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.
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
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.
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 6.90 V for a
230V 1-ph circuit
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.
Therefore the total voltage drop per run
Vd = mV x I x L
1000
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 xL
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