10T RTUs...maybe 15kw heat, usually 208/3
thanks

Answer already given.