NEC 240.86 and how it pertains to rack mounted PDUs in Data center

R

Ruckusmtb

Member
Location
RR - TX
Occupation
Data Center / Master Electrician
Customer question:
"Long story short, the rPDUs we have 5KA rated breakers within them and I'm needing to reduce the fault current to an acceptable level from my busway.
To reduce fault current to a level acceptable for the PDU breakers, I planned to use a Bussmann fuse upstream. However, since the breakers are active components, I understand that NEC 240.86 requires a tested and listed series-rated combination between the fuse and breaker, not just fuse sizing based on let-through energy."

Would the breakers in the manufactured rack mounted PDU need to be considered under this article? Its a 50amp 14.4 kW rPDU with 6 branch circuit breakers.
 
Ruckusmtb said:
Customer question:
"Long story short, the rPDUs we have 5KA rated breakers within them and I'm needing to reduce the fault current to an acceptable level from my busway.
To reduce fault current to a level acceptable for the PDU breakers, I planned to use a Bussmann fuse upstream. However, since the breakers are active components, I understand that NEC 240.86 requires a tested and listed series-rated combination between the fuse and breaker, not just fuse sizing based on let-through energy."

Would the breakers in the manufactured rack mounted PDU need to be considered under this article? Its a 50amp 14.4 kW rPDU with 6 branch circuit breakers.
Click to expand...
Is possible submit NTRL testing and relabeling the install with Busmann fuse upstream?
 
How long is the power cord between the busway plug and the PDU? Did you include that in you available fault current calculation? What voltage are these operating at (120, 208, ??).
 
You are absolutely on target for checking on this. You can take the impedance of the whip into account when calculating available fault current, but you cannot use any "current limiting" from the busway fuse unless it has been series listed (tested in combination) with the breaker / fuse in the in rack PDU (plugstrip) due to dynamic impedance from the breaker in the plugstrip. To make things worse, the breaker in the plugstrip is often a hydraulic type rather than a typical thermal magnetic.
 
ron said:
To make things worse, the breaker in the plugstrip is often a hydraulic type rather than a typical thermal magnetic.
Click to expand...
Why would that make it worse? I have never worked with a hydraulic breaker and the only think I know about them is that they are not temperature sensitive like thermal magnetic breakers.
 
don_resqcapt19 said:
Why would that make it worse? I have never worked with a hydraulic breaker and the only think I know about them is that they are not temperature sensitive like thermal magnetic breakers.
Click to expand...
There are few if any existing series ratings with hydraulic breakers and regular thermal magnetic breakers to build a list from without have to do new combinations.
 
suemarkp said:
How long is the power cord between the busway plug and the PDU? Did you include that in you available fault current calculation? What voltage are these operating at (120, 208, ??).
Click to expand...
Here is the specs on the rPDU - 208 4w 3ph PDU with a 10ft 8 awg SO cable
 
Not sure if you know the true AFC and impedance of the busway and source transformer, but assuming it is infinite if you had a longer run of #8 you can cut it down pretty fast. Maybe a ~30' branch circuit instead of 10'. Does it make sense to use a cord that long or to put a drop box using an additional 25' or so of #8 to do nothing but adding impedance for this install?
 
You must log in or register to reply here.
Top