MINIMUM ACCEPTABLE LEVELS OF SAFETY & RELIABILITY THE WIRING CODE OF TRINIDAD AND TOBAGO TTS 171 PART ONE

PRESENTED BY HAYDEN BATSON ELECTRICAL INSPECTOR 1 GOVERNMENT ELECTRICAL INSPECTORATE

Annex C Provides Sub-Circuit rating and requirements. C.1 Gives the maximum sub-circuit ratings for domestic, commercial and industrial premises. C.2 Gives the sub-circuit requirements.

Table C-1

Section B gives those that are not shown before

C.2.1 Ground-fault Circuit Interrupter protection shall be provided for personnel (ref sec 210-8 NEC 1999) in: a) Bathrooms b) Outdoor receptacles c) Garages d) Counter tops e) Other required locations.

C.2.2 Additional requirements for conformity. Receptacle outlets shall be spaced approx. 4M along walls in rooms. Receptacle outlet and lighting circuits shall be separated except by special permission of the CEI.

The rating for a general purpose domestic receptacle is 180 VA. The minimum height of the service entrance connection shall be 4M, observing the minimum clearance of 6M over road ways. We will maintain the IEE color code.

Two examples are are shown using table D.5.D.1 INSTALLATION METHODS The exact current carrying capacity of conductors is affected by a number of factors besides CSA, one of which is the way they are installed. Two examples are are shown using table D.5.D.1

Installing a run of 10 mm2 XLPE insulated multi-core armored cable: Example 1 Installing a run of 10 mm2 XLPE insulated multi-core armored cable: Using Method 1, a 3 or 4 core cable connected to a 3-ph ac supply will carry 73A. Using Method 11, a 3 or 4 core cable connected as above will carry 78A.

Installing a run of 95mm2 XLPE insulated multi-core armoured cable. Example 2 Installing a run of 95mm2 XLPE insulated multi-core armoured cable. Using Method 1, a 3 or 4 core cable connected to a 3-ph ac supply will carry 289A. Using Method 11, a 3 or 4 core cable connected to a 3-ph ac supply will carry 304A.

When using cards with current rating tables the method of installation for those values must be observed.

VOLTAGE DROP CONSIDERATIONS Apart from voltage drop and installation methods, factors such as ambient temperature grouping thermal insulation operating temperature of the cable also weigh in cable selection.

Emphasis however must be placed on voltage drop. WHY? This value should not exceed 3% in the circuit of which that cable forms apart.

Not more than 14.4V for a 480V 3-ph circuit.

Note also that the maximum allowable drop from the POE to fixed current using equipment must not exceed 4%.

VOLTAGE DROP TABLES These are tabulated for a current of 1 Amp and a length of 1 Metre along the route taken.

For a balanced 3-ph system the tables relate to line voltage.

Vd = mV x I x L 1000 Therefore the total voltage drop per run Where I = current in amps L = length in metres & mV = approx voltdrop/amp/metre.

Example: Consider a run of 200M to be installed in air (ref method 13) to carry a current of 100A from a supply of 415V 3-ph ac, the cable being of copper conductor, XLPE insulated, armoured and PVC sheathed.

Let mV be voltage drop in millivolts Then Vd = mV x I x L 1000 Transposing mV= Vd x 1000 I x L

Max permissible voltage drop= 3% of 415V =12.45V Inserting values for I, L and max permissible voltage drop Then mV= 12.45 x 1000 =0.62 mV 100 x 200 Referring to the following table.

Select value = to or just less than 0.62mV i.e. 60mV.

Apply current carrying capacity table.

This shows that we may have used a 25 mm2 cable without voltage drop consideration.

THE END