minimum size sub panel

[Daja7]

Question on calculating 3 phase loads. I have a friend that owns an office / warehouse and wants to transfer a bunch of circuits to a different new sub panel and would like to know the min size required. It has been years since i did any calculations and cannot seem to get it right.

he has a MDP to feed sub panel 3 phase 120/208

A/C and air handler 3 ph total amperage when running 50.3 amp

water heater 4500 watt 208 volt 1 ph

lights in office 52- 4 tube t-8 electronic 120 volt

62 receptacles general purpose

2 bath fans 1/3 hp each 120 volt

That is it. he would like to get away with a 100 amp 3 phase panel

He would need a load letter for the inspector

I realize this should be easy but as i say, I have been out of this for many, many years.
Thanks in advance

 

[charlie b]

Since all the loads are known, this should be a straight forward exercise. Just create a list of loads, such as you have already started. But show the load values in units of power (VA), not in units of amps. For the HVAC equipment, that would be 50.3 amps times 208 volts times 1.732 (the square root of 3), or 18,108 VA. For the water heater that would be 4,500 VA. For the lights, you need to know the wattage of each bulb, and then multiply by 52 and again by 4. Then multiply the total by 1.25, because you need to count lights at 125%. The receptacles get counted at 180 VA each, but you can reduce the total by 50% of the amount that exceeds 10,000 VA. Each of the bath fans would be counted at 864 VA (7.2 amps at 120 volts).

 

[Smart $]

Basically agree with Charlie, but you will have to use nameplate values for the HVAC equipment (Minimum Circuit Ampacity or MCA). And for the lighting, you'll have to use nameplate value on the ballasts. That will likely be in amps, so just multiply by voltage and number of ballasts.

 

[mwm1752]

Curious - since most loads described are single phase type would you multiply the final wattage total by .866(inverse of 1.73) to convert to 3 phase load calc?

 

[petersonra]

because you need to count lights at 125%.

why is that?

 

[Smart $]

why is that?

I think he was assuming, being it is office lighting, all the lights would be on, at least at times, for more than 3 hours.

 

[petersonra]

I think he was assuming, being it is office lighting, all the lights would be on, at least at times, for more than 3 hours.

isn't that only for calculating the wire size though?

 

[Smart $]

isn't that only for calculating the wire size though?

Panel ocpd, too... subsequently minimum panel size.

 

[Smart $]

Curious - since most loads described are single phase type would you multiply the final wattage total by .866(inverse of 1.73) to convert to 3 phase load calc?

Actually, the load calculation result is in kVA. Determining amperes from that value is for wire and ocpd sizing, which is technically not part of the load calculation.

 

[petersonra]

Panel ocpd, too... subsequently minimum panel size.

but not for the actual load.

 

[Smart $]

but not for the actual load.

Correct, not the actual calculated load... but you have to track continuous loads through the load calculation so you can apply 125% when determining wire and ocpd sizes. Some just prefer to do it within the load calculation.

 

[kwired]

Question on calculating 3 phase loads. I have a friend that owns an office / warehouse and wants to transfer a bunch of circuits to a different new sub panel and would like to know the min size required. It has been years since i did any calculations and cannot seem to get it right.

he has a MDP to feed sub panel 3 phase 120/208

A/C and air handler 3 ph total amperage when running 50.3 amp

water heater 4500 watt 208 volt 1 ph

lights in office 52- 4 tube t-8 electronic 120 volt

62 receptacles general purpose

2 bath fans 1/3 hp each 120 volt

That is it. he would like to get away with a 100 amp 3 phase panel

He would need a load letter for the inspector

I realize this should be easy but as i say, I have been out of this for many, many years.
Thanks in advance

Seems like a pretty big AC unit, unless it is cooling the warehouse, is single phase or you are misunderstanding what to look for, 4500 watt water heater is typical rating @ 240 volts but will draw less @ 208 volts - maybe around 3800, but you can get 4500 watt 208 elements if you wish they just are not all that common.

As some have mentioned you need to look at VA not just amps and balance is as well as possible across the three phases.

100 A x 208V x 1.732 = 36026 VA If you were dealing with 120/240 single phase you would need a 150 amp feed to be able to handle the same VA.

 

[kwired]

Quick figures with what was mentioned so far:

AC - (assuming it does draw 50.3 amps @ 208v 3 phase) = 18,120 VA

Water heater 4500 watts = 4500 VA

lights (figuring a 4 lamp ballast draws approximately 1 amp @ 120 volts) 52amps x120V = 6240 VA

62 receptacles @ 180VA each = 11,160VA

2 - 1/3 hp motors (@ NEC full load values) = 1728VA

sum of all the above is 41,748 VA which is about 116 amps, but we don't have enough details to know for certain that it the load we need to account for.

Even before I did any of above calculating, I was thinking one should run the AC on a separate feed and the rest is probably fine.