I am faced with a situation that I'm sure many others are faced with when loading Power Distribution Units (PDU) in a computer facility. Utilizing name plate ratings of PC's, printers, and computer monitors you very quickly run out of breaker spaces and end up loading a PDU to about 10% of its load capacity. Does anyone know of a code compliant method that can be used to apply diversity factors to the name plate ratings of computer equipment thus enabling you to put multiple computers on a 20 amp branch circuit based on actual running loads?
Diversity Factors and Branch Circuits
- Thread starter smoke
- Start date
- Status
Re: Diversity Factors and Branch Circuits
If article 645 applies, branches are to be 125% rated. Based on 220.3, outlets loads are based on "ampere rating of the appliance (utilization equipment per article 100) OR load served. Where the outlets are used for PC's that are moved around, we have found that 400VA covers the monitor and average computer. If you check with the manufacturer, you will probably find that the label covers the largest power supply and aux. load possible and not the actual of the unit. For other equipment, and printers, the nameplate should be accurate. Article 220-35 permits using the existing load plus 25%. As far as a NEC demand factor, I don't know of one, unless the IS guys have set up housekeeping with a bed and stove!
If article 645 applies, branches are to be 125% rated. Based on 220.3, outlets loads are based on "ampere rating of the appliance (utilization equipment per article 100) OR load served. Where the outlets are used for PC's that are moved around, we have found that 400VA covers the monitor and average computer. If you check with the manufacturer, you will probably find that the label covers the largest power supply and aux. load possible and not the actual of the unit. For other equipment, and printers, the nameplate should be accurate. Article 220-35 permits using the existing load plus 25%. As far as a NEC demand factor, I don't know of one, unless the IS guys have set up housekeeping with a bed and stove!
Re: Diversity Factors and Branch Circuits
I assume these computers and printers, etc, are cord and plug connected. If each receptacle is installed specifically to power a paticular computer or printer or monitor, I would guess you have to follow 220.3(B)(1).
However, I find that very unlikely. I would assume the outlets are general purpose and count them at 180VA per outlet per 220.3(B)(9). This would allow up to 13 outlets per 20A circuit. But then I would limit this to maybe 2 or 3 workstations per circuit to prevent breakers from tripping.
Steve
I assume these computers and printers, etc, are cord and plug connected. If each receptacle is installed specifically to power a paticular computer or printer or monitor, I would guess you have to follow 220.3(B)(1).
However, I find that very unlikely. I would assume the outlets are general purpose and count them at 180VA per outlet per 220.3(B)(9). This would allow up to 13 outlets per 20A circuit. But then I would limit this to maybe 2 or 3 workstations per circuit to prevent breakers from tripping.
Steve
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Diversity Factors and Branch Circuits
Are these installed machines, or are you laying out for design?
We almost always run out of branch breakers in a RPP/PDU before we run out of capacity.
Are the branch breakers serving a duplex receptacle or a power strip?
Are these installed machines, or are you laying out for design?
We almost always run out of branch breakers in a RPP/PDU before we run out of capacity.
Are the branch breakers serving a duplex receptacle or a power strip?
Re: Diversity Factors and Branch Circuits
The receptacles are mostly duplex, no plugstrips at all. The computers are powered up 24-7 (NASA Facility), all equipment runs no where near nameplate, as is true in almost every nameplate rating that exists (proven fact). I understand the implications of harmonics and impact on distribution systems, but it really seems like overkill and waste when you can only load a 125 KVA PDU to 30-50% of it's rating because of all the 20 amp code required breaker spaces calculated using nameplate ratings on equipment.
Smoke
The receptacles are mostly duplex, no plugstrips at all. The computers are powered up 24-7 (NASA Facility), all equipment runs no where near nameplate, as is true in almost every nameplate rating that exists (proven fact). I understand the implications of harmonics and impact on distribution systems, but it really seems like overkill and waste when you can only load a 125 KVA PDU to 30-50% of it's rating because of all the 20 amp code required breaker spaces calculated using nameplate ratings on equipment.
Smoke
caosesvida
Senior Member
Re: Diversity Factors and Branch Circuits
I have also run across this in schools. I have gone with running for nameplate. Yes it was overkill. BUt things change and you may need it in the future, allow for addding another subpanel next to one you install if more space is needed. Is there an actual load calculation that can be done after they are installed, as opposed to calculated load? Upsize you neutral also.
I have also run across this in schools. I have gone with running for nameplate. Yes it was overkill. BUt things change and you may need it in the future, allow for addding another subpanel next to one you install if more space is needed. Is there an actual load calculation that can be done after they are installed, as opposed to calculated load? Upsize you neutral also.
wanderer20001us
Senior Member
Re: Diversity Factors and Branch Circuits
I have taken the approach of running a circuit(s) to a protected plug strip(s) in a rack and then designating the capacity of the rack at the NEC circuit capacity in VA. The relative loading of the rack is then determined as equipment comes and goes. You are typically not going to get more than 50% PDU utilization, though.
I typically put (2) 30A, 208V circuits to each rack. It uses the same breaker space as (4) 20A, 120V circuits but delivers for more VA (~30%)to the rack. There are many protected plug strips out there for this type of circuit design.
Sam A. - PE
I have taken the approach of running a circuit(s) to a protected plug strip(s) in a rack and then designating the capacity of the rack at the NEC circuit capacity in VA. The relative loading of the rack is then determined as equipment comes and goes. You are typically not going to get more than 50% PDU utilization, though.
I typically put (2) 30A, 208V circuits to each rack. It uses the same breaker space as (4) 20A, 120V circuits but delivers for more VA (~30%)to the rack. There are many protected plug strips out there for this type of circuit design.
Sam A. - PE
- Status