Wireway - computer rooms

[aries]

I need to install a bunch of outlets in a computer room underneath their raised floor. The intent is to use wiremold 6000 raceway (see: http://www.legrand.us/wiremold/race...-raceway/6000-large-raceway.aspx#.UHyIt7TNWS1 ) with receptacles installed at each point where a computer cabinet is located. Each receptacle is to be on its own dedicated 30A branch circuit. They have 30 computer cabinets in a row.

This application would seem to run up against the limits on no more than 30ccc in the raceway, which in this application would be 15 circuits. Client needs more than 15 circuits (theres 30 cabinets).

Wiremold provides a fill chart for this product which suggests considerably more conductors can be placed into their product than NEC would seem to allow (see this fill chart PDF: http://www.legrand.us/~/media/C7A780E6EE1342FF8724D0D608901A1F.ashx ). Given the wiremold product is specifically designed for this sort of purpose; do these code limits apply here? I know also that there are some code variations where IT equipment rooms are involved. How might one resolve this issue?

Also to note, this wiremold will run underneath their raised computer flooring which is also used for the rooms cooling.

Any comments are appreciated.

 

[__dan]

Laborwise, I'm thinking five runs of 3/4" EMT, maybe 4 11/16" deep boxes ?, three circuits each with dedicated ground, H H G, would be a lot less material cost and less labor. Just guessing. No one will see it below the floor. Savings might be half, easier to work with.

Looks like surface metal raceway with snap covers is listed in 300.22 C 1 so you're OK there. Don't know about the conductor limit, what code reference?

 

[__dan]

Edit: I'm looking at 386.22 generally it only says derating may apply unless conditions 1, 2, and 3 are met. It does not limit the total to 30 CCC, only that derating will apply.

 

[infinity]

The 30 CCC limit is only for derating purposes (no derating required) and can be exceeded when the conductor ampacity is adjusted and when the conductor fill is not more than 20%. Here's the relvant article under ARTICLE 386
Surface Metal Raceways:

386.22 Number of Conductors or Cables. The number of
conductors or cables installed in surface metal raceway
shall not be greater than the number for which the raceway
is designed. Cables shall be permitted to be installed where
such use is not prohibited by the respective cable articles.
The adjustment factors of 310.15(B)(3)(a) shall not ap-
ply to conductors installed in surface metal raceways where
all of the following conditions are met:
(1) The cross-sectional area of the raceway exceeds
2500 mm2(4 in.2).
(2) The current-carrying conductors do not exceed 30 in
number.
(3) The sum of the cross-sectional areas of all contained
conductors does not exceed 20 percent of the interior
cross-sectional area of the surface metal raceway.

 

[aries]

The 30 CCC limit is only for derating purposes (no derating required) and can be exceeded when the conductor ampacity is adjusted and when the conductor fill is not more than 20%. Here's the relvant article under ARTICLE 386
Surface Metal Raceways:

Indeed, this was my interpretation .. that de-rating adds up fast! My interpretation of how to apply the adjustment factors is that they apply to all conductors in _excess_ of the 30 with the conductor count in the first column of that table being adjusted down by 3. For example, in a scenario with 31 conductors you get:

Conductors 1 - 30: not derated
Conductors 31: 80%

With say 40 conductors you get

Conductors 1-30: not derated
Conductors 31-33: 80%
Conductors 34-36: 70%
Conductors 37-40: 50%

Thoughts?

 

[Smart $]

Indeed, this was my interpretation .. that de-rating adds up fast! My interpretation of how to apply the adjustment factors is that they apply to all conductors in _excess_ of the 30 with the conductor count in the first column of that table being adjusted down by 3. For example, in a scenario with 31 conductors you get:

Conductors 1 - 30: not derated
Conductors 31: 80%

With say 40 conductors you get

Conductors 1-30: not derated
Conductors 31-33: 80%
Conductors 34-36: 70%
Conductors 37-40: 50%

Thoughts?

I don't believe that is the intent, but I do see how it can be interpreted that way. I've always been of the impression that any required derating applied to all 30+ conductors.

 

[fmtjfw]

I don't believe that is the intent, but I do see how it can be interpreted that way. I've always been of the impression that any required derating applied to all 30+ conductors.

The derating factor based on the number of conductors applies to all the current carrying conductors. Consider the following: If you had 3 4/0, 3 3/0, and 30 #12 you would have a total of 36 conductors in a conduit which would give you 40% of full value for each conductor. If you thought of it the other way would you derate 3 conductors at 100%, 3 at 80%, 10 at 50%, 10 at 45%, and 6 at 40%? If that were the case Which conductors would you assign to which percentage?

This leads to madness.

 

[Smart $]

The derating factor based on the number of conductors applies to all the current carrying conductors. Consider the following: If you had 3 4/0, 3 3/0, and 30 #12 you would have a total of 36 conductors in a conduit which would give you 40% of full value for each conductor. If you thought of it the other way would you derate 3 conductors at 100%, 3 at 80%, 10 at 50%, 10 at 45%, and 6 at 40%? If that were the case Which conductors would you assign to which percentage?

This leads to madness.

I think you are replying to the wrong person ... I'm already past that point :happyyes:

My reply was an attempt to put it mildly and avoid explanations coupled with examples... and you stepped right into the gaping hole I left :jawdrop:

But since you have taken it to the next level, the jump from no derating fof 30 ccc's to 40% for 31 ccc's defies the logical application of physics. Why not a more gradual change in derating percentage??? Another gaping hole, or rhetorical remark... you choose.

 

[infinity]

I've always been of the impression that any required derating applied to all 30+ conductors.

I agree once you exceed the 30 CCC's then they all require derating.

 

[fmtjfw]

I think you are replying to the wrong person ... I'm already past that point :happyyes:

My reply was an attempt to put it mildly and avoid explanations coupled with examples... and you stepped right into the gaping hole I left :jawdrop:

But since you have taken it to the next level, the jump from no derating fof 30 ccc's to 40% for 31 ccc's defies the logical application of physics. Why not a more gradual change in derating percentage??? Another gaping hole, or rhetorical remark... you choose.

My partner will not let me near ditches, I'm always stepping into them. Too much thinking, not enough looking.

I agree about the steps in the table. But these are:
1) produced by committee,
2) not for electrical engineers, hence no differential equations,
3) probably good enough.

To do it "right" it would vary on the type of raceway (plastic versus metallic, steel versus brass, aluminum), arrangement of wires (pile in the bottom of the wireway or trained back against the back wall), thermal conductivity of the wall the raceway was mounted on, thermal conductivity of the boxes of toilet paper piled in front of and on top of the raceway, altitude, .... This also leads to madness and confusion.:?

 

[aries]

Thanks guys for the awesome replies; this is very helpful.

One more item - is there any way around this limitation such as having a EE do the design and sign off on it? The derating is quite detrimental in this project...

 

[__dan]

Thanks guys for the awesome replies; this is very helpful.

One more item - is there any way around this limitation such as having a EE do the design and sign off on it? The derating is quite detrimental in this project...

What code cycle are you on and is the room wiring method complying with 300.22 C or D + 645.4 (2005). Are the outlets and portable cords allowed in the underfloor space? There are threads on this site discussing this.

 

[aries]

What code cycle are you on and is the room wiring method complying with 300.22 C or D + 645.4 (2005). Are the outlets and portable cords allowed in the underfloor space? There are threads on this site discussing this.

2008, yes room methods are complying with 300.22(d) and 645.

 

[iwire]

This application would seem to run up against the limits on no more than 30ccc in the raceway, which in this application would be 15 circuits. Client needs more than 15 circuits (theres 30 cabinets).

Make two runs or use one run but supply from two ends, or an end and the middle etc.

 

[barclayd]

Make two runs or use one run but supply from two ends, or an end and the middle etc.

Agree - split it up.
NOTE: A lot of data centers are embracing 'dual-corded' equipment that requires circuits from two sources. That might just double everything - has there been any discussion in that regard?
db

 

[aries]

Agree - split it up.
NOTE: A lot of data centers are embracing 'dual-corded' equipment that requires circuits from two sources. That might just double everything - has there been any discussion in that regard?
db

Indeed, and that is part of the issue here. There will already be two wireways as there is an "A side" and a "B side" electrical system which are entirely independent. Client is very sensitive about the underfloor and everything being neat and tidy which is why a whole mess of emt (etc) feeding the raceway at various points is undesirable. Every angle we look at we keep coming up against the 30ccc limit and all the "workarounds" are less tidy. I wonder if installing a divider in the raceway could accomplish this. Ive never used divider before but that's an angle I will investigate today. The 6000 wiremold wireway is definitely large enough to accomodate a divider with some circuits and devices on one side and the other half on the other. Thoughts here?

This would seem to be a very common problem in datacenters, wonder what others do...

 

[fmtjfw]

Have you already bid this and are stuck with a fixed (too low) price?

Have you already bid this and are stuck with a fixed (too low) price?

I'm having a hard time understanding why you don't want to do this right, using the derating as required by the code.
If your customer wants the area under the raised floor to look neat, he will not like the looks of it after the electrical fire.

That said.

Assuming you are supplying single phase to 30 outlets and you are not sharing neutrals, that takes 60 ccc. If you just place 2 WM6000 runs side by side with half of the circuits in each, you are down to 30 ccc for each and no derating.

Assuming you are supplying three-phase to 30 outlets, that takes 90 ccc (unless you have high harmonic content which brings it up to 120). With 3 side by side with 1/3 of the circuits in each you are once again back to 30 and no derating.

Now we have the problem of how to feed the WM. One way is to terminate the WM at the panelboard. If you use conduit then you are back to even more derating.

If you can feed the WM from the center (think two ELLs back to back) then you need 2 6000s from the panelboard to the center of the area for the receptacle WM. Then you only have 30 ccc per WM.

The three-phase configuration is left to the student.

By the way WM4000 would work and save you a little money. WM3000 would also work for the single-phase version and would save you even more money. WM3000 would save space under the floor as well.

 

[weressl]

I need to install a bunch of outlets in a computer room underneath their raised floor. The intent is to use wiremold 6000 raceway (see: http://www.legrand.us/wiremold/race...-raceway/6000-large-raceway.aspx#.UHyIt7TNWS1 ) with receptacles installed at each point where a computer cabinet is located. Each receptacle is to be on its own dedicated 30A branch circuit. They have 30 computer cabinets in a row.

This application would seem to run up against the limits on no more than 30ccc in the raceway, which in this application would be 15 circuits. Client needs more than 15 circuits (theres 30 cabinets).

Wiremold provides a fill chart for this product which suggests considerably more conductors can be placed into their product than NEC would seem to allow (see this fill chart PDF: http://www.legrand.us/~/media/C7A780E6EE1342FF8724D0D608901A1F.ashx ). Given the wiremold product is specifically designed for this sort of purpose; do these code limits apply here? I know also that there are some code variations where IT equipment rooms are involved. How might one resolve this issue?

Also to note, this wiremold will run underneath their raised computer flooring which is also used for the rooms cooling.

Any comments are appreciated.

Wouldn't wirebasket/tray with tray cable a more flexible installation?

 

[steve66]

Indeed, and that is part of the issue here. There will already be two wireways as there is an "A side" and a "B side" electrical system which are entirely independent. Client is very sensitive about the underfloor and everything being neat and tidy which is why a whole mess of emt (etc) feeding the raceway at various points is undesirable. Every angle we look at we keep coming up against the 30ccc limit and all the "workarounds" are less tidy. I wonder if installing a divider in the raceway could accomplish this. Ive never used divider before but that's an angle I will investigate today. The 6000 wiremold wireway is definitely large enough to accomodate a divider with some circuits and devices on one side and the other half on the other. Thoughts here?

This would seem to be a very common problem in datacenters, wonder what others do...

I generally agree with Iwire and others - split it up.

But I will add - I think you could get some relief from the requirement if the design is such that not all the wires will be current carrying at the same time. For example, if half the racks are operating off the A side, and the B wires going to those racks will only be loaded if the A side fails. (This doesn't work if all the wires are only half loaded. It may not make sense, but that's the way the requirement was worded.)

I would not do this for a datacenter, but I believe a divider in the wireway creates 2 wireways. So you could have 60 conductors without derating. But I could be wrong.

For a data center, I would probably suggest going above and beyond the code unless you are absolutely sure the heat won't be a problem. Just following the code doesn't guarentee something will work.

Others split up the feeds, often with conduits. 200 amp bus ducts with outlet boxes that include receptacles and branch breakers are also common.

 

[fmtjfw]

Wouldn't wirebasket/tray with tray cable a more flexible installation?

These would be a perfect dust bunny trap