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 Topic Title: Max Zs tables in 7671 17th Topic Summary: Created On: 02 August 2008 01:49 PM Status: Read Only Linear : Threading : Single : Branch
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 02 August 2008 01:49 PM nickperkins Posts: 162 Joined: 25 July 2008 I am used to reducing the tabulated values max Zs in 7671 16th by 80% to alow for heat in the circuit. Now that the tables are calculated differently in the 17th, do i still need to reduce the new values by 80%? NP 02 August 2008 01:59 PM Testit Posts: 2962 Joined: 06 August 2007 yep.... ------------------------- Online Services - http://propertydevelopment.org.ukExperience can sometimes show that cost prevails over quality and safety, such little self-value that people hold. 02 August 2008 02:35 PM Legh Posts: 4066 Joined: 17 December 2004 By measurement Zs(meas) <= 0.8 Zs(tab) or by calculation Ze + 1.2*(R1+R2)*L = Zs(Calc) <= Zs(tab) (R1+R2 are taken from the tables in GN3 (and I assume, OSG 17th)) Legh ------------------------- http://www.leghrichardson.co.ukde-avatared 02 August 2008 11:31 PM Pauldf1 Posts: 54 Joined: 09 May 2007 Just to confuse the matter, the figures in the back of guidance note 3 (2008 ed) appear to have been calculated by dividing the tabulated figure by 1.24. (The 1.24(F) is given by the calculation F = 1 + (60 x 0.004) where 60 is the difference of conductor temperatures between max and measured and 0.004 is the simplified resistance coefficient of copper or alu.) 02 August 2008 11:44 PM alancapon Posts: 6941 Joined: 27 December 2005 Originally posted by: Pauldf1 Just to confuse the matter, the figures in the back of guidance note 3 (2008 ed) appear to have been calculated by dividing the tabulated figure by 1.24. Yes, but dividing by 1.24 is the same as muliplying by 0.8065, which is almost 0.8. That is where we started. Regards, Alan. 02 August 2008 11:50 PM Pauldf1 Posts: 54 Joined: 09 May 2007 Yep, x 0.8 is almost right. I just thought I'd add the / 1.24 as the figures don't match up with the ones in GN3 when you do the x 0.8 thing. In the old GN3 they just used x 0.8.
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