Service Caculations

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bob

bob

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Alabama
I use 220 to caculate service ampacity. In doing so I add any A/C units or Heat at 100%. The question came up about adding the 1.25 factor to the motor load when making this caculation.
The reference used was 220.3B(3). I pointed out that this was for branch circuits as the heading of 220.3 indicates. However example D1b adds the
1.25 factor in the caculations. I do not think it is correct to do so but what are your thoughts.
 
Re: Service Caculations

It will depend on what unit you are allowed to eliminate per 220.21.

If the A/C is omitted as being the smaller of two loads, than the 25% increase of this load will not be required. You will then have to go to the next largest motor load in the dwelling.

In most cases, the heater unit is the larger load and may need to be inceased by 25% if it is the largest motor load. If it is a non-motor heater unit, then it would not have to be increased 25% per 430.22 or 430.24. :)
 
Re: Service Caculations

Bryan
I may have confused the issue by mentioned the heat. My main question was when using 220 to caculate service size, should you add the 1.25 factor the A/C unit load. Forget the heat.
If you look at 220.14 Motors, it refers you to 430.24, ,25 and .26 and 430.26 for A/C units.
430.24 Several motors and Other Loads adds the 1.25 factor to the largest motor load. 440.6 for hermetic motors does not mention the 1.25 factor. The 1.25 factor must be applied to feeders and branch circuits. The question is does it apply to a service. In section 230.42 Services, A General states the ampacity must me not less that that calculated in (1) or (2). Number (1) calls for the use of the 1.25 factor.
Number (2) does not. I think most services would meet number (2).

[ June 25, 2003, 01:16 PM: Message edited by: bob ]
 
Re: Service Caculations

You shouldn't multiply an A/C load by 1.25 for determining heating vs A/C loading. They are both calculated at 100% for this comparison.

I don't agree with not having to include an A/C unit for the largest motor load. If it is not included for heating vs A/C I don't see where is is excluded for largest motor calc.
 
Re: Service Caculations

Originally posted by Thom Peterson:
I don't agree with not having to include an A/C unit for the largest motor load. If it is not included for heating vs A/C I don't see where is is excluded for largest motor calc.
Click to expand...
If you arent using the A/C load for your calculation, then what value are you increasing by 25%? You will already be sizing the service large enough for the LARGER heating load so adding in 25% more of the A/C load is unecessary.

If you are ommitting the A/C load per the allowance of 220.21, then you are allowed to ommit any other calculation that may be assoicated with it in regard to service sizing. :)
 
Re: Service Caculations

I will provide a clear example to show my point numericaly.

Lets say you have a 12 KW Heater and a 9.6 KW A/C unit. (50amp, 240V heat / 40amp, 240V A/C)

The code allows you to omitt the 9,600 watts and only requires you to add the 12,000 watts.

By increasing the A/C load by 25%, you would have to add an additional 2,400 watt to the calculation. (9,600 X 25%)

This means that you are essentially increasing the load of the A/C to 12 KW. Since you are already have used the 12KW for the heat, you have covered this.

Does this clear it up any? In any case, you can go ahead and size your service to the connected load for all I care, I am just trying to provide what I think the minimal requirement is. :)

[ June 25, 2003, 01:49 PM: Message edited by: bphgravity ]
 
Re: Service Caculations

Bryan
I understand and agree with you statement. Assume
you have gas heat and a 4 ton A/C unit. When adding the load per 220 do you multiply the A/C
load by 1.25 and add it to the total caculated
in the methods of 220? I do not think you need to do this.
 
Re: Service Caculations

Bryan,

I don't understand what code excludes the A/C motor from the largest motor @ 125%? It shouldn't be just because the heater won out. If it didn't win the A/C would be the largest motor.
 
Re: Service Caculations

Article 220.21 does.

The code allows you to omit the A/C from the calculation. As far as the service is concerned, it doesnt exist.

You base the calculation on the larger load which ensures the service is large enough for either one. What would be the purpose of increasing a load that isnt even being used for the calculation? It doesnt make any sense.

In the case of the A/C being the larger load, than I do feel it would need to be increased the 25% since it is being used to determine service size.

This does not exclude the branch-circuit requirements to the A/C unit provided in 440.32 or 440.34, just the service calculation.
 
Re: Service Caculations

If you look at 220.14 Motors, it refers you to 440.6 for A/C units.
440.6 for hermetic motors does not mention the 1.25 factor. Section 230.42 Services, A General states the ampacity must not be less than that calculated in (1) or (2). Number (1) calls for the use of the 1.25 factor.
Number (2) does not. I think most services would meet number (2). If this is correct then the 1.25
factor is not used for the A/C load. It is added
at 100% to the other caculated load per 220.
 
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