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Topic Title: DC Cable Calculation-1250amps
Topic Summary: DC cable calcs-48v
Created On: 30 October 2013 09:19 PM
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 30 October 2013 09:19 PM
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BILLYBOY65

Posts: 35
Joined: 11 May 2011

Hi

Can any one help me do a cable calculation, need to size a set of DC cables in a Data centre, 48v ,max 4 cables in parallel each Pos and Neg, Design Current 1250amps. Cable run under a suspended floor clipped direct. HO7 single core flex.
My Castline Cablecalc freaks out when I put the Data in.

Alternatively can anyone put me in contact with someone that deals with DC supplies and wants some freelance Design work ?

Cheers
Billy
 30 October 2013 10:24 PM
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HarryJMacdonald

Posts: 254
Joined: 15 May 2002

At 48V volt drop is the key thing to watch for. Current calcs can be done from the tables quite easily.

What parameters have you been given?

The 4% commonly used is derived from a general requirement that AC equipment can tolerate +- 10% and the Supply authority can use 6% of this leaving 4% for the user. These criteria do not apply in your case.

DC equipment is often very tolerant of reduced voltages, (protection relays operate at -20% of nominal) but your source may use up some of this (battery end of discharge is often defined as when it produces nominal -10%).
 31 October 2013 03:28 AM
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Jaymack

Posts: 4585
Joined: 07 April 2004

Originally posted by: BILLYBOY65
Need to size a set of DC cables in a Data centre, 48v ,max 4 cables in parallel each Pos and Neg, Design Current 1250amps. Cable run under a suspended floor clipped direct. HO7 single core flex.

I suggest that you contact cable suppliers, or the ERA for guidance; The method of installation, including containment and bracing the cables under fault conditions, is an important consideration.

Regards
 31 October 2013 08:34 AM
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broadgage

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Joined: 07 August 2007

Something else for you or your customer to consider is the value of energy wasted by voltage drop, this can be significant for a long hour load.

4 volts voltage drop at 1250 amps is 5KW lost.
And to produce this 5KW at 48 volts DC might require 6KW from the mains.
That could cost between 50 pence and £1 per hour, or many thousands of pounds a year, and liable to increase as energy costs rise.

Spending a few thousand pounds on cables larger than the minimum could be well worthwhile.

If the space is air conditioned as is likely, then reducing losses is even more important since the waste of say 6KW in cable losses may result in the consumption of another 6KW by air conditioning plant to remove the heat.
 31 October 2013 09:52 AM
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HarryJMacdonald

Posts: 254
Joined: 15 May 2002

I agree with broadgage and this implies that you should engage the overall design team in your considerations. When they see the sort of kW losses being talked about they may want to redesign the layout or use several smaller 48V power supplies. This is specialist stuff.
 31 October 2013 11:34 AM
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gkenyon

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Originally posted by: HarryJMacdonald
DC equipment is often very tolerant of reduced voltages, (protection relays operate at -20% of nominal) but your source may use up some of this (battery end of discharge is often defined as when it produces nominal -10%).
This isn't always the case for end-use equipment - some I've come across for 24 V supplies (including switching relays) may well require 90 % of nominal voltage to operate, whereas others may state 80 % to 110 % or 70 % to 110 %.

Another consideration for ELV d.c. systems, is that assumptions for LV supplies based on "faults of negligible impedance" no longer applies - and the impedance of faults becomes a real issue.

Protection doesn't always operate properly, and if not provided by fuses, may well "weld closed"; fire becomes a real risk.

Hence, consideration of mechanical protection for the heavy-current cabling is well worth it, and in the past we've used SWA for 240 V supplies as low as 20 A (in-line battery backup providing high prospective short circuit currents, even though the SMPSUs had crowbar protection).

When considering mechanical protection, where prospective short-circuit currents on ELV supplies may exceed a few 10s of Amperes, the fault may act as a "welder" and tray or trunking may well become damaged.

High current ELV supplies, particularly if battery-backed, can be really dangerous, more so when you get down to 24 V d.c., but still plenty of things to consider at 48 V d.c.

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 20 January 2014 02:46 PM
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BILLYBOY65

Posts: 35
Joined: 11 May 2011

Thanks Guys,

The client is feeding 6 PDU's in a DATA environment, to each have the capacity of 1250amps.They want to run from an existing DC unit in the hall to feed these units. Out
going circuits by others.

Yes it is specialist stuff, does anyone know a freelance engineer who could do a design and spec on a fee paying basis ?

Regards

Billy
 20 January 2014 09:14 PM
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jamieblatant

Posts: 511
Joined: 11 January 2006

Not much help but I enjoyed the banter / Knowlage in this post I fid DC a bit voodoo over a 50 amps

Loads of people ask me to do stuff on cars/ vans and seem baffeld when as a skilled electrical engineer I tell them batteries scare me

Am I the only one ???? Who dreads ?...........THE DIRECT CURRENT ......MMMMMWWWHAHAHAHAHAHAHAHAHH

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 20 January 2014 09:35 PM
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slittle

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I'm not keen either.

We've got involved in a job which is now going to get a large UPS (wasn't in the original brief) and I was asked if we wanted to install......

Er, no thanks....

40 odd Batteries equals lots of DC volts and Amps which I decided where best left to the UPS company. Their install cost was £300 which I reckoned was pretty fair.


Stu
 21 January 2014 07:53 PM
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peteTLM

Posts: 3119
Joined: 31 March 2005

BT have DC cabling such as this (and much much bigger) in big switching centres. From observation, these are multiple Busbars 20mm thick and 100mm deep (at least) and occasionally bolted together in parallel, double and triple per conductor dependent on current demand. Scary stuff at DC.

BUT, im surprised you are still doing a centralised system as individual PER racks per suite have been the norm for 20 years. Smaller batteries, easier to maintain, and not 1 point of failure.

DC Data centre cabling is a specialist subject. Get someone to design it properly. It will costs thousands to get designed, but this isnt a corner shop its going into. Would i touch it, no, not on your life.

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Lack of planning on your part doesn't make it an emergency on mine....

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 24 January 2014 01:10 PM
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BILLYBOY65

Posts: 35
Joined: 11 May 2011

Thanks guys,

it was a strange request and obviously a specialist subject, I have thrown it back to the client for them to obtain a full design and spec and I will price from that.

Cheers
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