fuse/wire sizing

[tulum]

Here is the situation (in canada)...

The electrcian installed a squared 600v 60a switch with a 50A fuses. The switch feeds a 45kVA (80degc) 600V-208/120 transformer. The transformer feeds a 100A 120/208v squared panel c/w a 100a main.

The electrical safety authority says that because of rule 14-606 the primary fuses need to be changed to 40A and the secondary cables need to be upsized from #2 to #1.

Does the 100A breaker intergral to the panel board not count as suuffecient overload protection for the panel?

I do not understand...

Regards,
TULUM

 

[wareagle]

I am not familar with the Canadian Code. I am assuming you have a 3 phase transformer. 45 kva/.600 x 1.73 = 43 amps.
1.25 x 43 amps = 54 amps. You could use a 60 amp under the US NEC. The full load rating of the transformer is 125 amps. If the code require that the conductor be rated for the full load amps , then you need to use #1 cu. The NEC 75C rating of #2 is 115 amps which would be adequate since the main panel has a 100 amp breaker. There are other NEC rules that dictate the conductors size, these being how long the circit is. If its 10 ft or less the #2 would be correct. The Canadian Code Conductor ratings may be different.

"The electrical safety authority says that because of rule 14-606 the primary fuses need to be changed to 40A and the secondary cables need to be upsized from #2 to #1."
A 40 amps appears to be too small.

 

[tulum]

Thanks wareagle the safety inspector actually requires a 1/0 on the secondary ( I was miss informed).

From looking at 14-606, which basically says if you have a panel at 100A then the primary overcurrent device needs to be 100/(600/208) =35A or 40A fuses.

It does not specify in the code but I believe if an integral main is in the panel sized for the conductor this does not apply?

Thanks
TULUM

 

[l3city]

I don't have the NEC code in front of me, but I will say that if the secondary feeder ampacity is rated less than your (100A) panel main, than there will be a problem since your secondary feeder will not be protected. Maybe, that is why he is asking to change from #2 to #1.
Don't know what rule 14-606 is, but something similar might be happening.
Regards

 

[tulum]

Sorry guys,

I was a little vague...in CEC world a #2 is good for 115A...

 

[tulum]

Just to add the confusion...

We are using type J (AJT50) fusing and the inspector requires that it be changed to type RK 40A fuses..

 

[l3city]

hi there, as wareagle said, 60A fuses will be ok, 50A might be ok but "pushing it" (under NEC). On the other hand, "Cooper-Bussman" recommends the RK type fuses for inductive and resistive loads, transfomers, motors, capacitor banks, ask your inspector exactly why, but that might be a reason.
Regards

 

[tulum]

I think I get it now...

Because the cable on the secondary is good for 115A.. the fusing on the primary cannot be set at 125%. 125% fuses on the primary protect a cable that is sized at 125% on the secondary (up to the OCD). Therefore the primary fuses must be set at 115/(600/208) = 39A...

Regards,
TULUM

 

[wareagle]

tulum posted

"Because the cable on the secondary is good for 115A.. the fusing on the primary cannot be set at 125%. 125% fuses on the primary protect a cable that is sized at 125% on the secondary (up to the OCD). Therefore the primary fuses must be set at 115/(600/208) = 39A... "

The NEC will not allow you to protecdt the secondary with the primary protection except as noted in 240.21.C.1. In
other cases the secondar conductors may be protected by the
OC device in the panel if the installation meets all of the requirements of 240.21(C)(2 and 3)