Need opinions on heat

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rald277

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Dallas Tx
We have a design and build on a 5000 sq ft doctors office here in Texas. The service is 120/208v 3 phase. Connected amps are 759 which puts me just under the 10% mark for an 800a service. I need to find another 10 or 11% Any of you with hvac experince feel like 5 15kva heat units on this building is a bit excessive? 75kva for this size building seems pretty high to me. Thoughts?:?
 
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kwired

kwired

Electron manager
Location
NE Nebraska
rald277 said:
We have a design and build on a 5000 sq ft doctors office here in Texas. The service is 120/208v 3 phase. Connected amps are 759 which puts me just under the 10% mark for an 800a service. I need to find another 10 or 11% Any of you with hvac experince feel like 5 15kva heat units on this building is a bit excessive? 75kva for this size building seems pretty high to me. Thoughts?:?
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that is 15 watts per square foot. If it were in northern Alaska it may not be too high, but probably is in Texas.
 
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GoldDigger

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kwired said:
that is 15 watts per square foot. If it were in northern Alaska it may not be too high, but probably is in Texas.
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The only justification I can think of for oversizing the heat would to be to do allow it to warm the office very quickly when they open up in the morning. A timer would tame care of that more economically.
 
kwired

kwired

Electron manager
Location
NE Nebraska
GoldDigger said:
The only justification I can think of for oversizing the heat would to be to do allow it to warm the office very quickly when they open up in the morning. A timer would tame care of that more economically.
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True, but just how much heat loss is there going to be in Texas overnight is also something to consider. Recovery time should still not be all that long in Texas as compared to Alaska.
 
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rald277

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Location
Dallas Tx
ptonsparky said:
My daughter, the Energy Analyst, says "Don't guess. Look it up! Do the calculations."
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These units were in the specs. As it is a holiday weekend we can't question anybody about who sized them and why. Just trying to get a feel for if we should argue for them to be down sized or if we need to argue for a bump to 1000a service. Sizing HVAC is not really our thing so I was hoping some on here might know.
 
kwired

kwired

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Location
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We have kind of gone away from the electrical and into the HVAC side of things and questioning if the units are sized properly. I don't blame you for questioning that, but back to electrical - if you have 759 amps calculated load on an 800 amp service - you have no NEC violations if you have at least 759 amps worth of service conductors. I can understand you may have design concerns over possible future capacity.
 
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rald277

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Location
Dallas Tx
kwired said:
We have kind of gone away from the electrical and into the HVAC side of things and questioning if the units are sized properly. I don't blame you for questioning that, but back to electrical - if you have 759 amps calculated load on an 800 amp service - you have no NEC violations if you have at least 759 amps worth of service conductors. I can understand you may have design concerns over possible future capacity.
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Future load is part of the concern but what about loading the breaker at over 80%? BTW I realize I have strayed from the intent of the board but there always seems to be someone here with the knowledge.
 
kwired

kwired

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Location
NE Nebraska
rald277 said:
Future load is part of the concern but what about loading the breaker at over 80%? BTW I realize I have strayed from the intent of the board but there always seems to be someone here with the knowledge.
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Your art 220 load calculations for feeders and services already have adjustments for continuous loading included in them where necessary, you do not need to increase again after coming up with a final result.
 
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texie

Senior Member
Location
Fort Collins, Colorado
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Electrician, Contractor, Inspector
kwired said:
Your art 220 load calculations for feeders and services already have adjustments for continuous loading included in them where necessary, you do not need to increase again after coming up with a final result.
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Are we sure the OP means calculated load or is it connected load as he said? If he really meant connected load then he needs to rethink this as that is a meaningless number. 800 amps @208Y sounds like an awful lot of service for a dentist office of this size, even with 75 KW of heat (which is also questionable).
 
kwired

kwired

Electron manager
Location
NE Nebraska
texie said:
Are we sure the OP means calculated load or is it connected load as he said? If he really meant connected load then he needs to rethink this as that is a meaningless number. 800 amps @208Y sounds like an awful lot of service for a dentist office of this size, even with 75 KW of heat (which is also questionable).
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If he really means connected load, then he needs to go through art 220 and apply any demand factors allowed to come up with calculated load, or to omit the smaller of heating/cooling or other loads that do not run simultaneously where permitted to come up with calculated load.

Mgraw said:
The first question I would ask is if those 15kw heaters are 15kw @ 208v. Very common to have 15kw @ 240v heaters on 208v systems.
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If they are 20 kW heaters@ 240 volts and are connected to 208 volts they will draw right about 15 kW. But the HVAC guys don't always realize this.


But if they do realize the difference then they maybe supplied 15 kW @ 240 volt units, but are getting about 11.25 kW per unit @ 208 volts.

Add: If OP doesn't realize this he may have roughly 25% less heat load than he realizes.
 
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rald277

Member
Location
Dallas Tx
kwired said:
If he really means connected load, then he needs to go through art 220 and apply any demand factors allowed to come up with calculated load, or to omit the smaller of heating/cooling or other loads that do not run simultaneously where permitted to come up with calculated load.



If they are 20 kW heaters@ 240 volts and are connected to 208 volts they will draw right about 15 kW. But the HVAC guys don't always realize this.


But if they do realize the difference then they maybe supplied 15 kW @ 240 volt units, but are getting about 11.25 kW per unit @ 208 volts.

Add: If OP doesn't realize this he may have roughly 25% less heat load than he realizes.
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The 759 amps is calculated load. As whether they are 240v units I have know way of knowing at this time but it is something I had not taken into consideration. Thanks for that thought.
 
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gar

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Location
Ann Arbor, Michigan
Occupation
EE
130902-0045 EDT

A 25 kW resistive load at 120 V is 209 A. Connect 3 25 kW (208 V) loads in delta across a 208 delta, and the line current is still 209 A. Where is the problem? With 5 15 kW units you won't be balanced. Go to 6 for balance and the current is still only 250 A. Where is all this extra current load in a dentist's office?

.
 
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topgone

Senior Member
rald277 said:
We have a design and build on a 5000 sq ft doctors office here in Texas. The service is 120/208v 3 phase. Connected amps are 759 which puts me just under the 10% mark for an 800a service. I need to find another 10 or 11% Any of you with hvac experince feel like 5 15kva heat units on this building is a bit excessive? 75kva for this size building seems pretty high to me. Thoughts?:?
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I agree that the 5-15kVA heat pump units are excessive for a 5,000 sq. ft. office building. Even using a lower rule of thumb of 190 sq. ft. per ton, you will only need about 26.3 tons of heating (or cooling, as the case maybe), which will require you 26.3 kW of electrical input to the heat pumps (using SEER=12).
 
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Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
gar said:
130902-0045 EDT

A 25 kW resistive load at 120 V is 209 A. Connect 3 25 kW (208 V) loads in delta across a 208 delta, and the line current is still 209 A. Where is the problem? With 5 15 kW units you won't be balanced. Go to 6 for balance and the current is still only 250 A. Where is all this extra current load in a dentist's office?

.
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5 15 kw units would be 15 5 kw strips.
 
kwired

kwired

Electron manager
Location
NE Nebraska
david luchini said:
There is nothing in the article 220 calculations that mentions continuous loading.
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Fair enough. My point was that you do not need to add 25% for continuous loading, and maybe I should have said it in a different way.

As an example lets look at 220.12 for calculating general lighting load. It is possible the lighting may be continuous in some of the occupancies in table 220.12. but we do not add an additional 25% to the value we come up with, we just plug 100% in to our load calculation, and maybe even apply demand factors to it at some point.
 
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