Cable calculation parameters

KeenPensioner · Dec 12, 2020

Hi, as I've said many times on this forum, I'm not an electrician and now retired. I'm still keen to learn and spend a lot of time studying all things electrical (especially during this lock down madness - it keeps my brain active!!).

Anyway - I understand what Ib and In are and how they relate to each other when calculating cable size but I'm confused whether there is a difference between It and Iz?

Following on from that can you help explain why "the protective device operating current (I2) cannot be bigger than 1.45 times the current carrying capacity of the cable (Iz)."

Any help would be appreciated.

Lister1987 · Dec 13, 2020

Coordination goes Ib (Design Current) must be less than or equal to In (Protective device) must be less than or equal to Iz (Current Carrying Capacity of conductor or It (Tabulated/Corrected Current carrying Capacity) (Electricians drive their Mercedes BeNZ uphill to Town (B N Z T, uphill = getting bigger)

You're wanting to ensure that your protective device and cable can handle the design current, and in the case of I2T not exceeding 1.45x Iz it's relating to the energy that the device lets though and, unlike It/Iz doesn't take into account resistance (hence the 1.45 factor to them take account of this resistance).

KeenPensioner · Dec 13, 2020

Thank you for taking the time to reply.

You say in your reply "be less than or equal to Iz (Current Carrying Capacity of conductor or It (Tabulated/Corrected Current carrying Capacity"). Am I correct in assuming that after correcting "Ib" to account for environmental and installation methods you arrive at "It". Then using "It" you look up the tables to get" Iz"? In other words "It" and "Iz" have different values - "Iz" equal to or greater than "It"

Still not sure about the 1.45 theory, You've given me something to work on though.

Thanks again for your help

Zerax · Dec 21, 2020

Iz = It x Derating factors from App4 of BS7671

Wilko · Dec 21, 2020

Hi - here’s an article that might help

.

Requirements for Protection Against Overload

The primary purpose of a protective device is to protect the circuit from damage, and for such purposes Regulation 433.1.1 requires the operating characteristics of a device protecting a conductor aga

professional-electrician.com

KeenPensioner · Dec 21, 2020

Wilko said:
Hi - here’s an article that might help .

Requirements for Protection Against Overload

The primary purpose of a protective device is to protect the circuit from damage, and for such purposes Regulation 433.1.1 requires the operating characteristics of a device protecting a conductor aga

professional-electrician.com

Thank you for pointing me to this article but I've already read that. I fully understand conditions (1 and 2) but I'm struggling with condition (3) - I2 ≤ 1.45 Iz. I've followed several worked examples elsewhere (Chris Kitcher, SparkNinja) and they refer to It being derived from Ib and In making allowances for environmental and installation methods to determine cable size and I understand that as well, My question - is there a difference between It and Iz and can you explain what I2 is. This is purely a learning exercise for me but I'd appreciate any help you can give.