OT...transformers

fishenguy

Member
Join Date
Nov 2007
Location
WESTERN PA
Posts
125
How do you figure how many amps a control voltage transformer is rated for?
The label says 250va. It's 480v primary and 120v secondary.

What size fuses should I use on the primary and secondary?

Thanks for your help, Dave
 
VA is Volts x Amps
this is similar to watts
works equally for input or output
120/250 = 2amps approx Output
OR
480/250 = 0.5 approx input
 
most transformers can handle more -
VA is the rating but allow for spikes
an old style valve may draw 1 amp but can see a spike of 3 amps.
you only need to protect the secondary spike as the primary sees 1/4 or (120/480) of that.
the Primary is different - I assume it is 480 V Phase to Phase
use a double pole CB prob 2 amp - it is unlikely the trans will burn out
 
For overcurrent protection of transformers, consult Article 450 of NFPA 70 (The National Electrical Code). There are special rules for 'less than 2 amps', and special rules if both Primary and Secondary are protected.
Now, on a little one like this, magnetizing surge will be minimal, but, on a big one, the initial surge of energization can be large. I almost always end up using circuit breakers, even though I love fuses. You need to oversize the fuses, and use pure time-delay types, if its a big one (5000 VA ++).
IMHO
 
For overcurrent protection of transformers, consult Article 450 of NFPA 70 (The National Electrical Code). There are special rules for 'less than 2 amps', and special rules if both Primary and Secondary are protected.
Now, on a little one like this, magnetizing surge will be minimal, but, on a big one, the initial surge of energization can be large. I almost always end up using circuit breakers, even though I love fuses. You need to oversize the fuses, and use pure time-delay types, if its a big one (5000 VA ++).
IMHO

I would always use C/B's - quicker trip time - but you can use 'D' curve if needed.
the 5000va transformers - the larger CB's can have their magnetics and thermals set.

I don't know your regulations - is there a link so i can read them?
 
Thanks Annon
We have clearly documented regulations specifically for electrical, machine safety, cable sizing etc. these are law over here.
 
The NEC codifies the requirements for safe electrical installations into a single, standardized source. It is part of the National Fire Codes series published by the National Fire Protection Association, and while not itself a U.S. law, NEC use is commonly mandated by state or local law.
I have never met any U.S. electrician, electrical designer, or electrical engineer who would dare to intentionly violate the National Electrical Code. For all intents, it has the FORCE of law in the US, and those who use the arguement that "it is not law" are living a fantasy that only the ignorant can afford.

States and cities often have an earlier version of the NEC that is their official law, and sometimes are slow to adopt the newer editions. Sometimes they also impose requirements in addition to those in the NEC. But overall, it is a national standard just like it says in the title.
 
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