I took the Mike Holt 25 question practice test HERE. There are a few questions I got wrong, but on many, I am able to figure out why I got them wrong. This thread is for those I am unable to figure out.
Q14. An existing junction box is located 65 ft from the panelboard and contains 4 THHN aluminum conductors. What size copper conductor can be used to extend this circuit 85 ft and supply a 50A, 208V load? Note: Apply the NEC recommended voltage-drop limits.
A. 8 THHN
B. 6 THHN
C. 4 THHN
D. 10 THHN
I chose B, but the answer is C. The original 65 ft length of wire only has a 3.3V voltage drop, which is 1.6% of 208V. To arrive at my answer, I simply did a voltage drop calc using 6.24V which is 3% (this is the NEC recommended voltage-drop limit, isn't it?) of 208V and using the 85 ft length. I arrived at 17572 kcmil which is #6. However, since the answer is #4, I'm guessing I should have added the 65 ft and 85 ft together for a total of 150 ft. When I do that, I arrive at a kcmil of 31009, which bumps me up to #4. Is it correct that I am to add up the original length of wire and the new length of wire for my total voltage drop distance? Thanks.
Roy
more to follow I'm sure.
Q14. An existing junction box is located 65 ft from the panelboard and contains 4 THHN aluminum conductors. What size copper conductor can be used to extend this circuit 85 ft and supply a 50A, 208V load? Note: Apply the NEC recommended voltage-drop limits.
A. 8 THHN
B. 6 THHN
C. 4 THHN
D. 10 THHN
I chose B, but the answer is C. The original 65 ft length of wire only has a 3.3V voltage drop, which is 1.6% of 208V. To arrive at my answer, I simply did a voltage drop calc using 6.24V which is 3% (this is the NEC recommended voltage-drop limit, isn't it?) of 208V and using the 85 ft length. I arrived at 17572 kcmil which is #6. However, since the answer is #4, I'm guessing I should have added the 65 ft and 85 ft together for a total of 150 ft. When I do that, I arrive at a kcmil of 31009, which bumps me up to #4. Is it correct that I am to add up the original length of wire and the new length of wire for my total voltage drop distance? Thanks.
Roy
more to follow I'm sure.