Are you using a shield with the circuit configuration from the ‘Learn’ section?
I have no experience with this particular Arduino, but I don’t see why there should be a problem.
You design the circuit like this:
From what little information I can quickly find, the MKR runs at 3.3 V, like our emonTx.
Choose R1 & R2 to be any convenient value. What the value is, is not critical. What is critical is both are the same value. You feed those resistors from 3.3 V, not from the 5 V.
Choose the burden so that you get about 1.1 V rms across it at the maximum current you want to measure. For your YHDC SCT-013-000, it’s 22 Ω to measure 100 A. That 1.1 V rms gives you close to 3.3 V peak - peak to feed into the analogue input pin. The calibration constant is the actual current that gives you 1 V (both measured as rms) across the burden resistor.
1480 might not be the ‘best’ number to use. That was the best for the Uno when it was written, but it depends on the speed of sampling and on emonLib. (See Sampling rate of emonLib) But that should not make a big difference to the numbers.
Is “an amperometric clamp” one something like this:
https://www.edwardes.co.uk/media/image/5096/161/133/0/Fluke-323-400a-600v-AC-Digital-Clamp-Meter/jpg
Three possible reasons for the difference:
1 - Your heater is not what it says it is!
2 - What is the current range of your meter? If the current you are measuring is small compare to the maximum on the range you were using, the meter will not be accurate.
3 - Were the jaws clean and closed completely? Any dirt will cause a big error.
Unless you have cracked the ferrite core, it is not likely that it is broken. I have tested one to 250 A and it is still good. If you use a multimeter, or the Ohms range on your clamp ammeter, you should measure approximately 100 Ω between tip and sleeve of the plug. If you do, the c.t. is OK.