Anyone getting complaints or doing the complaining about building service load calculations? I've had a string of electricians complain to me telling me that they can get away with a much lesser service size and I respond to them that I have to follow the NEC. We all know that 99% of the time a building doesn't use close to the service size, but I can't base my calculations on "in my XX years of experience".
Load Calcs recently
- Thread starter Rock86
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it has been my experience that the people using that argument cannot do a service load calculation and they hide behind "I've been doing this for XX years".
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
One would have to see specifics. I have certainly seen jobs where it was obvious the Engineer just covered their butt. I have also seen jobs where the service size was apparently based on the connected load. OTOH, usually the services are reasonably calculated and the service size is, as you stated, NEC sized and far exceeding actual usage. On a connected note. I teach second year apprenticeship and I tell my students that in the real world, it isn't likely two people would come up with the same figures on any fair sized job, so a certain amount of fudge is always best.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
The rules are what they are. They don't have to make sense.
- Location
- Bremerton, Washington
Are you using the alternate method for resi calculations?
One job is a firehouse, and other job is an apartment building. Now, the apartment units are easy to calculate via the NEC. An example for you, the largest unit is all electric, range, dryer, heat pump, water heater. I came up with 186A, so I am calling for a 200A. The EC is saying he is going to put in 100A panels because he hasn't had a problem with them on previous jobs. Maybe I'm venting... I know engineers oversize often... but a single apartment unit has to be one of the easiest calcs you can do.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
How does the EC get to decide to put in 100 Amp panels if the calcs show they need to be 200 A? Does the AHJ accept this?
The job has not yet started. The EC has been part of the clients team, so he was telling them I was over sizing things and he could save them money buy using what they always use.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
You are not part of the team?
My opinion, do your drawings, put whatever size you think is correct on them and don't worry much about what they actually do.
It used to offend me when the electricians would just ignore the drawings I made and do what ever they felt like, but over time I have realized my power to force them to do it right is very limited, and as often as not there is little real supervision of what is going on in the field and the electricians know this and use it to their advantage.
- Location
- Connecticut
- Occupation
- Engineer
How big is the apartment to get a 186A load?
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
I'm betting he is used to split units. A heat pump always has a higher calculation because of the need to run the compressor and the heat strips at the same time when it is cold. I agree with you that 100 seems small for the loads.
Averaging 1200sgft. All electric units, and the heat pumps are what brings the numbers up like Strathead mentioned. Trust me, the first few all electric apartment projects, we were questioning our numbers... but the numbers are the numbers in the end.
- Location
- Connecticut
- Occupation
- Engineer
208V or 240V? Even at 1200sf, most of the all electric apartments we've seen don't get anywhere near 186A.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
kwired
Electron manager
- Location
- NE Nebraska
I have never seen a heat pump draw more than maybe 30% it's RLA in heating mode, and especially when temp is really cold. There just isn't enough heat in the air to load it that much compared to when it is in cooling mode and there is plenty of heat for it to be moving. How much heat is being moved is a big factor in loading. NEC only recognizes the max rating and whether it is able to run at same time as other load but not actual conditions of use.
Some units do turn on electric heat while in defrost. That can be a period of high use, but at same time is somewhat limited time - but no demand factors exist in NEC to apply that either, though it probably can be something that seldom causes troubles when marginally sized supply is run.
If you see an error, please let me know. I never claim to be perfect.
kwired
Electron manager
- Location
- NE Nebraska
3 ton compressor @ 14456VA?
That sounding like backup heat, though still a lot for only 1200 SF. For that matter 3 ton compressor sort of a lot for only 1200 SF. Particularly if multi-units with at least some non exterior walls between units. Stand alone structure this size maybe has more need for that size of unit. Doors and windows make a difference in heat/cooling load.
- Location
- Connecticut
- Occupation
- Engineer
Any reason not to use the Optional Method?
Thanks to you, I noticed a mistake.... I had the 7.5kW electric heat air handler and the compressor condenser totaled together. I can take the larger of the two being they wont use heating and cooling at the same time.
nope...
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