Optional & Standard Service Calc - Different Service Sizes

[designer82]

So I did an optional and a standard service load calc. The standard resulted in a higher service size than the optional.

Either I did something wrong or this is something that isn't uncommon.

If it is something that isn't uncommon, which method would you select the final size based on... the more conservative (standard) or less conservative (optional)?

Thanks

 

[suemarkp]

The optional typically gives a smaller result, but you just can't use it for everything. Both produce a size larger than really necessary. Compare your service conductors to what the utility provides for the same thing. I think the utilities know the reality, but they may start to get bit now with car chargers and maybe tankless electric water heaters.

 

[paullmullen]

I'm not licensed in Massachusetts (Call me when you move to Wisconsin), but I did a quick check of the on-line Massachusetts State rules. There is no modification to the NEC that I could find in that code, so you can use either method. Which you choose depends a lot on what you're trying to optimize. Most businesses will try to minimize costs and thus use the load calc with the smaller calculated load, but you're free to choose.

 

[jaggedben]

I always use one of the optional methods for a single family home (most of my work). What would be the point of a different optional method if didn't come out different? Or actually, if it didn't come out lower. I also typically still find that the optional methods for a single family home overestimate actual usage by a factor of 3 or 4. For an all electric home, it only overestimates by a factor of 2.

 

[paullmullen]

I also typically still find that the optional methods for a single family home overestimate actual usage by a factor of 3 or 4. For an all electric home, it only overestimates by a factor of 2.

Good to know. How do you measure/calculate the amount of overestimate? This could be a useful rule of thumb.

 

[JoeStillman]

For multi family buildings, the only time your Article 220, Part III method beats the optional Part IV method is when the units have gas for all possible appliances - WH, Dryer, Range, Heater; and there are 30 units or less. The more electric appliances and heat, the bigger the gap between Part III and Part IV.

 

[suemarkp]

Good to know. How do you measure/calculate the amount of overestimate? This could be a useful rule of thumb.

You could do a 220.86 calculation based on actual demand. Do enough of them and maybe you'll see a pattern to make a rule of thumb.

This should be getting easier for residential now with the number of utilities using smart meters. I have a device that wireless communicates with my electric meter and shows the current demand, but not 15 minute interval data and it has no history. I don't know if the poco website has 15 minute interval data available though. Maybe if I asked for a dump of 1 year's data they would have it. Utilities have this for commercial services as they need the info to calculate the demand charges.

 

[jaggedben]

Good to know. How do you measure/calculate the amount of overestimate? This could be a useful rule of thumb.

As a solar/battery installer I have a lot of access to customer data, both before install, which is typically hourly utility data, and 15min intervals after install from our systems. I haven't done a study or anything, what I said above is just my subjective take.

 

[paullmullen]

Great to know. Actually designing a solar + battery install for a current project and your 2:1 ratio could be really helpful for sizing. Thanks.