Mike Scarchilli
Member
- Location
- Fishkill, NY. USA
I have a job at an Air National Guard base job in Scotia NY in a a bit of a jamb. The Engineers specified 460V 3ph. HVAC RTUs They missed the fact that the existing units were 208V. :blink:
So the facility electrician and the idea to put transformers on the roof. Sounded great, except the engineer was coming up with crazy pipe and wire upgrades i did not feel was necessary. can you guys just review my math in my emails to them and let me know who is right here. Wouldn't the nameplate max breaker size be the safe calculation?
[FONT="]Please review my math with me[/FONT]
[FONT="] [/FONT]
[FONT="]RTU1 Load calculations ( all KVA was based off MAX CB as i did not have actual Full load current[/FONT])
[FONT="] [/FONT]
[FONT="]Secondary side (Unit load)[/FONT]
[FONT="]480v* x 90a x 1.732[/FONT][FONT="] =74.83 KVA [/FONT][FONT="]Need 75 KVA trans[/FONT]
[FONT="] 1000[/FONT]
[FONT="] [/FONT][FONT="](*Need to check w. Mfr for tolerance of Unit as it?s rated for 460V) **This was my comment as I wanted to be sure that applying 480v to a 460 unit wouldn't damage the equipment[/FONT].
[FONT="] [/FONT]
[FONT="]Primary side[/FONT]
[FONT="]75 KVA x 1000[/FONT][FONT="] = 208 A (Primary breaker size) Actually it is the load size, now multiply by 1.25 to get the breaker size. [/FONT]
[FONT="](SQ RT3) X 208v [/FONT]
[FONT="]208*1.25 = 260amps (engineer's comment)[/FONT]
[FONT="]If I remember the existing wiring for RTU1 was 3/0 with existing breaker 225. As per your sketch, why would we need a new 300A 3p breaker and run new 350 mcm wire? [/FONT]
[FONT="] [/FONT]
[FONT="] [/FONT]
[FONT="]RTU2[/FONT]
[FONT="]Secondary [/FONT]
[FONT="]480x80x1.732 [/FONT][FONT="]= 66.52 KVA Use same 75 KVA trans. Can use same primary load calcs. at 208 Amps. [/FONT]
1000
[FONT="] [/FONT]
[FONT="]Existing primary wiring and breaker is the same as above. Why would we need to tap into that other panel, run 350 mcm and a new 400 amp disc w/ 300 amp fuse[/FONT]
**the engineer made a sketch to upsize the wire to 350 MCM and 300 amps???? wouldn't 208 amps be correct?
[FONT="]
RTU3[/FONT]
[FONT="]Secondary[/FONT]
[FONT="]480x50x1.732[/FONT][FONT="] = 41.56 KVA use 45 kva trans.[/FONT]
[FONT="] 1000[/FONT]
[FONT="]Primary[/FONT]
[FONT="]45kva x 1000[/FONT][FONT="] = 124.9A Has 100amp existing breaker and I believe #1 AWG (155A) wire . Will need 125A CB upsizing but why would we need to install 175 breaker and 3/0 wiring?[/FONT]
[FONT="]1.732 x 208[/FONT]
[FONT="] [/FONT]
[FONT="]RTU 4[/FONT]
[FONT="]Secondary [/FONT]
[FONT="]480x30x1.732[/FONT][FONT="] = 24.94 [/FONT]
[FONT="] 1000 Use 30 KVA trans[/FONT]
[FONT="] [/FONT]
[FONT="]Primary[/FONT]
[FONT="]30 KVa x 1000 [/FONT][FONT="]= 83 amps [/FONT]
[FONT="]1.732 x 208[/FONT]
[FONT="] [/FONT]
[FONT="]The existing unit had a 50 amp breaker but ihave to guess that the wiring is rated for the same. This wiring will need to be upgraded. [/FONT]
End of my first email
Her response
Hi Michael. Your math is good but remember you have to size per inrush current too. According to the equipment documents, you have to size it at an additional 1.2 * the load. We also sized per the actual demand on the units not the circuit breaker rating. Kevin has that if you would like. Also you need to multiply the load by 1.25 to get the breaker sizes. Once you do that you find that the breaker sizes go up and in order to protect the wire correctly by the breaker you have to upsize. Otherwise you won?t have any protection on your wire. If you have any other questions please feel free to let me know. Thank you!!
And my reply back
Yes Michelle, I see your point using the actual KVA, I did not have that actual figure. However I figured that by using the MAX breaker size in the equipment, it would be the safe calculation knowing the equip engineers figure for start up currents. Also Transformers can basically able to be ?overused? by 125% so any inrush currents would probably not affect the transformer. No?
Lastly you are correct about the wire in a HVAC installation we have to over rate the wire by 125% But I figured that the nameplate Max breaker size was already figuring for this over-sizing requirement.
Take for example HV-23 on Bldg 7 has a 30 amp load with a max breaker set for 60a , this which is already set at 200% (max of 225% by NEC). Wire sizing was set at 125% of the 30 amps (=37 amps) allowing us to run #8 AWG rated at 55 amps at 90 deg C.
So basically, final calculated KVA is KVA whether Primary or Secondary and I figured for the max?. or as I thought???
Am i not correct in using the nameplate max CB size to use when calculating the KVA and transfomer sizing? Or is the Engineer with all her schooling correct? If we used actual KVA of the units then i could see adding 125% for wire and breaker sizing. But I thought the name plate is the law... Finally, KVA is KVA is it not?
Your responses will be much appreciated. Sorry for the long post:ashamed:
So the facility electrician and the idea to put transformers on the roof. Sounded great, except the engineer was coming up with crazy pipe and wire upgrades i did not feel was necessary. can you guys just review my math in my emails to them and let me know who is right here. Wouldn't the nameplate max breaker size be the safe calculation?
[FONT="]Please review my math with me[/FONT]
[FONT="] [/FONT]
[FONT="]RTU1 Load calculations ( all KVA was based off MAX CB as i did not have actual Full load current[/FONT])
[FONT="] [/FONT]
[FONT="]Secondary side (Unit load)[/FONT]
[FONT="]480v* x 90a x 1.732[/FONT][FONT="] =74.83 KVA [/FONT][FONT="]Need 75 KVA trans[/FONT]
[FONT="] 1000[/FONT]
[FONT="] [/FONT][FONT="](*Need to check w. Mfr for tolerance of Unit as it?s rated for 460V) **This was my comment as I wanted to be sure that applying 480v to a 460 unit wouldn't damage the equipment[/FONT].
[FONT="] [/FONT]
[FONT="]Primary side[/FONT]
[FONT="]75 KVA x 1000[/FONT][FONT="] = 208 A (Primary breaker size) Actually it is the load size, now multiply by 1.25 to get the breaker size. [/FONT]
[FONT="](SQ RT3) X 208v [/FONT]
[FONT="]208*1.25 = 260amps (engineer's comment)[/FONT]
[FONT="]If I remember the existing wiring for RTU1 was 3/0 with existing breaker 225. As per your sketch, why would we need a new 300A 3p breaker and run new 350 mcm wire? [/FONT]
[FONT="] [/FONT]
[FONT="] [/FONT]
[FONT="]RTU2[/FONT]
[FONT="]Secondary [/FONT]
[FONT="]480x80x1.732 [/FONT][FONT="]= 66.52 KVA Use same 75 KVA trans. Can use same primary load calcs. at 208 Amps. [/FONT]
1000
[FONT="] [/FONT]
[FONT="]Existing primary wiring and breaker is the same as above. Why would we need to tap into that other panel, run 350 mcm and a new 400 amp disc w/ 300 amp fuse[/FONT]
**the engineer made a sketch to upsize the wire to 350 MCM and 300 amps???? wouldn't 208 amps be correct?
[FONT="]
RTU3[/FONT]
[FONT="]Secondary[/FONT]
[FONT="]480x50x1.732[/FONT][FONT="] = 41.56 KVA use 45 kva trans.[/FONT]
[FONT="] 1000[/FONT]
[FONT="]Primary[/FONT]
[FONT="]45kva x 1000[/FONT][FONT="] = 124.9A Has 100amp existing breaker and I believe #1 AWG (155A) wire . Will need 125A CB upsizing but why would we need to install 175 breaker and 3/0 wiring?[/FONT]
[FONT="]1.732 x 208[/FONT]
[FONT="] [/FONT]
[FONT="]RTU 4[/FONT]
[FONT="]Secondary [/FONT]
[FONT="]480x30x1.732[/FONT][FONT="] = 24.94 [/FONT]
[FONT="] 1000 Use 30 KVA trans[/FONT]
[FONT="] [/FONT]
[FONT="]Primary[/FONT]
[FONT="]30 KVa x 1000 [/FONT][FONT="]= 83 amps [/FONT]
[FONT="]1.732 x 208[/FONT]
[FONT="] [/FONT]
[FONT="]The existing unit had a 50 amp breaker but ihave to guess that the wiring is rated for the same. This wiring will need to be upgraded. [/FONT]
End of my first email
Her response
Hi Michael. Your math is good but remember you have to size per inrush current too. According to the equipment documents, you have to size it at an additional 1.2 * the load. We also sized per the actual demand on the units not the circuit breaker rating. Kevin has that if you would like. Also you need to multiply the load by 1.25 to get the breaker sizes. Once you do that you find that the breaker sizes go up and in order to protect the wire correctly by the breaker you have to upsize. Otherwise you won?t have any protection on your wire. If you have any other questions please feel free to let me know. Thank you!!
And my reply back
Yes Michelle, I see your point using the actual KVA, I did not have that actual figure. However I figured that by using the MAX breaker size in the equipment, it would be the safe calculation knowing the equip engineers figure for start up currents. Also Transformers can basically able to be ?overused? by 125% so any inrush currents would probably not affect the transformer. No?
Lastly you are correct about the wire in a HVAC installation we have to over rate the wire by 125% But I figured that the nameplate Max breaker size was already figuring for this over-sizing requirement.
Take for example HV-23 on Bldg 7 has a 30 amp load with a max breaker set for 60a , this which is already set at 200% (max of 225% by NEC). Wire sizing was set at 125% of the 30 amps (=37 amps) allowing us to run #8 AWG rated at 55 amps at 90 deg C.
So basically, final calculated KVA is KVA whether Primary or Secondary and I figured for the max?. or as I thought???
Am i not correct in using the nameplate max CB size to use when calculating the KVA and transfomer sizing? Or is the Engineer with all her schooling correct? If we used actual KVA of the units then i could see adding 125% for wire and breaker sizing. But I thought the name plate is the law... Finally, KVA is KVA is it not?
Your responses will be much appreciated. Sorry for the long post:ashamed: