Document 2.4.126, Revision 4
October 2017
31
Three-Phase Padmounted Distribution Transformers
2. Re-install the hand-hole cover. Re-
install fasteners according to the torque
recommendations in Table 1. After tightening
all fasteners, re-torque each one to ensure
proper torque.
3. Pressurize the headspace to 3-4 PSIG and
check for fluid leaks. This pressure should be
maintained for at least four hours.
Torque Guidelines
Tables 1 through 4 below contain recommended torque values for tightening various connections on the
transformer. Connections with gaskets and those involving rubber components (such as high-voltage busing
inserts) will normally relax after initial tightening. Nominal torque values listed below include an allowance
for normal relaxation.
Do not over-tighten any connection; otherwise, gaskets may split due to over-compression, and components
may break. Fluid leaks may result if tank-mounted components are over tightened. Check with the Howard
Industries Transformer Division for recommended torque values for any devices or connections not listed
below. Use the manufacturer’s recommended torque values for any customer-supplied devices.
Fastener Type
Nominal Torque (in-lbs)
Torque Range (in-lbs)
Penta-head security bolt
100
80-120
3/8” bolt
550
500-600
Hand-hole cover bolt
190
170-210
Table 1: Torque Guidelines for External Cabinet Fasteners
Mounting Type
Nominal Torque (in-lbs)
Torque Range (in-lbs)
Low-voltage bushing, molded Tri-Clamp (without
clamp ring), 3/8” mounting studs
60
40-80
Low-voltage bushing, molded (with clamp ring),
3/8” mounting studs
120
90-150
Low-voltage bushing, porcelain (with clamp
ring), 1/2” mounting studs
80
70-90
High-voltage bushing, molded Tri-Clamp (with-
out clamp ring), 3/8” mounting studs
60
40-80
High-voltage bushing, molded (with clamp ring),
3/8” mounting studs
120
90-150
High-voltage bushing, porcelain
80
70-90
When checking tightness of gasketed components, the measured torque will normally be less than the nominal torque listed in the
table above due to relaxation of the gasket material. Additional tightening of bushing mounting hardware may cause the component
to crack or the gasket to become over-compressed.
Table 2: Torque Guidelines for External Bushing Mounting Hardware
