Service Sizing

[mike k]

I have a disagreement with an engineer regarding my service calculations. The project is a small physical therapy suite. I come up with 188 A for my service over current protection and 3/0 for a feeder size.

The engineer wants me to put a 225-A panel boar in and 4/0 feeders because he considers the RTU (s) which are A/C gas fired heat to be a continues load. And that ?a panelboard may only be loaded to 80% of its rating?. He actually says that ?all equipment may only have 80% connected load of its rating?. Like a disconnect for instance ?many only have a circuit which has up to an 80-amp load?.

SQ. FT. 4247
NON CONTINUOUS LOADS

NEC RECEPTACLE LOADS QTY. Total VA
220.44 Receptacles at 180VA 56 10080

NEC Specific Applaiances or Loads QTY. Total VA
220.14 (A) Tread Mills at 1800VA 6 10800
Elliptical/Cross Trainers at 180 6 1080
Hot Packs at 1200 2 2400
Microwave at 1800 VA 1 1800
Refrigerator at 1800 VA 1 1800
Water Heater Power Vent at 360 1 360
220.52 (B) Washer Recpt at 1800 VA 1 1500
220.54 Dryer Recpt. At 5000VA 1 5000
Total 34820
NEC Demand Factors - Non Dwelling Recpt. Loads
Table 220.44 First 10 KVA 100% 10000 10000
Remainder Over 10KVA 50% 24820 12410
Sub Total Non Continuous 12410


CONTINUOUS LOADS

Table 220.12 General Lighting 3.5 VA per Sq Ft 4247 14864.5 * See Note
220.14 (F) Outside Sign Circuit at 1200 VA 1 1200
Sub Total Continuous 16064.5

OTHER NONCONTINUOUS LOADS
Water Heater at 1800 VA 1 1800
RTU #1 1 12248
RTU #2 1 12248
220.51 Wall Heatrer at 3000VA 3 9000
Sub Total Other Non Continuous 35296

* Note Actual lighting load is less than the load from table 220.12 so the minimum lighting load from table 220,12 is used in the calculation.


Sub Total Non Continuous 12410
Sub Total Continuous 16064.5
Sub Total Other Non Continuous 35296
Total Continuous + Non Continuous Loads 63770.5


MINIMUM SIZE FEEDER (OR SERVICE) OVERCURRENT PROTECTION 215.3 OR 230.90

Sub Total Non Continuous 12410
Sub Total Other Non Continuous 35296
Sub Total Continuous at 125% 20080.625
Total 67786.625

67786 VA / 208 V / 1.732 = 188 A
See 240.6 The Next Higher Standard size is 200-A



MINIMUM SIZE FEEDER (OR SERVICE CONDUCTORS) 215.2 & 230.42 (A)

67786 VA / 208 V / 1.732 = 188 A

215.3 & 310.16 Service or feeder Conductor is 3/0


All Calculation shown above are take from example "D3 Store Building" in Annex D of the 2005 NEC


Thanks,
Mike

 

[kingpb]

OK, lets just say the 188A is correct. A few questions:
1. Why is an engineer involved but your doing the calc?
2. How did you bid the job if your doing the calc e.g. based on what size?
3. Is an engineer going to seal and sign this design and calc?
4. Based on your 188A, using the 80% you would need 250A panel not 225A.
5. Finally, if your coming up with 188A, for a business with potential for future growth I wouldn't put in anything less than 300A service, but I've been known to be conservative.

"220/221, whatever it takes" - Quote from Mr. Mom

 

[jtester]

Mike

I agree with your theory of loading, and disagree with what I think your engineer said. There seem to be some errors in your calculation, and you'll probably be at or over 225 amps anyway.

For example, you used 5000 va for a dryer and cited 220.54. That applies to dwelling unit calculations. You should use the nameplate data, which for even common residential dryers is almost 7000va.

A second thing that I disagree with is your use of diversity according to 220.44 on specific appliances. I believe those appliances should be calculated at their nameplate and not diversified.

I calculate about 227 amps with those changes alone.

Jim T

 

[jeff43222]

Perhaps the engineer is thinking about voltage drop in addition to continuous use issues. As for the 80% policy, maybe this is the engineer's policy or a design decision rather than a code rule. I could understand your concern if the engineer is asking you to put in something that you believe is inadequate, but the opposite is the case here.

As long as you aren't violating code with what you install, let the engineer worry about the design considerations.

 

[bob]

It is hard to follow your caculations because you have several NON CONTINUOUS LOADS and several CONTINUOUS LOADS listed at different locations in the post. Go back and put all non continuous together and all
continuous together. Also the A/C units should be considered as Continuous
Loads but only the larger of the cooling or heat is listed. The loading on the panel may be 125% of the cont loads plus 100% of the Non Cont loads. You have listed the following:
OTHER NONCONTINUOUS LOADS
Water Heater at 1800 VA 1 1800
RTU #1 1 12248
RTU #2 1 12248
220.51 Wall Heatrer at 3000VA 3 9000
Sub Total Other Non Continuous 35296

220.51 requires heating to be at 100% and no derating. You have it listed under Non continuous Loads. Go back and re-organize these loads in the post.