Load Calculation Question

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  #1  
Old 09-03-07, 01:34 AM
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Load Calculation Question

I have a condo with a 50 amp service (yes, only 50 amps). The condo was built in 1987. I'm trying to figure out how much I have available for A/C. The A/C unit I want has a max rating of 16 amps and requires a dedicated circuit with a 20 amp time delay breaker (per the manufacturer's spec sheet).

My city has adopted the 1993 NEC for reference. My question is with the following loads, can I legally add the 20 amp circuit (with a 16 amp @ 220V load) for the A/C?



My load is as follows:

1035 sq ft

2 small appliance circuits

1 laundry circuit

Fixed in place appliances are disposal (720VA), dishwasher (1080VA), microwave (1580VA). (The dishwasher and microwave will be removed if needed to make room for the A/C).

All heating and cooking appliances (wall heater, water heater, dryer, and oven) are gas. The motors for the A/C are included in the main unit load, there is no separate air handler (mini-split system).

Using an online load calc tool, I found that the load (including my proposed A/C load of 3840VA) was exactly 50 amps. Is that correct and if so can I legally add my intended circuit?
 
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Old 09-03-07, 10:20 AM
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Man, I thought Michigan was behind because they had not accepted the '05 code yet.

Typically it is not so much as a load calc limiting an addition but actual use. I have not had an inspector give me a problem about adding circuits but the local POCO may be concerned if you overload their supply. I doubt the load you are speaking of would overload the POCO though. If it would, they are already due to upgrade.

What I would prefer to do is take an actual reading of the current draw. A recording ammeter would be great but if not available, then multiple readings and many times of the dday. The idea is to find out if your main will actually carry everything you use at the same time PLUS the a/c.

Any chance of upgrading the service to 100-200 amp service?
 
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Old 09-03-07, 12:11 PM
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A service upgrade is not an option. As far as actual use, I know with everything I run at any one time including the A/C I would be safely below the 50 A capacity. All the other equipment is rated above the 50 amps, my bottleneck is the service entrance. What I'm looking for is someone to double check my load calc and tell me if they come up with the same information, that adding 3900VA or so to my current load will not be something my local authority can just look at and say "no". I know it'd be safe the way it is given my use but I'm trying to look at it from their point of view since they'll be issuing the permit. I don't think they'd take "I don't use that much power at any one time" into consideration when issuing/signing off a permit.

Is my load calc right? Is it legal (per the NEC) to add my proposed A/C load of 3900VA given my existing load (included in original post)?
 
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Old 09-03-07, 01:28 PM
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ok, I don't do resi work so do not take this as definative BUT I get an answer of no, it is not acceptable.

I get a total of 50.85833 amps. (12205 va)

12000 va would be 50 amps.

this is per 2005 NEC and my calculations.

Now, do I think an inspector would allow it? In my area, yes, as long as there are no other problems that jump out at them.
 
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